Saltwater intrusion in Southern Eyre Peninsula, December 2009

Saltwater intrusion in Southern
Eyre Peninsula
December 2009
i

This paper has been produced under the Eyre Peninsula Hydrogeology Research Fellowship
as a part of the Eyre Peninsula Groundwater, Allocation and Planning Project.
Authors: James Ward, Adrian Werner and Brenton Howe
Date of publication: December 2009
Front cover image: Sea cliffs and wind farm near Uley South
An appropriate citation for this report is:
Ward, J.D., A.D. Werner and B. Howe 2009, Saltwater Intrusion in Southern Eyre Peninsula.
Report developed through the Eyre Peninsula Groundwater, Allocation and Planning Project.
ii

Contents
Executive Summary
v
1. Project Background 1
1.1 Eyre Peninsula Water Resources 1
1.2 Report Context and the National Water Commission 2
1.3 Eyre Peninsula Hydrogeology Research Fellowship 3
2. Introduction 4
2.1 Broad EP Hydrogeology 4
2.2 Significance of Seawater Intrusion on EP 6
3. Past Investigations of Seawater Intrusion 8
3.1 Monitoring (drilling and sonding) 8
3.2 Airborne Electromagnetic Survey 10
3.3 Student Research Projects 13
4. Methods 15
4.1 Conceptual Models of Saline Intrusion 15
4.1.1 General Concepts 15
4.1.2 Eyre Peninsula Examples 16
4.2 Application of an Analytical Model 20
4.2.1 Basic Model 20
4.2.2 Dimensionless Formulation 22
4.2.3 Steady-State Seawater Intrusion – Key Factors 24
4.2.4 Steady-State Seawater Intrusion and Climate Change 25
4.3 Parameterisation of EP Aquifers 26
4.3.1 Aquifer Properties 26
4.3.2 Regional Water Balance 28
4.3.3 Inland Boundary Condition 28
4.3.4 Parameter Summary 29
4.3.5 Climate Change Scenarios 29
5. Results and Discussion 30
5.1 Seawater Intrusion Vulnerability of EP Aquifers 30
5.2 Comparison with Observations 32
5.2.1 Evidence of Up-coning 32
5.2.2 Depth to Saltwater Interface 33
5.2.3 Saline Interface Predicted by AEM Survey 35
5.3 Future Impacts 36
5.3.1 Climate Change 36
5.3.2 Increased Extraction 37
5.4 Confidence in Predictions 39
5.4.1 Uncertainty in Climate Change Scenarios 39
5.4.2 Outstanding Data Gaps 40
6. Conclusions 41
7. Future Work 42
7.1 Ongoing Work 42
7.1.1 Monitoring of Saltwater Interface in Coastal Wells 42
7.1.2 Scenario Testing Using a Numerical Model 42
7.2 Suggested Further Work 43
7.2.1 Investigation of Heterogeneity 43
7.2.2 Investigation of Transient Behaviour and Reversibility 44
7.2.3 Investigation into Saltwater Up-coning 44
7.2.4 Uncertainty Analysis of Seawater Intrusion Approaches 44
7.2.5 Assessment of Sources of Salinity 44
References 45
Appendix - Data and interpretation provided by SA Water 48
iii

Tables
Table 1. Summary of hydrogeologic units ............................................................................5
Table 2. Aquifer parameters adopted for analytical modelling .............................................. 29
Table 3. Dimensionless parameters for seawater intrusion extent in Southern
Basins .......................................................................................................................... 30
Figures
Figure 1. Regional map showing major groundwater resources on EP ..................................1
Figure 2. Southern Basins PWA, showing location of individual groundwater
basins and SA Water production bores. ..........................................................................6
Figure 3. Typical groundwater level trends in three coastal aquifers (Coffin Bay,
Uley South and Lincoln Basin) and one inland aquifer (Uley Wanilla). .............................7
Figure 4. Salinity profiles in coastal Uley South observation wells (both ~500m
inland) ............................................................................................................................9
Figure 5. Conductivity slice at -53m ADH, annotated features include resistive
basement ridges (blue) and conductive saline groundwater (pink/red) (After
Fitzpatrick et al. 2009). ................................................................................................ 11
Figure 6. Conductivity slice at -23m AHD showing potential small seawater
wedge extending inland into Uley South........................................................................ 12
Figure 7. Interpreted surface for base of Bridgewater formation; annotation
shows area below mean sea level. ............................................................................... 13
Figure 8. Two Conceptual models for seawater intrusion in Uley South Basin. .................... 16
Figure 9. Conceptual model of fresh/saline water interactions in Lincoln B Lens
(above) and Coffin Bay (below). .................................................................................... 18
Figure 10. Conceptual model for Robison Lens; pre and post pumping (Brown
and Harrington, 2003) .................................................................................................. 19
Figure 11. Diagram of parameters for analytical model of seawater intrusion ...................... 21
Figure 12. Contours of x
T
for different M and F .................................................................. 25
Figure 13. Contours of x
T
for: (a) Uley South (SE) – QL only, (b) Uley South
(SE) – QL+TS, (c) Uley South (NW), (d) Coffin Bay A lens, (e) Lincoln B
lens. ............................................................................................................................. 31
Figure 14. Evidence of seasonal saline up-coning in Lincoln B Lens. .................................. 32
Figure 15. Plot of theoretical freshwater/saltwater interface determined from (2)
for the southeast portion of Uley South, for the two conceptual models (QL
only vs. QL + TS), showing the approximate location and screened interval
of observation well SLE069. ........................................................................................ 34
Figure 16. Plot of theoretical freshwater/saltwater interface determined from (2)
for the northwest portion of Uley South assuming a continuous clay aquitard
at a depth of 15m. ........................................................................................................ 35
Figure 17. Contours of x
T
across a range of climate change scenarios for the
three aquifer conceptualisations in the Uley South basin. .............................................. 37
Figure 18. Basic assessment of steady-state seawater intrusion extent under
different increased pumping scenarios, assuming a coastline width of 5km. .................. 38
iv

Executive Summary
The Eyre Peninsula (EP) in South Australia is home to some 55,000 people, with an
economy supported significantly by agriculture and aquaculture exports. A network of
mains pipelines covers much of the region with the majority of this water being used
for domestic consumption and stock watering. EP sources approximately 85% of its
mains water from unconfined aquifers within the Quaternary limestone (Bridgewater)
formation, the most significant resource being the Uley South Basin near the
southern tip of the peninsula. Regional groundwater levels are heavily influenced by
seasonal recharge, however the aquifers typically exhibit low hydraulic gradients due
to high hydraulic conductivities. Many of the aquifers on the southern EP are in direct
hydraulic connection with the sea, and the low hydraulic heads suggest a potential
seawater intrusion threat, particularly during periods of low recharge.
This report utilises a simple analytical solution to model the flow of freshwater
overlying a stationary saltwater wedge adjacent to the sea. The steady-state model
solves the inland extent of the wedge (reported as the toe of the freshwater-saltwater
interface) as a function of the aquifer properties and the rate of freshwater discharge
to the sea; high discharge results in a saltwater wedge that is closer to the coast,
whereas reductions in discharge (due to recharge decline and/or increased
extraction) lead to a landward shift in the saltwater wedge, i.e. seawater intrusion.
The model is presented as a useful “first step” to help build intuition around some of
the processes that influence the position of the saltwater wedge in the southern EP.
Future efforts will move towards a numerical groundwater model for the Uley South
basin to provide for transient, spatially variable predictions of seawater intrusion. The
simplistic method adopted here provides only a preliminary assessment of seawater
intrusion in relative terms; three-dimensional numerical modeling is required to
properly account for the complexities of the system and to allow for seawater
intrusion characterisation for the purposes of under-pinning water resources
planning.
The application of the analytical solution to steady-state seawater intrusion broadly
supports the findings of a recent Airborne Electromagnetic (AEM) survey, which
indicates that seawater generally extends only a limited distance (~500m) into the
Uley South Basin (at the time of the survey in 2006). The AEM survey also identified
a region of deep basement in the southeast portion of the basin as having a higher
potential for seawater intrusion. The analytical modelling presented in this report has
compared the different conditions found in different portions of Uley South. The
model indicates that there is an elevated risk in the southeast portion due to the
relatively deep basement and the possible absence of a clay aquitard, while in other
areas the presence of a clay aquitard underneath the Bridgewater Formation leads to
a lower potential for seawater intrusion. Note that information from a recent
investigation relating to the presence of a clay aquitard (including in the southeast
portion of Uley South) is contained in the Appendix.
The modelling is extended to form a preliminary analysis of seawater intrusion under
changes in conditions (sea-level rise / recharge changes) as might be induced under
climate change scenarios. For Uley South, it is found that a possible future decline in
recharge is likely to contribute far more to the long-term seawater intrusion threat
v

than increases in sea-level. This assumes that the aquifer water levels are allowed to
rise at a similar rate to the rise in sea-level.
Other groundwater resources on EP such as the Coffin Bay “A” and Lincoln “B”
Lenses are at risk of intrusion via a different process - saltwater up-coning. This
mode of intrusion is not investigated quantitatively in this study although up-coning
has been observed in both of these aquifers. It is therefore recommended that a
study be undertaken into the likely threat of up-coning in those basins under present
and possible future recharge and pumping scenarios.
To date, the threat of seawater intrusion in the Southern Basins has not been studied
thoroughly in the context of resource management. Groundwater extraction volumes
have been set according to historical recharge (“flux-based management”), despite
falling trends in groundwater levels, and some very low water levels developing near
the coast in places. The dynamic nature of the saltwater interface needs to be
investigated to properly assess seawater intrusion under different management
approaches. To achieve this, a more in-depth analysis of seawater intrusion is
required, involving the application of numerical models that account for the 3D
dispersive nature of the process.
To improve confidence in the sustainable use of the groundwater resources there
must be an increase in the frequency of targeted monitoring; in the higher-risk area in
the southeast of Uley South, salinity profiles for coastal wells have so far been
obtained only twice since 2004. There is a need for an improved understanding of the
movement of the freshwater/saltwater interface over time, and preferably a link to
more proactive (such as “trigger-level”) management styles that can encompass the
dynamic nature of seawater intrusion risk and ensure protection of these crucial
groundwater resources.
vi

1. Project Background
1.1 Eyre Peninsula Water Resources
Eyre Peninsula (EP) is a diverse region with extensive coastline, numerous native
vegetation communities, various land uses and water systems and a Mediterraneantype
climate ranging from arid to temperate conditions. The economy is based on
agriculture (traditionally grain and wool), seafood (fishing and aquaculture), tourism,
mining and mineral processing industries.
Water resources are relatively scarce on EP and major groundwater resources are
confined to the south and west coasts (Figure 1). Cities, towns, and industry rely on
potable mains (or reticulated) water provided by SA Water via an extensive network
of pipelines that cover much of the west and east coasts of the Peninsula. The region
is also home to some 1.6 million sheep and 25,000 cattle (ABS, 2008), which
consume approximately the same volume of reticulated water as all domestic and
industrial users combined. The vast majority of mains water (~85%) is sourced from
prescribed groundwater resources 1 which are shown in Figure 1. The sustainable
management of these resources is the joint responsibility of the EP Natural
Resources Management Board (EPNRM) and the Department of Water, Land and
Biodiversity Conservation (DWLBC). In addition to groundwater, a small proportion of
mains water (~15%) is supplied from the River Murray to the EP, via the Whyalla to
Kimba pipeline.
Figure 1. Regional map showing major groundwater resources on EP
1 Prescribed resources are designated under the Natural Resources Management Act ( 2004)
1

The security of public water supplies on Eyre Peninsula is managed through the SA
Water Long Term Plan 2 . Under a scenario of continued population growth and
subsequent demand increase, augmentation of the existing groundwater based
supply will be required. Desalination of seawater has been identified as the preferred
supply augmentation option rather than drawing further on the River Murray.
However, given the extent of capital already invested in infrastructure, the continued
use of groundwater resources is the preferred water source option (more cost
effective and less energy intensive). For this reason, the proportion of the public
water supply being sourced from groundwater should continue to be maximized.
It is important that groundwater resources are managed to ensure their long term
sustainable use and to protect the natural ecosystems that depend on them. This is
particularly important as we face the considerable risk of a changing climate.
Incorporating climate change into the management of the region’s water resources is
vital and requires the consideration of the vulnerability or resilience of each resource.
1.2 Report Context and the National Water
Commission
In Prescribed Wells Areas (PWAs), groundwater use is currently managed by way of
annual allocations under a Water Allocation Plan (WAP). When undertaking its 5
yearly review of the WAPs in 2006, EPNRM consulted the community, who
expressed concern regarding the sustainability of the groundwater basins. Key
issues included (but were not limited to): the impacts of climate change, how much
water the environment needs, the impact of vegetation on recharge and the risk of
seawater intrusion.
In 2008, the EP Groundwater Allocation, Planning & Management (GAPM) Project
was launched to attempt to address the community’s concerns. The National Water
Commission (NWC) committed $700,000 over ~2 years and this was matched
through cash and in kind contributions from the state-based partners: Department of
Water, Land and Biodiversity Conservation, SA Water Corporation and Eyre
Peninsula Natural Resources Management Board.
The main objective of the GAPM Project is to improve the scientific understanding
that underpins the sustainable management of the groundwater resources. The
project involves a suite of detailed investigations, which are tied to reporting
milestones as follows:
2 “Meeting Future Demand : SA Water's Long Term Plan for Eyre Region”, completed in November 2008 and
accessible via the SA Water website (www.sawater.com.au)
2

Milestones 1-3 Project management objectives
Milestone 4
Milestone 5
Milestone 6
Milestone 7
Milestone 8
Milestone 9
A detailed review of the current monitoring of the groundwater
resources
The assessment of the impacts of climate change on rainfall recharge
Investigating the impacts of extraction and reduced aquifer recharge
on seawater/groundwater interaction
Investigating the relationship between soils, vegetation and recharge
The mapping of water-dependent ecosystems and an assessment of
their water requirements
The preparation of conceptual groundwater models
Milestone 10 The preparation of numerical groundwater models and predictive
scenarios to inform future management and allocation of groundwater
resources.
The project will assist EPNRM to develop new WAPs that will include a much more
detailed assessment of the capacity of the resources to meet the demands for water
on a continuing basis.
This report aims to deliver on Milestone 6 of the GAPM project. This report presents
a conceptual understanding of the nature of seawater intrusion, specific to aquifer
conditions in EP. The likely impacts of increased extraction, reduced recharge and
sea level rise are explored using a simplified approach involving analytical models.
1.3 Eyre Peninsula Hydrogeology Research
Fellowship
The Eyre Peninsula Hydrogeology Research Fellowship (EPHRF) was established to
create a focal point of expertise at Flinders University and to address key strategic
research and investigation issues related to EP’s water resources. The Fellowship is
driven by three industry partners: EPNRM, DWLBC and SA Water with further
funding support from the Flinders Centre for Coastal and Catchment Environments
(FR3CE) and the Centre for Groundwater Studies. The collaboration model is one
where a postdoctoral research fellow has been the focal point of interactions between
a collective working team comprising Flinders University students (at Honours,
Masters and PhD levels), as well as academic staff and industry partner members.
The broad research themes of the fellowship align closely with the reporting
milestones of the NWC GAPM project.
3

2. Introduction
In coastal aquifers with a hydraulic connection to the sea, assessment of “sustainable
yield” must consider both the quantity and the quality of extracted water. It is a
common misconception that extraction can be sustained provided it does not exceed
recharge; however this alone is not sufficient for sustaining acceptable water quality
(e.g. low salinity). In order to prevent seawater advancing inland in a coastal aquifer,
there must be an adequate discharge of fresh groundwater to the sea. This implies
that withdrawals from the coastal aquifer must be substantially less than total
recharge, and that a certain quantity of fresh groundwater must therefore be
“expended” (i.e. via ocean discharge) to ensure protection of the quality of the
remaining freshwater.
Under possible scenarios of climate change, the seawater intrusion threat could
increase in several distinct (though interrelated) ways: (i) sea-level rise causes a
greater hydraulic forcing on the seaward boundary of the aquifer and drives seawater
further inland; (ii) declining rainfall leads to a reduction in recharge and thus a decline
in discharge to the sea; (iii) pumping increases in response to a warmer and drier
climate, reducing the net discharge to the sea. Note that the latter change (an
increase in pumping) may also be anticipated due to ongoing population and
economic growth in the region, irrespective of climate change.
2.1 Broad EP Hydrogeology
The various hydrogeologic units are summarised in Table 1. More detailed
descriptions of EP hydrogeology can be obtained from numerous investigative
reports produced through the SA Mines and Energy Department. Barnett (1980)
produced a useful bibliography of hydrogeology that can be used to locate early
reports specific to each individual area.
The primary target for reticulated public water supply is the Bridgewater Formation,
an unconfined Quaternary Limestone aquifer. The aquifer occurs as isolated lenses
with generally high yields and low salinity. Skeletal soils, surface calcrete, sparse
(sometimes absent) vegetation, the presence of surface dissolution features and
karst limestone all contribute to enhanced recharge. Groundwater levels in these
aquifers typically rise rapidly in response to episodes of high rainfall and decline
during extended dry periods.
The secondary aquifer is the Wanilla Formation, a semi-confined Tertiary Sand
aquifer characterised by generally lower yields and more brackish water than in the
Bridgewater Formation. The Tertiary sands are usually (but not always) overlain by a
relatively thin Tertiary Clay (the Uley Formation) of low permeability. Hydraulic
gradients between the aquifers are generally low, indicating that extensive inter-layer
flows probably occur.
4

Table 1. Summary of hydrogeologic units 3
Era Unit, Lithology Hydrogeology
Recent (Holocene)
Cainozoic
Coastal Dunes: Fine-grained
aeolianites, unconsolidated, actively
mobile. Grains comprise calcite and
shell fragments.
Unconfined Aquifer: seasonal, small
yielding, thin, low salinity supplies
located at the base of the mobile
sand dune systems
Quaternary
(Pleistocene)
Cainozoic
Bridgewater Formation: Aeolianites, Unconfined Aquifer: generally low
fine to medium-grained, cross bedded, salinity. Permeability ranges from
weakly to moderately cemented, Grains low to very high. Hydraulic
are calcite and shell fragments, mainly conductivity ranges typically up to
0.1 - 1.5 mm. Generally calcrete at 1000 m/day. The usual target
surface.
aquifer for large water supplies on
EP.
Tertiary (Eocene)
Cainozoic
Wanilla, Poelpena, and Pidinga
Formations: Clays, sands (quartz) and
gravels with thin lignite layers. Sand is
generally fine-grained, less than 0.5
mm, uncemented or weakly cemented.
Semi-confined to Confined Aquifer:
low to moderate permeability but
with marked variations vertically and
laterally. Salinity variable and
generally higher that the overlying
unconfined aquifer.
Jurassic
Mesozoic
Polda Formation: Sands (quartz), silts
and clays. Sand grains usually less
than 0.5 mm, occasionally up to 3 mm.
Sediments generally carbonaceous and
contain lignite beds.
Confined Aquifer: very low
permeability, high groundwater
salinity generally exceeding 14,000
mg/L.
Neo-Proterozoic
Proterozoic
Pre-Cambrian Basement: Schists,
gneisses and quartzites intruded by
granites and basic rocks. Deeply
weathered in places.
Semi-confined to Confined Aquifers:
groundwater occurs in the
weathered profile or within the
fracture spaces of these rocks.
Salinity generally exceeds 7,000
mg/L, occasionally lower.
3 Table compiled by DWLBC (Fact Sheet #3)
5

2.2 Significance of Seawater Intrusion on EP
Several of EP’s most important groundwater sources occur in coastal settings. These
include the Uley South and Lincoln Basins, which respectively provide ~70% and
~10% of total reticulated water, as well as Coffin Bay “A” Lens which currently
provides 100% of reticulated water to the township of Coffin Bay (i.e. an isolated
supply system). These resources lie within the Southern Basins PWA (Figure 2).
Given the overall importance of these aquifers, it is important to understand and
manage against the risk of seawater intrusion due to either climate change or overextraction.
Figure 2. Southern Basins PWA, showing locations of individual groundwater basins and SA
Water production bores. Note that water is not extracted from all basins; however three of the
key production areas (Uley South, Lincoln and Coffin Bay A) are located adjacent to the
coast.
6

Water Level (m AHD)
Water Level (m AHD)
Water Level (m AHD)
Water Level (m AHD)
Hydrographs from the four key water supply areas in the Southern Basins are shown
(Figure 2.1b) for comparison. Of the four areas shown, the three coastal aquifers
(Coffin Bay, Uley South and Lincoln B) currently have relatively steady water levels at
around 1m AHD (i.e. 1m above mean sea level). Uley Wanilla is an inland system
occurring at a higher elevation, and is not connected to the coast, however it is
included here to show the contrast in water level trends. The past two decades of
below-average recharge 4 are likely to have contributed to the downwards trend in
Uley Wanilla (~4m drop over 20 years, and continuing to fall). Meanwhile, the
downward trend is far less pronounced in the three coastal zones, where there is a
“boundary condition” of 0 m AHD imposed by the sea. In these areas a reduction in
recharge is likely to have led to a reduction in freshwater outflow to the sea. This can
theoretically cause inland movement of the saltwater wedge, which serves as an
inflow to the aquifer thereby helping to “buffer” further water level decline. The extent
of this effect is currently untested, and requires more sophisticated methods than
those adopted here. It is clear from other studies of seawater intrusion that stable
groundwater levels do not necessarily indicate a stabilised saltwater wedge, because
of the difference in lag time between the watertable (e.g. days-months) and saltwater
wedge (e.g. months-years) following a change in the hydrological conditions (e.g.
recharge, pumping, etc). In coastal aquifers, stable water levels may appear to imply
that an equilibrium between recharge and discharge has occurred and that the
aquifer provides a sufficient supply of water in terms of quantity, however if seawater
is intruding the aquifer from the coast, then water quality may be at risk.
2
Coffin Bay - LKW027
73
Uley Wanila - ULE007
72
1
71
70
0
69
68
67
-1
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
66
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
5
Uley South - ULE102
2
Lincoln B Lens - SLE030
4
3
1
2
0
1
0
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
-1
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
Figure 3. Typical groundwater level 5 trends in three coastal aquifers (Coffin Bay, Uley South
and Lincoln Basin) and one inland aquifer (Uley Wanilla). Note the “levelling off” of coastal
water levels at around 1m AHD, while inland levels have continued to drop.
4
Note that in the Uley Wanilla basin, pumping rates have been greatly reduced over the past two decades in an effort
to arrest the decline in water levels. Despite these efforts, water levels have continued to drop sharply relative to the
long-term average. As the fall in water levels are not easily attributable to over-extraction, it is thoughts that a
reduction in recharge has occurred.
5
Data obtained from “ObsWell”, Primary Industries and Resources SA (PIRSA) https://obswell.pir.sa.gov.au/
7

3. Past Investigations of
Seawater Intrusion
Consideration of seawater intrusion on the Eyre Peninsula has predominantly
focused on Uley South due to the large volumes of water extracted and thus its
overall importance to water supply. Morton and Steel (1968) noted the presence of
coastal springs at Shoal Point (Figure 2.1) and used what was thought to be a
“conservative” value of hydraulic conductivity (0.011 cusecs/ft 2 , or ~300 m/d) to
estimate an aquifer discharge to the coast of ~17 GL/a (roughly 20% of rainfall).
Barnett (1978) revised this estimate up to 30 GL/a and considered that 50% of this
could be extracted without fear of seawater intrusion, citing favorable basement
configuration (sloping up from the coast to a relative high point near the extraction
area) as the key to the minimal risk.
Evans (1997) further revised the estimate of discharge to the sea, this time
considering only the central portion of the basin (where extraction was occurring at
the time) and using the Ghyben-Herzberg principle to account for a saline wedge at
the coastal boundary. Discharge through this portion of the basin was calculated to
be ~11GL/a prior to the inclusion of the saltwater wedge, which was found to reduce
the discharge estimate to ~5GL/a. Evans also used pressure transducers to
demonstrate that water levels in the Tertiary Sand aquifer respond to tidal
fluctuations, confirming the strong hydraulic connection to the sea.
3.1 Monitoring (drilling and sonding)
In response to expansion of the Uley South bore field and increased extraction in
2000, monitoring infrastructure was upgraded (Clarke et al., 2003; Clarke, 2005).
This involved the construction of two long screened coastal monitoring wells, allowing
the direct measurement of a saline interface by salinity profiling (sonding). In 2004,
the first sonding identified a relatively sharp interface at approximately 24 and 17
meters below sea level in ULE205 and SLE69 respectively (Figure 4).
Sonding of these two wells was conducted again in 2008, with the aim of observing
any trend changes in the system. The results, however, have proven inconclusive
due to the long gap between monitoring and the unknown impacts of tidal fluctuations
or dispersion of the interface. Further, the approach to sampling the long-screen
wells was not consistent in the 2004 and 2008 sonde profiles – further
measurements are needed that adopt a consistent approach. The data may indicate
that there has been no significant change in the depth of the interface between fresh
and saltwater, although the nature of the mixing zone may have changed in ULE205.
Note that Clarke (2005) recommended monitoring of the seawater-freshwater
interface via sonding at intervals of 2-3 months initially. A more intensive interface
monitoring program is now being planned (see Section 7.1.1).
8

Elevation m AHD
Elevation m AHD
ULE 205
EC uS/cm
-10000 0 10000 20000 30000 40000 50000 60000
2
SLE 69
EC uS/cm
-10000 0 10000 20000 30000 40000 50000 60000
2
0
0
-2
-4
Jun 2004
Jul 2008
-2
-4
Nov 2004
Jul 2008
-6
-6
-8
-8
-10
-10
-12
-12
-14
-16
-18
-20
-14
-16
-18
-20
Mixed Zone
-22
-24
-26
-28
-30
-32
Mixed Zone
-22
-24
-26
-28
-30
-32
Figure 4. Salinity profiles in coastal Uley South observation wells (both are approximately
500m inland)
The interpretation of salinity profiles from fully slotted wells is complicated by the
potential for significant movement of water within the well itself (Shalev et al., 2009).
Due to density differences, the interaction between freshwater and saltwater
commonly creates strong vertical components of flow near the coast, and these are
usually upwards near the interface. This means that the level of the interface in the
well is not necessarily an accurate representation of the interface in the aquifer.
During 2009, drilling in coastal areas of Uley South was conducted to install
additional monitoring infrastructure and to assist in the “ground truthing” of AEM data
(see Section 3.2). SA Water funded drilling at two locations in the northwestern
coastal area of the basin where previously very few wells have been drilled and
hence understanding has been poor. EPNRM, through the complementary state
funding program, drilled two wells (ULE156 & ULE209) in a single location in the
southeastern coastal area (where Fitzpatrick et al., 2009, identified an area
“particularly susceptible to salt water intrusion”). At the time of this report, SA Water
drilling operations had been postponed due to technical difficulties. ULE156 was
drilled to basement and did not encounter any water of salinity resembling seawater,
indicating an absence of seawater intrusion in that area (Evans et al. 2009), although
monitoring is ongoing.
9

3.2 Airborne Electromagnetic Survey
In 2006, an airborne electromagnetic (AEM) survey was conducted over an area that
covers the Uley South and Coffin Bay A lenses, including the area between them
under the Coffin Bay National Park, where access for drilling is severely limited and
few boreholes exist. The major purposes of the study were to help define the bounds
of the aquifers (stratigraphy) and to define the extent of saline groundwater within the
aquifers. The following section serves as a summary of the Airborne Electromagnetic
project and results. For further detail please refer to the final report (Fitzpatrick et al.
2009).
The AEM method provides a non-invasive technique that collects information about
the conductivity (or inversely, the resistivity) of the sub-surface. Conductivity is
primarily controlled by a combination of the rock porosity and the properties of the
groundwater (in particular the presence of salts). Conductivity is also commonly
influenced by the degree of saturation and the presence of clay. In this survey, areas
of suspected saline groundwater were considered relatively easy to identify as they
exhibited extremely high conductivities (Fitzpatrick et al., 2009).
The raw conductivity data were manipulated using advanced processing (inversion)
techniques, and plausible interpretations were made for the base and thickness of
the Quaternary Limestone aquifer and base and thickness of the combined Tertiary
Clay, Sand and weathered basement sequences. Results were constrained using
existing stratigraphic logs and down-hole logging. Interpretations of aquifer bounds
compared well with interpretations made by bore-logs alone, indicating that the
technique provided a useful insight in areas where bore logs are sparse or absent.
Deeper sequences (basement topography) were defined with lower confidence and
were not defined at all below highly saline water due to masking of the signal by the
salt water.
In the Coffin Bay National Park area, AEM results suggested the presence of highsalinity
groundwater (presumed to be seawater) at depth, extending from the
southern coastline to Coffin Bay (Figure 5). Limited drilling at two locations in the
National Park confirmed the presence of a significant thickness (~40m) of freshwater
overlying this saline water (Smith et al. 2007). Results of the AEM and drilling were
used to determine an approximate depth to the freshwater/ saltwater interface but
could not accurately define the depth to basement. The AEM method struggled to
resolve below the saline aquifer, and drilling ceased at a depth of ~60m without
encountering basement.
10

Figure 5. Conductivity slice at -53m ADH, annotated features include resistive basement
ridges (blue) and conductive saline groundwater (pink/red) (from Fitzpatrick et al. 2009).
AEM results also hinted at a saline water wedge extending a limited distance inland
along the coast in Uley South (Figure 6). The high conductivity zones interpreted and
annotated as saline groundwater generally occur in the thick sedimentary “troughs’
between basement ridges. Wells drilled to sufficient depth and completed with long
screens (ULE205, SLE69) confirm the presence of seawater beneath freshwater and
the two datasets provide reasonable agreement. The AEM survey indicates (at least
at the time of the survey in 2006) that seawater is likely to extend only a short
distance, in the order of 500 metres into the Uley South aquifer.
11

Figure 6. Conductivity slice at -23m AHD showing potential small seawater wedge extending
inland into Uley South. Note: missing data (white strip adjacent to the coast in the
southeastern region) due to wind farm along flight lines (from Fitzpatrick et al. 2009).
The AEM survey also highlighted an area along the southeastern region of Uley
South where the base of the Quaternary limestone aquifer appears to be a
considerable thickness, possibly extending >30m below sea level (Figure 7). These
areas present a higher risk for seawater intrusion, and are in contrast to other areas
where basement is much closer to sea-level (at several points along the coast, the
basement is even understood to be well above sea level, implying no direct
connection to the sea in those locations). The southeastern area identified as highest
risk for seawater intrusion due to the combination of deep basement, thick sediments
and a thin or absent aquitard, is considered the most urgent area for targeted,
ongoing monitoring.
12

Figure 7. Interpreted surface for base of Bridgewater formation; annotation shows area below
mean sea level (from Fitzpatrick et al. 2009).
3.3 Student Research Projects
The EP Hydrogeology Fellowship (see Section 1.3) is making progress towards the
first body of research dedicated to seawater intrusion on EP. A significant focus of
this effort has been the training of students at Honours level. Prior to the current
report, two Honours theses have been written (Seidel, 2008; Alcoe, 2009) relating to
seawater intrusion on EP.
Seidel (2008) presented a simple assessment of seawater intrusion and used
steady-state analytical methods to investigate sensitivity of seawater intrusion
extent to aquifer parameters and different conceptual models.
Alcoe (2009) considered saline intrusion the primary management risk in his
parsimonious assessment of trigger-level management compared to the
current flux-based management. A steady-state (monthly) water balance
model incorporated both extent of seawater incursion and volumes of
seawater into its framework.
13

Research is continuing under the EP Hydrogeology Fellowship, and at the time of
preparing this report, a journal paper (Ward et al., In Prep.) has been simultaneously
in preparation, considering climate change impacts on seawater intrusion. The
analytical methods used by Seidel (2008) and Alcoe (2009), which are extended in
Ward et al. (In Prep.), have formed the basis for the assessment method that is
heavily relied upon in subsequent sections of this report. While the method is applied
here for several idealized EP aquifer settings, it is also used by Ward et al., (In Prep.)
to gain more generalized insights into the driving factors behind seawater intrusion
impacts under combined recharge decline and sea-level rise. This builds
substantially on the international literature, in which the impact of sea-level rise on
seawater intrusion has been generally studied (e.g. Werner and Simmons, 2009), but
generalized studies of the possible impacts of declining recharge have not been
done.
14

4. Methods
Owing to the very high proportion of groundwater that continues to be supplied by
Uley South and the close proximity of the basin to the sea, this study will focus on the
potential threats to this system from seawater intrusion. This means that the
analytical solution employed is for the impermeable basement conceptual model
described above. The issue of saltwater up-coning under Coffin Bay or Lincoln Basin
will not be quantitatively investigated in this study but will be investigated qualitatively
based on observations of well salinity. It is recommended that a more quantitative
analysis of the up-coning risk in those basins be conducted under possible future
works.
4.1 Conceptual Models of Saline Intrusion
4.1.1 General Concepts
A fundamental aspect of the interaction between freshwater and saltwater is the
density difference present between the fluids (i.e. seawater is denser than
freshwater). Some level of mixing between the fluids usually occurs, the extent of
which is dictated by heterogeneity of aquifer properties and time-varying flow
processes (e.g. recharge, tides). However, provided relatively stable conditions are
present there is a tendency for freshwater to stratify above saltwater.
Several different conceptual models for the saltwater/freshwater interactions are
applicable to the different aquifer settings on EP. We will make a distinction here
between “lateral seawater intrusion” and “saltwater up-coning”, with each term being
defined below. The two different concepts require the application of different
analytical models 6 and are likely to result in different extraction management and
monitoring targets.
Lateral seawater intrusion is likely to occur as a result of over-extraction (the most
commonly considered case), but could also be caused by a decrease in recharge, or
sea-level rise. In other words, it is caused by a disruption of the hydraulic equilibrium
of the coastal groundwater system. If the equilibrium is disturbed, e.g. due to
extraction, the interface will move inland toward the point of extraction. Seawater
intrusion typically takes the form of a “wedge” of saltwater underlying the discharging
freshwater (Figure 8), hence seawater intrusion moves furthest at the base of the
aquifer. Analytical models can be used to predict the position of the wedge ”toe” (the
inland tip of the wedge), which can be used to assist management decisions. If the
toe intrudes inland past the location of production bores but the freshwater-saltwater
interface remains well below the production bore screen, saltwater up-coning may
become a more significant threat.
Saltwater up-coning has the potential to occur when freshwater is extracted from a
fresh groundwater body that is underlain by saline water (Dagan and Bear, 1968;
Reilly and Goodman, 1987). If extraction results in an upward vertical gradient
sufficient to overcome the density-induced stratification, it creates the potential for
6
The analytical methods described later in this report (section 3.1) do not apply to the saltwater up-coning concept
15

upward flow of saline water into the production well. Broadly speaking, the greater
the drawdown at the point of extraction, the greater the potential for up-coning.
Saltwater up-coning can potentially occur relatively quickly, and – depending on the
particular combination of aquifer properties and extraction rates – may possibly lead
to anything from a widespread rise in the saline interface to a highly localized
upwelling immediately around the point of over-extraction. The processes,
timeframes and reversibility of saltwater up-coning are not well understood in a
fundamental way, and research in this field is ongoing (e.g. Werner et al., 2009).
4.1.2 Eyre Peninsula Examples
Several examples of different aquifer settings, and the likely resultant interaction
between fresh and saltwater are presented below with reference to specific EP
cases.
Aquifer underlain by an impermeable basement
In this conceptual model the coastal aquifer is considered to be underlain by a
continuous, completely impermeable medium. For example, two similar variations of
this concept exist in Uley South (Figure 8). In some areas of Uley South (“clay
present”) the Bridgewater Formation is underlain by the Uley Formation Tertiary Clay
which is presumed here to be impermeable for the purposes of applying the
analytical solution. In other areas (“clay absent”), the clay is absent and the regional
basement complex acts as the impermeable basement. Both variations of this
conceptual model will be explored in more detail using analytical methods later in this
report.
Figure 8. Two Conceptual models for seawater intrusion in Uley South Basin.
16

On EP, seawater intrusion in unconfined aquifers underlain by an impermeable
basement or aquitard broadly present in the following manner:
- Solution features and other surface features has lead to enhanced recharge
to upper aquifer
- Fresh groundwater discharges to the coast
- Aquitard is sometimes present and confining, sometimes absent.
- Undulating impermeable crystalline basement underlies the aquifers
- Under steady-state conditions, a saline wedge exists that is held in place by
freshwater discharge to the sea
- Over-extraction and/or declining recharge will reduce discharge to the sea,
causing the “toe” of the interface to migrate towards a new steady-state
position further inland.
- An increase in sea-level creates a larger saltwater head at the coast, and
without an increase in discharge this will force the wedge to tend towards a
new steady-state position further inland
Coastal freshwater lens
In cases where the basement is sufficiently deep, it is possible for the saltwater to
completely underlie the freshwater body but does not necessarily contaminate
shallow pumping wells (e.g. Coffin Bay A Lens, Lincoln B Lens). This results in the
fresh groundwater behaving like a stratified layer (i.e. lens) overlying seawater. On
EP, these systems broadly present as follows:
- Fresh groundwater discharges to the coast
- Semi permeable clay layer (not shown) is present in some areas, helping to
separate waters of different quality
- Denser saline groundwater (maybe seawater) underlies most or all of the
fresh groundwater and is closest to the surface near the coast
- Extraction leads to a local depression in water table, inducing lateral and
upward groundwater flow
- Saltwater up-coning occurs below the point of extraction; the speed and
severity is linked to the thickness of freshwater between the bottom of the
production zone and the freshwater-saltwater interface.
17

Figure 9. Conceptual model of fresh/saline water interactions in Lincoln B Lens (above) and
Coffin Bay (below).
Observations and further discussion of saltwater up-coning in Lincoln B and Coffin
Bay A are provided in Section 5.2.1.
18

Inland freshwater lens
It is worth noting that saline intrusion can also occur in inland aquifers that are
disconnected from the coast. In certain situations, a freshwater lens may reside over
brackish or saline groundwater (e.g. Robinson Lens). This is conceptually different to
a coastal lens where the underlying saline water is seawater (due to hydraulic
connection with the coast), although similar principles apply in regards to density
effects, saltwater stratification and up-coning. Figure 10 shows an example for the
Robinson lens on EP presented by Brown and Harrington (2003). The Robinson lens
can be broadly described as follows:
- Formation of lens due to preferential recharge via solution features.
- Displacement of higher salinity water and some freshwater-saltwater mixing
- Presence of semi permeable clay layer (not shown) helps to separate waters
of different quality
- Deep rooted trees access fresh/brackish groundwater (shown in Figure 10 for
Robinson Lens)
- Extraction depletes freshwater reserves and causes saltwater up-coning at
the point of extraction
- Small reserves of freshwater may still be present but cannot be accessed
using existing infrastructure
Figure 10. Conceptual model for Robison Lens; pre and post pumping (Brown and
Harrington, 2003)
19

4.2 Application of an Analytical Model
The following sections outline an analytical method for rapid and simplified
assessment of vulnerability to seawater intrusion, especially under scenarios of
climate change. Analytical modelling can be considered a “bare minimum” approach
to the characterization of coastal aquifers for seawater intrusion studies, representing
a useful starting point before one undertakes more detailed analyses such as
numerical modelling. Furthermore, the rapid assessment of seawater intrusion using
analytical approaches allows one to prioritise data collection and modelling efforts.
This work draws heavily on journal paper content from Ward et al. (In Prep.), which
has been produced under the Fellowship. Full derivations of the analytical solutions
are covered in the journal paper.
4.2.1 Basic Model
Following Werner and Simmons (2009), we adopt a steady-state, sharp-interface
analytical solution. Figure 11 shows a cross-sectional representation of flow to the
coast, as adopted by previous studies (e.g. Strack, 1976, 1989; Naji et al., 1998;
Mantoglou, 2003; Werner and Simmons, 2009). The situation is an unconfined
coastal aquifer, bounded below by an impervious horizontal layer at a depth B [L]
below sea level. The inland distance x i [L] represents the coastal fringe, with any
pumping activities presumed to occur inland of x i . x i is expected to be on the order of
several hundred to a few thousand metres and would be chosen such that it is of a
scale relevant to planning – e.g. the maximum allowable seawater intrusion extent.
The aquifer is assumed to receive uniform recharge R [L/T] in the coastal fringe. Note
that R is net recharge, after considering losses due to evapotranspiration from the
water table. It is assumed that R ≥ 0. The system receives a lateral inflow q i [L 2 /T]
through the inland boundary. q i may be considered to be the result of net recharge,
accounting for losses due to pumping inland from x i . The discharge q o [L 2 /T] of
freshwater to the ocean is equal to the sum of the inflows (Rx i + q i ).
As a result of the density contrast between freshwater and seawater, a wedge of
dense seawater penetrates the coastal region of the aquifer, held in place by the
discharge q o . In this model, the wedge is assumed to be defined by a sharp interface.
The depth of the interface below sea-level is denoted d [L] and the wedge penetrates
inland to a maximum distance x T [L].
20

Figure 11. Diagram of parameters for analytical model of seawater intrusion
The freshwater-saltwater interface is assumed to intercept the coastline (x = 0) at sea
level, while in reality the interface must be slightly below sea level at the coast, to
allow freshwater to discharge. This assumption has little impact on the interface toe
length x T (Custodio, 1987; Cheng and Ouazar, 1999). x T is commonly adopted as a
means of characterising the extent of seawater intrusion (e.g. Naji et al., 1998;
Cheng et al., 2000; Werner and Simmons, 2009), and is found using the single
potential method (Strack, 1976; Mantoglou, 2003). Thus x T is given by:
x
q
Rx
R
q
Rx
R
K
R
2
i i
i i
2
T
1 B
(1)
2
where the density difference ratio
s
f
seawater and freshwater densities [M/L 3 ].
f
, and where
s
and
f
are respectively
While x T is useful for characterising seawater intrusion extent, it is not readily
measurable in the field. The most common measurement in seawater intrusion
monitoring is the depth to the freshwater-saltwater interface, which is found by
sonding (see Figure 4). The single potential method can also be used to determine
the depth d [L] to the interface at a distance x from the coast, after Mantoglou (2003):
d
2x
q
i
K
R
1
x
i
x
2
(2)
If the depth to the freshwater-saltwater interface is known from field measurements,
then d calculated from (2) could be used to infer the inland extent and distribution of
the freshwater-saltwater interface. If sufficient interface measurements are available
along a transect, it may be possible to crudely calibrate the other parameters in the
analytical model, or (perhaps more likely) to broadly cross-check the validity of the
analytical solution.
It should be noted that in their sea-level rise study, Werner and Simmons (2009)
identified two different inland boundary conditions as end-members to the types of
21

conditions expected in field settings – a ”flux-controlled case”, and a ”head-controlled
case”. Head-controlled boundaries may arise in situations where, for instance, an
inland water body (e.g. a canal, lake or wetland) of constant head occurs at the
inland boundary of the model (e.g. in the case considered by Dausman and
Langevin, 2005), or where pumping occurs such that the inland head is constant
despite sea-level rise. In Ward et al. (In Prep.), both flux-controlled and headcontrolled
settings are investigated further, however for the present study the fluxcontrolled
boundary is considered to be the only relevant inland boundary condition.
4.2.2 Dimensionless Formulation
Naji et al. (1998) presented (1) in dimensionless form, and we adopt a similar form.
The dimensionless clusters used below are slightly different to those of Naji et al.
(1998) because they allow the physical processes to be described in terms of
meaningful ratios of the different flow types (as described below). The dimensionless
seawater intrusion extent is defined as
x
T
x
T
(3)
xi
2
such that: x T
F 1 F 1 M F 1
(4)
where:
q
Rx
i
F ,
i
M
K
x
i
1
q
i
B
Rx
i
2
(5)
Here x
T
is the seawater intrusion extent, expressed as a proportion of the inland
distance x i . F and M are two dimensionless ratios describing the governing physical
controls. F is the ratio of lateral inflow to coastal rainfall-recharge (which we term
“mixed inflow ratio” from herein). M is a ratio of free-convective components to
forced-convective (advective) components that we term a “mixed convection ratio”, in
a similar fashion to Massmann et al. (2006) and Ward et al. (2007). In M, the
numerator lumps together those parameters which we know contribute to increasing
seawater intrusion (most significantly K and B, as the density ratio is relatively
constant for freshwater/seawater interactions). The denominator in M contains the
summed inflows, i.e. the discharge to the sea (which leads to the retardation of
seawater intrusion). The reason for using a dimensionless formulation is to reduce
the number of critical parameters, so as to allow different systems to be more rapidly
compared against each other. In this case, all of the hydrogeological variables have
been reduced to just two parameters (M and F), and as a result the idealised
seawater intrusion extent can be rapidly compared across multiple unconfined
coastal aquifer systems which vary widely in terms of recharge and inland inflows,
hydraulic conductivity, thickness and inland extent, by simply comparing their
respective M and F values.
Mathematically, x
T
is defined over the interval 0 M F 1 . What this means in
practical terms is that when M > F + 1 it implies that the toe of the wedge has
intruded inland of the point of maximum hydraulic head (the peak of the coastal
groundwater mound), and the situation becomes unstable (Strack, 1989).
22

A Note on Analytical vs Numerical Modelling
A numerical model of the Uley South / Uley Basins is being constructed as part of the
GAPM Project (Milestone 10), however in this report we seek to explore the factors that
are likely to govern seawater intrusion on Eyre Peninsula fundamentally using
analytical solutions. The following discussion explores the fundamental differences
between analytical and numerical modelling and the benefits that both offer.
The methods for modelling groundwater systems can broadly be divided into two classes:
analytical and numerical. Analytical methods typically involve an equation (or set of
equations) describing a whole domain (e.g. the depth to the freshwater-saltwater interface
being represented as a mathematical function of the distance inland). Numerical methods
typically involve dividing the domain into smaller parts which are solved one-by-one and then
reassembled into a cohesive whole.
The key advantage of analytical modelling over numerical modelling is that the former is very
fast to construct and execute – a basic analytical model of a seawater intrusion interface may
be developed from first principles in a matter of minutes and then yields an answer in a matter
of seconds. Defining a problem in a single set of equations also allows one to obtain explicit
parameter groupings that can be used to broadly characterise the underlying physics
governing system behaviour, for example the relationships between K, B, R and x T can be
obtained with relative ease.
On the other hand, analytical modelling requires certain elements of the conceptual model to
be heavily constrained: for example, often the aquifer must be assumed homogeneous and
the interface must be assumed to be sharp (no dispersive mixing). Moreover, analytical
solutions to seawater intrusion problems are usually steady-state and therefore cannot be
used to simulate transient changes in conditions (such as the timing of the response to
specific changes in recharge or sea-level, or different transient pumping regimes).
Numerical models, while more time-consuming than analytical solutions, offer far greater
flexibility as each discretized “cell” can be assigned individual properties, making it possible to
simulate different geological zones (heterogeneity). Furthermore, numerical solutions typically
have a transient formulation, allowing simulation of changing conditions over time, so that
aspects of time scales, reversibility and remediation can be investigated. Numerical models
can readily be established in 3-D and for complex aquifer geometries, making them an
attractive tool in basin management.
It must be remembered that a model can only ever be as reliable as the parameters used to
construct, calibrate and validate it. In hydrogeology, parameter measurement almost always
carries substantial uncertainty. Furthermore, a highly complex model with many “calibrateable”
parameters may yield an excellent fit to the relatively small number of observations used
to calibrate it, but may actually be less accurate between those points (Hill, 2006).
It is thus useful to approach modelling by starting with the simplest and easiest method (i.e.
analytical) and investigating sensitivities and uncertainties there. Only once the system is
characterised and well-understood in this way is it appropriate to extend the modelling effort
into more complex schemes (e.g. quasi-3D sharp interface solutions, or fully dispersive 3D
numerical models). In some cases, the inherent uncertainty in the data may make it
impossible to gain any significant advantage by increasing complexity, despite the apparent
gain in terms of more sophisticated graphical output. Anderson (1983) draws an intriguing
parallel between the faith (among groundwater modellers) in the graphically spectacular
results of complex models, and the well-known story “The Emperor’s New Clothes”: the
characters in the fable all claimed to be able to see clothes that did not exist – out of a fear of
appearing incompetent to their peers if they admitted they couldn’t see the clothes. We must
resist the temptation to “believe” modelling results that are presented in a more intricate
fashion, and remind ourselves that the uncertainty in parameters used to develop complex
numerical models may mean that the results are no more “correct” than answers from the
most simplistic analytical solutions.
In seawater intrusion, analytical models can be used to rapidly build up a fundamental
understanding of the physics governing e.g. the steady-state (long-term) position of the saline
interface under various scenarios (such as climate change scenarios), using a simplified
hydrogeological setting. The knowledge gained through this simplified approach can then be
carried into the more complex modelling environment, to better inform both our expectations
and our understanding of system behaviour.
23

4.2.3 Steady-State Seawater Intrusion – Key Factors
If x i is chosen to be a distance of practical significance (such as the approximate
distance from the coast to a pumping well), then x
T
can be considered a measure of
the natural risk or susceptibility of that aquifer (defined by M and F) to seawater
intrusion; in other words, if x
T
is close to 1, then this implies that the inland extent of
the wedge is close to x i under natural conditions and a relatively minor change in
parameters could possibly induce intrusion of the pumping well. On the other hand, if
pumping wells are located at x i and x
T
Rx i ),
x
T
will be smaller than in cases where coastal recharge dominates the aquifer
inflows, even if the total aquifer inflows were the same in each case. This is a
significant point, as it demonstrates that recharge within the coastal fringe may not
necessarily have the same influence on seawater intrusion extent as inflows to the
aquifer that occur up-gradient of the interface toe. Hence the water balance of inflows
to the aquifer is critical to seawater intrusion.
Figure 12 shows contours of x
T
plotted for different M and F values. Generally, we
see that x
T
increases with increasing M, and also with decreasing F when M > 0.3
(approx.). x
T
is largely independent of F where M < 0.3 (approx.). We will use the
term “M-F space” to describe the domain of the plot in Figure 12. M-F space is useful
as it captures a wide range of possible parameter combinations in unconfined coastal
aquifers, and the “location” of a particular aquifer within M-F space provides insight
into (a) the aquifer’s vulnerability to seawater intrusion and (b) the likely sensitivity to
changes in M and F. In a later section of this report, aquifers from the Southern
Basins will be located in M-F space to evaluate their respective vulnerabilities to
seawater intrusion.
Where the combination of M and F values approaches the unstable region in the
lower-right of the plot in Figure 12 (i.e. where M approaches F + 1), the behaviour
becomes asymptotic. In such cases, a small increase in M or decrease in F could
lead to extensive intrusion due to salt migrating into the region of landward flow
inland of the mound. Hence aquifers that fall near (and especially within) the unstable
zone in M-F space should be seen as highly vulnerable to seawater intrusion.
24

Figure 12. Contours of x’ T for different M and F
The dimensionless ratios M and F can be seen to “summarise” the parameters that
are needed in order to achieve a basic understanding of seawater intrusion
processes. For M, one needs data on coastal aquifer properties (namely hydraulic
conductivity K, aquifer depth B and the distance x i from the coast to the inland
boundary), as well as the total inflows to the system (q i + Rx i ). Furthermore, for F one
needs to obtain a regional water balance (to give the ratio of total recharge volume
Rx i to the lateral inflows coming from the inland aquifer).
4.2.4 Steady-State Seawater Intrusion and Climate Change
Changes in B, R or q i will induce changes in M, F and x
T
. In the following, initial
conditions and final conditions are denoted by subscripts “init” and “final”,
representing (respectively) the steady states before and after changes in B, R and/or
q i . The final steady-state condition can be thought of as the situation that would occur
a considerable time 7 after the change in B, R and/or q i .
Sea-level rise (resulting from climate change) can be represented by an increase in
B, which leads to an increase in M, with a resulting increase in x
T
according to (4). A
reduction in R leads to an increase in both M and F, while a reduction in q i increases
M and reduces F. x
T
increases with an increase in M, but decreases as F increases
(see Figure 12), and therefore, whilst a general reduction in inflows leads to an
increase in M, the resultant impact on x
T
is critically dependent on the nature of
changes in coastal aquifer inflows (i.e. changes in coastal fringe recharge, lateral
7
Further work is under way at Flinders University to investigate the transient nature of seawater intrusion occurring
on timescales relevant to climate change, e.g. decades to centuries (see section 6.1.5)
25

inflow from inland, or both). It becomes obvious here that the consideration of climate
change impacts on seawater intrusion is not necessarily straightforward.
The expression for the change in dimensionless steady-state seawater intrusion
extent will be denoted x
T
and can be derived from (4). We define changes in B, R
and q i in dimensionless form as follows:
B
B
final
B 1 ,
init
B
B
init
R
R
final
R 1 ,
init
R
R
init
q
i
q
i , final
q
i , init
q
q
i
1 (6)
i , init
The change in seawater intrusion extent x
T
can be shown to be a function of the
initial conditions M
init
, F
init
, and the parameter changes B , R , and q
i
:
x
T
F
init
qi
R
1
F
init
qi
R
1
2
M
init
(7)
B
2
F
R
init
1
F
init
1
2
M
init
F
init
1
Equation (7) allows us to simulate a wide range of possible future seawater intrusion
scenarios in a generic manner. In cases where either R
init
0 , R
final
0 or q
i , init
0 ,
(7) will be undefined, although again, in practice (7) can be used by approximating
zero values. (Ward et al. (In Prep.) also provide exact derivations for the special
zero-value cases.) Thus using (7), it is possible to evaluate the relative “threats” to an
aquifer under various climate change scenarios defined by B , R , and q
i
.
4.3 Parameterisation of EP Aquifers
The analytical model presented in Section 4.2 is considered most suitable for
application to the “aquifer underlain by an impermeable basement” concept
introduced in Section 4.1.2, that is, the Uley South area. Note that two separate
coastal portions of Uley South have been tested here: the northwest (NW) near
Shoal Point and the southeast (SE) in the vicinity of ULE205. Parameters
representing other coastal aquifers (Coffin Bay and Lincoln Basin) have also been
applied using the same methodology. Note that in reality, these aquifers do not
necessarily have an impermeable basement underlying the freshwater, so this
analysis is used to test the relevance of the impermeable basement conceptual
model in these cases.
4.3.1 Aquifer Properties
A discussion of parameter selection follows, with all parameters listed in Table 2
(Section 4.3.4).
Hydraulic Conductivity (K)
On EP, pump tests have generally been conducted in localized areas targeted for
groundwater extraction, that is, areas confined mostly to where pumping wells
currently exist in the Quaternary Limestone aquifer. In Uley South, inferred K is in the
approximate range 100-1200 m/d in the central portion of the lens (see Sibenaler,
26

1976) but no data exists closer to the coast where the terrain rises steeply and is
generally considered to be too high to make groundwater extraction feasible. In
Coffin Bay A Lens, pump tests in the current production well field yielded K values of
50-2000 m/day (Dennis, 1989) and a single pump test in the Lincoln B Lens yielded a
K value of 765 m/d (Selby, 1972). Numerical modelling can be used to further inform
determination of representative “aquifer scale” hydraulic conductivity values. In the
Uley South area, findings of previous (James-Smith and Brown, 2002; Zulfic et al.,
2006) and current (Werner, 2010) numerical modelling exercises have been drawn
upon for selection of K values for subsequent sections of this report (Table 2).
Aquifer Thickness (B)
Typical aquifer thicknesses have been derived by assessment of lithological logs
from the state drillhole database 8 , as well as existing reports and geophysics where
available. Where more than one drillhole had intercepted basement, an average
value for the area was selected (a flat basement was assumed). In Lincoln B and
Coffin Bay, limited drillholes are sufficiently deep to encounter basement. In the case
of Coffin Bay, several drillholes to 60m did not intersect basement, hence best
estimate of B=65m (Table 2) is adopted. In the case of Lincoln B, Shepherd (1959)
inferred a basement elevation of approximately -60m AHD based on a limited
number of drillholes that actually encountered basement.
In the Uley South (SE) area, several possible conceptualisations exist due to the
possible presence or absence of a clay aquitard (see Figure 8 and appendix for
further detail). Following Seidel (2008), both concepts will be tested here:
Clay Present – only the quaternary thickness and parameters will be
considered (QL only)
Clay Absent – the combined thickness and parameters of the quaternary and
tertiary sediments will be considered (QL + TS)
Inland Boundary (x i )
The inland boundary has typically been selected as the distance between pumping
wells and the coast, with the exception of Lincoln B, and Uley South NW where the
distance between the coast and a relevant flow divide was selected. Setting x i to the
typical inland distance of near-coastal production bores is logical, as the model
output ( x
T
) indicates the position of the saltwater toe relative to production bore
location, and hence provides a measure of susceptibility ( xT
0 implies minimal risk,
xT
1 implies saltwater in close proximity to pumping wells). Similarly, in cases
where x i is selected as the distance to the peak of the groundwater mound, x
T
again
represents a measure of the vulnerability of the aquifer – in these cases, if x
T
>1, it
would imply that seawater migrates inland of the groundwater mound, and enters the
zone where hydraulic gradients are directed inland (thus salt could be carried
towards the pumping wells).
Recharge (R)
8
Drillhole Enquiry System (DES), Primary Industries and Resources SA (PIRSA)
https://des.pir.sa.gov.au/deshome.html
27

Recharge estimates were adopted from numerical modelling where it exists (Uley
South) and from the Water Allocation Plan in other areas (Coffin Bay and Lincoln B).
In Uley South, numerous and varying recharge predictions exist. Zulfic et al. (2006)
used a uniform recharge value of 100 mm/yr, while the value of ~140 mm/yr has
been adopted by DWLBC for the purpose of setting water allocations. Recent work
under the Research Fellowship investigating the spatial variability of recharge
indicates that recharge along the coastal fringe of Uley South may be lower than
other areas of the basin, therefore we will adopt the value of 100 mm/yr for the
purpose of this modelling exercise.
Lateral inflows from inland (q i )
Once the other parameters have been measured or estimated, the lateral inflows can
be calculated from average hydraulic head h i observed at the inland boundary of the
model according to the following formula (from Ward et al., In Prep.):
q
i
K
2x
i
h
i
2
1
B
2
Rxi
K
2
(8)
Please note that q i is not necessarily the total inflow to the “basin”, but is the inflow at
the inland boundary of the cross-section defined for the analytical model (see Figure
11).
4.3.2 Regional Water Balance
From (4) and (5) we can see that the ratio of inflows entering the aquifer from inland
(q i ) to inflows entering the coastal zone as recharge (Rx i ) is critical to the resultant
seawater intrusion extent. This means that the breakdown of the water balance in the
coastal zone is significant. Along the whole coast in Uley South, the heavily
vegetated hilltop is understood to lead to lower recharge (Ward et al., 2009). Given
the large area of comparatively high recharge inland in the less vegetated central
portion of the lens, this would suggest q i > Rx i and hence F > 1.
4.3.3 Inland Boundary Condition
When considering the impact of climate change, one has a choice of inland boundary
conditions (specified head versus specified inflow) as investigated by Werner and
Simmons (2009) and Ward et al. (In Prep.). A specified head condition could occur
due to a high inland water table that allows evapotranspiration to maintain the
groundwater at an approximately constant level near the ground surface.
Alternatively, excessive pumping could create drawdown of groundwater to an
approximately constant level. However, in the present study, the most appropriate
inland boundary is thought to be one of specified inflow (q i ), dominated by recharge
occurring inland. For this reason, the inflow q i is specified to reduce by the same
proportion as any reduction in coastal recharge (R) under climate change. In other
words q = R . Climate change scenarios are described in section 5.3.1.
i
28

4.3.4 Parameter Summary
Table 2. Aquifer parameters adopted for analytical modeling, as derived from Werner (2010),
Dennis (1989) and Selby (1972)
Uley South (SE)
QL only
Uley South (SE)
QL + TS
Uley South
(NW)
Coffin Bay A
lens
K (m/d)
B
(m)
R
(mm/yr)*
x i
(m)
h i
(m)
q i
(m 3 /d/m)
320 25 100 2000 1 2.56
164
(Average of 320
and 7)
50 100 2000 1 1.30
1300 15 100 2000 1 6.70
500
(Range 50-2000)
65 34 1500 1 4.16
Lincoln B Lens 750 60 56 3000 1 3.64
* Note: Recharge is reported here in mm/yr but is converted to m/d in the calculation of M and F
4.3.5 CLIMATE CHANGE SCENARIOS
Given the uncertainty associated with climate change projections, it is appropriate to
consider a range of possible scenarios. This enables the identification of dominant
modes of change (such as sea-level rise versus reduction in recharge).
The recharge scenarios to be tested here have been adopted from the work
conducted under Milestone 5 of the GAPM project (Ward et al., 2009). Ward et al.
(2009) estimated a reduction in recharge of up to approximately -50% in the
Southern Basins for the worst-case scenario, however under an assumed climate
change scenario an increase in rainfall intensity could give rise to a minor (up to
20%) increase in recharge. To capture the uncertainties in these predictions, the
range of relative recharge changes considered here is from -50% to +20% (i.e. 0.5 ≤
R ≤ 1.2), with q = R .
i
There is considerable conjecture surrounding the extent of future sea-level rise, and
authors such as Hansen (2007) propose that it could be on the order of several
metres within the 21 st century. The Intergovernmental Panel on Climate Change
(IPCC) estimates a rise in sea level of between 0.09 and 0.88 m by 2100 relative to
1990, or 0.8 to 8.0 mm per year (Church et al., 2001). Sea-level rise scenarios tested
in this study cover a continuous range up to 1 m to reflect the range of impacts that
may be possible.
29

5. Results and Discussion
5.1 Seawater Intrusion Vulnerability of EP
Aquifers
Considering the parameters above (Table 2), the resultant dimensionless numbers
are given in Table 3.
Table 3. Dimensionless parameters for seawater intrusion extent in Southern Basins
M F x
T
a. Uley South (SE) – QL only 0.83 4.66 0.43
b. Uley South (SE) – QL + TS 2.83 2.37 2.03
c. Uley South (NW) 0.44 14.6 0.22
d. Coffin Bay A lens 8.39 29.8 4.53
e. Lincoln B lens 5.62 7.92 3.49
Figure 13 shows a representation of five aquifers in the Southern Basins in the single
“M-F space” (similar to Figure 12), with seawater intrusion characterized by steadystate
dimensionless toe location ( x
T
).
As expected, Coffin Bay A lens and Lincoln B (marked “d” and “e” on the figure) both
result in x
T
> 1, implying that the mathematical model is predicting a wedge toe
extending well beyond the position of the inland boundary. The simplified conceptual
model is no longer relevant in these cases, since the aquifer would have to extend
many kilometres inland with uniform conditions for the conceptual model to hold. The
precise value of xT
is also less relevant in Coffin Bay A and Lincoln B lenses, but has
been included here to demonstrate that the model results tend to infer that the lenses
are extensively underlain by seawater. The factors contributing to the high M values
(namely large thickness and hydraulic conductivity, and relatively low inflows) can be
seen to be the dominant factors leading to the extensive seawater body observed
under the freshwater, and leading to an enhanced up-coning risk.
30

Figure 13. Contours of x’ T for: (a) Uley South (SE) – QL only, (b) Uley South (SE) – QL+TS,
(c) Uley South (NW), (d) Coffin Bay A lens, (e) Lincoln B lens. Note that values of x’ T >1 imply
a saltwater wedge toe that extends beyond the inland model boundary and should only be
used qualitatively to suggest “extensive intrusion”.
The analytical solution provides a more practically relevant result when applied to
Uley South (points “a”, “b” and “c” in Figure 13). Simulated seawater intrusion in the
southeast portion of the basin is highly dependent on whether the system is
considered to be a thin aquifer (Quaternary Limestone only, underlain by
impermeable clay) or a thicker aquifer (Quaternary Limestone and Tertiary Sand
considered together as one unit). In the former case, the toe of the seawater wedge
is likely to reside less than half way to the pumping wells ( x T
~ 0.4), however in the
latter case the seawater wedge appears likely to extend inland beyond the bore field
( x
T
~ 2). In that case, saltwater could theoretically underlie the southeastern
production bores (in a similar manner to Coffin Bay A lens or Lincoln B), and
excessive extraction could one day give rise to up-coning. Note that saline water has
not been observed inland of production wells and this supports “clay present”
conceptual model (point “a”). The proximity of point “b” (the thicker aquifer
conceptualization) to the mathematically undefined zone implies inherent instability;
relatively small changes in M or F could possibly push this aquifer system towards
very extensive intrusion. A key conclusion from this result is that there needs to be a
better appreciation of the presence (and hydraulic characteristics) of the Tertiary Clay
aquitard, especially near the coast. The smallest simulated intrusion extent is found
in the thin Quaternary Limestone aquifer in the northwest portion of Uley South,
where x
T
~ 0.2.
31

Salinity (uS/cm)
Water level (m AHD x 10 4 )
Pumping (hrs/day)
5.2 Comparison with Observations
5.2.1 Evidence of Up-coning
The results of the M-F analysis are in agreement with existing knowledge that Coffin
Bay A and Lincoln B lenses both have extensive seawater underlying them. Evidence
suggests that there is a potential up-coning risk under excessive pumping regimes. In
Figure 14 we see a time-series of salinity (EC, µS/cm), approximate pumping rate
and hydraulic head measured in (and near) a production bore (Bore F) of Lincoln
Basin B over a 10 year period. Pumping tends toward a regular annual cycle, with a
peak rate during summer and effectively zero extraction during winter; this is
reflected in the oscillating groundwater levels with seasonal depressions forming
each summer. The lowering of the groundwater levels clearly leads to up-coning
each year, as indicated by the sharp rise in salinity. It is interesting to note that the
salinity subsides as the groundwater levels are restored (i.e. as pumping stops and
the recharge season starts). Reilly and Goodman (1987) report that under certain
conditions, an almost linear relationship may exist between the well discharge rate
and the salinity of the discharge; this relationship is somewhat apparent at
Production Bore F in Lincoln B lens.
8000
7000
Conductivity (uS/cm)
SLE037
Pumping (hrs/day)
16
14
6000
12
5000
10
4000
8
3000
6
2000
4
1000
2
0
Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99 Jan-00 Jan-01 Jan-02 Jan-03
Figure 14. Evidence of seasonal saltwater up-coning in Lincoln B Lens. Plot shows
approximate extraction rate and measured salinity from Production Bore F. Observed
groundwater levels in nearby SLE037 are also shown (multiplied by 10 4 for display purposes,
e.g. a water level of 3000 in the figure represents 0.3m AHD).
In Coffin Bay, an observation well located near the production borefield (LKW40)
shows several recent salinity measurements 9 in the order of 11,000 mg/L TDS. The
depth of the slotted interval (approx. 40m below sea-level) suggests that the
seawater interface may be somewhat lower than 40m. The stratigraphic logs for the
0
9 Data obtained from “ObsWell”, Primary Industries and Resources SA (PIRSA) https://obswell.pir.sa.gov.au/
32

production bores and observation wells suggest a clay layer of variable thickness and
intermittent extent is present in this area that may help to protect the pumping wells
from drawing the saltwater upwards. However, in Coffin Bay, preferential pumping
from a single production (water supply) bore 10 (PB1) occurred during the 1980’s and
90’s. Around the year 2000, the salinity in PB1 increased to the point where the
extracted water became too saline to use. Attempts to redeem the water quality in
PB1 included extended rest periods and blocking the bottom portion of the well, but
were unsuccessful. This experience has fortunately not been repeated in the other
production bores in Coffin Bay and additional low yielding wells have subsequently
been drilled to spread the pumping load. The incident confirms that placement and
construction of production wells in this area and subsequent extraction should be
approached with caution.
5.2.2 Depth to Saltwater Interface
A comparison can be made between the modelled and the observed interface by
evaluating (2) to find the depth to the interface d, and comparing against the depth of
the mixed zone found during sonding (Figure 4). In the southeast of Uley South
(Figure 15), in SLE069 (~ 500 m inland), we find that according to the analytical
solution, d ~ 19.6 m below sea-level for the case of Quaternary Limestone only, and
d ~ 20.5 m below sea-level for the case of combined Quaternary Limestone and
Tertiary Sand. Both of these values compare (qualitatively) quite well with the sonded
interface (see Figure 4), which was located at approximately 18-20 m below sealevel.
It is interesting to note that the difference between the two aquifer conceptual
models (QL only vs QL + TS) is too small to infer whether either model is more
appropriate. However, the results of the AEM survey (Fitzpatrick et al., 2009) may
help to shed some light on which conceptual model is more applicable (see Section
5.2.3 below).
It should be noted that even if the interface resides under the pumping well, the firstorder
prediction here is that the depth of the saltwater would be in the order of 35m in
the absence of pumping, while the wells themselves are only screened to a depth of
~10m. Further, strata logs indicate the presence of semi-confining clay in the vicinity
of the production wells in this area, making up-coning of saltwater under these
conditions potentially unlikely, depending on the extensiveness and permeability of
the clay layer. It is pertinent to recall at this point that the analytical solutions
adopted in this study involve a sharp-interface solution and are not at all suitable to
assess the dynamic and dispersive salt movements near a production bore. This
matter should be the subject of further investigation (e.g. into the potential for upconing)
and increased monitoring efforts.
10 The base of this drill hole initially penetrated a brackish clay sequence and this is likely to have contributed to the
increasing salinity of pumped water
33

In the northwest portion of Uley South, modelling results indicate the saltwater wedge
appears to penetrate into the aquifer to a distance of roughly 440m, as shown in
Figure 16. This is broadly consistent with the preliminary observation that seawater
did not appear to be present in two newly-drilled wells ULE210 and ULE211, located
about 500m from the coast. Given the large distance between the theoretical wedge
and the location of the pumping wells, this simplified analysis would suggest that
saltwater intrusion is unlikely to present a major threat in the northwest area of Uley
South if the clay aquitard is indeed continuous, under current climatic and extraction
conditions. It should be noted that any saline interface in the Tertiary Sand aquifer
has not been considered here. Furthermore, the assumption of homogeneous aquifer
properties on the prediction of seawater intrusion has not been tested, and the
presence of preferential flow paths may introduce a departure in the freshwatersaltwater
interface distributions estimated using the current methodological
framework.
Figure 15. Plot of theoretical freshwater/saltwater interface determined from (2) for the
southeast portion of Uley South, for the two conceptual models (QL only vs. QL + TS),
showing the approximate location and screened interval of observation well SLE069.
34

Figure 16. Plot of theoretical freshwater/saltwater interface determined from (2) for the
northwest portion of Uley South assuming a continuous clay aquitard at a depth of 15m. The
approximate location and screened interval of the nearest pumping well are shown.
An interesting result of the modelling is that despite the higher K (1300 m/d) in the
northwest portion, the predicted seawater intrusion extent is greatest in the southeast
portion where K is at least four times lower. This is because, whilst it contributes to a
higher M value, a high K also leads to increased inflow from inland (according to (8))
and therefore increased discharge to the sea. It is the high discharge and the
relatively thin aquifer in this region that leads to the comparatively lower seawater
intrusion threat, whereas in the SE region (where the aquifer could be up to 50m
thick and the discharge to the sea is lower), there is a far greater propensity for
seawater intrusion according to the current simplified analysis. Further testing is
needed using more sophisticated methods that account for the spatial variability in
aquifer geometry and hydraulic properties, plus the regional impact of groundwater
pumping and discontinuities in the connectivity of the Quaternary aquifer to the sea,
before conclusions can be reached regarding the relative sensitivity of different parts
of the Uley South aquifers to seawater intrusion.
5.2.3 Saline Interface Predicted by AEM Survey
The results of the AEM survey (Fitzpatrick et al., 2009) suggested that the area most
at risk to seawater intrusion in Uley South was located in the southeast portion of the
basin (see Section 3.2). In Uley South (SE), the extent of intrusion of seawater (as
inferred by Fitzpatrick et al., 2009) appears to be on the order of ~0.5km at a depth of
23m, and ~2km at a depth of 53m. This broadly supports the thick-aquifer (QL + TS)
case presented in Figure 15, notwithstanding the gross simplification of the two-layer
aquifer to a single homogeneous unit.
35

5.3 Future Impacts
5.3.1 Climate Change
Figure 17 shows contours of x
T
(in metres) generated by evaluating (7) across the
range of climate change scenarios defined above (i.e. recharge varying from a 50%
decrease up to a 20% increase, and sea-level rise varying from 0 to 1 m). From
Figure 17, we see that the contours are far more sensitive to changes in recharge
than changes in sea-level (recall that q
i
= R , i.e. relative “changes in recharge”
are assumed to equally affect both coastal recharge and lateral inflows from inland).
This outcome is closely linked to the presumption of the inland boundary condition as
being one of a flow reduction proportional to the climate change-induced recharge
change. If the water levels of Uley South are not permitted to rise as sea-level rises,
then the inland boundary condition will more closely resemble a specified head
condition, and sea-level rise may have a larger impact, as identified by Ward et al. (In
Prep.). In the higher risk zone (southeast), even if one considers the more “desirable”
conceptual model in which a clay aquitard is continuous throughout the region, a
reduction in recharge of 50% was predicted to produce intrusion on the order of 1
km, which may begin to present a threat to the nearby pumping wells. In the thicker
(QL + TS) conceptual model, the current condition involves saltwater occurring
beneath pumping wells in SE Uley South, according to the analytical model. The
model predicts that any recharge reduction greater than about 20% produces an
inland shift in the toe beyond the theoretical groundwater mound (see Section 5.1),
resulting in substantial seawater intrusion.
In the northwest portion of Uley South, the predicted climate change impacts are
roughly half the impacts predicted for the southeast, with the model producing a
maximum likely intrusion distance of around 500 m. It isn’t clear whether this
intrusion distance would also occur in a highly heterogeneous aquifer, such as those
found in the Uley South basin, but the current estimates serve as first-order
approximations in the absence of more comprehensive modelling. The results of the
analytical modelling suggest that under these predicted levels of climate change and
assuming that the groundwater levels are allowed to rise commensurate with sealevel
rise, the saltwater toe could move significantly inland, although it is not likely to
intrude so far that it becomes a high risk to pumping wells in this part of the basin. If
the boundary head is maintained at current levels despite sea-level rise, more
extensive seawater intrusion is likely to occur in this part of the basin.
36

Figure 17. Contours of x T across a range of climate change scenarios for the three aquifer
conceptualisations in the Uley South basin.
5.3.2 Increased Extraction
Under the conceptual model and analytical solutions presented above, it’s assumed
that the main impact of increased extraction on seawater intrusion is to reduce q i , and
hence reduce the discharge to the sea. In reality, pumping will have a more
complicated impact on the aquifer than simply reducing the groundwater flow in the
coastal zone in a vertically and laterally uniform manner, due to aquifer
heterogeneities and the radial flow effects of pumping. For a given increase in the
basin-wide levels of extraction, the effect will be more significant in areas with low
initial q i , and less significant in areas with high q i . In the case of Uley South, in the
northwest portion of the basin, it is possible that q i is several times larger than in the
southeast area due to the large difference in K. However, these results don’t take into
account various complexities, including variability in the Quaternary aquifer-ocean
connectivity, preferential flow paths due to heterogeneities and the complex nature of
groundwater flow patterns at the basin-scale – further work is needed to confirm this
37

assertion. Nonetheless, if we assume that a potential increase in pumping affects
discharge to the sea along a localised portion of the coastline, we can compare the
impacts between these two regions. If a coastline width of 5km is arbitrarily selected
(coastal widths are the subject of conjecture), Figure 18 shows that increasing
pumping by 2.5 GL could substantially impact the seawater intrusion threat, and
therefore the southeast portion of the basin is considered to be susceptible to a major
decline in ocean discharge. The same change in pumping applied to a higher K
aquifer with similar hydraulic gradients as previously would be expected to have
minimal impacts. Therefore, as long as a high rate of discharge occurs and is
maintained in the NW part of the basin, the susceptibility to seawater intrusion is
consider low based on the simple analysis presented here.
It is worth noting that the Uley South cross-sections considered here are assumed to
be independent of each other, whereas in reality they are linked. The simplified
cross-sectional method provides no insight into how a change in extraction in one
area may impact water levels and seawater intrusion in other parts of the aquifer and
these results should be seen as exploratory only. Extreme caution should be
exercised around any considered increase in extraction. It is anticipated that the
impacts of different spatially-varying pumping scenarios should be able to be
addressed more thoroughly using the regional seawater intrusion model (currently
under development under Milestone 10 of the NWI project).
Figure 18. Pumping-induced seawater intrusion in a high discharge setting (labelled
“northwest”) and a low discharge setting (labelled “southeast”). Properties are somewhat
similar to those found in Uley South (albeit highly idealised),
38

5.4 Confidence in Predictions
The model assumes an aquifer of uniform thickness (horizontal basement),
homogeneous hydraulic conductivity and flow is assumed to be in steady state with
(therefore) a stationary saltwater wedge and zero dispersion (i.e. a completely sharp
interface). Furthermore, pumping is assumed to only influence a narrow strip of
aquifer, and the disconnection of the aquifer with the ocean along part of the
coastline is neglected. In reality, the freshwater-saltwater interface is likely to be
dispersed, due to mixing caused by small and large-scale heterogeneities in the
aquifer’s hydraulic conductivity (particularly in karst areas with the potential for
preferential flow paths), and flow processes are probably highly transient due to
tides, seasonal variability in recharge and pumping, and the undulating basement.
Further radial flow effects introduced by pumping negate the reliability of crosssectional
analysis, as has been undertaken in this study. Other factors that cannot be
considered using this preliminary method include the effects of spatially variable
pumping and recharge, and the effects of up-coning (especially in Coffin Bay and
Lincoln Basin).
Given the very different hydraulic conductivities in the Quaternary and Tertiary
aquifers, it is most unlikely that the interface would present as a continuous line
shown in Figure 15. However, the extent of seawater intrusion in the Tertiary Sand
aquifer – and its possible influence on the seawater intrusion extent in the upper
Quaternary Limestone aquifer – are not intuitive and cannot be resolved by this firstpass
analysis. Such issues are expected to be addressed in subsequent numerical
modelling efforts through the Fellowship.
Despite these limitations, the modelling approach used above does provide a useful
first-pass investigation into the physical factors driving seawater intrusion on the
southern EP. It has allowed us to see that in Coffin Bay A lens and Lincoln B, the
combination of aquifer depth and relatively low inland heads (meaning low flow
towards the coast) have most likely led to seawater extensively underlying the
coastal freshwater layer, as supported by the observed data. In Uley South, the
analytical model has shown that seawater intrusion (both present and future) is
governed by aquifer thickness, recharge and discharge to the sea. In the northwest
portion of the basin where the Tertiary Clay is thought to be relatively thick and
continuous, there is a reduced risk of seawater intrusion, whereas in the southeast
portion where the clay may be absent or irregular, it is possible that intrusion has a
greater potential to extend further inland.
5.4.1 Uncertainty in Climate Change Scenarios
Climate change projections are inherently uncertain, given the coupled, non-linear
nature of the climate system in general. In climate modelling, it is necessary to
approximate certain physical processes, and make assumptions of key parameters
such as aerosol and albedo forcings. Further, there are major computational
constraints on grid resolution that result in grid cell sizes on the order of several
hundred kilometres for global models. The combination of these factors means that
projections (especially for rainfall) can vary considerably between different models –
39

even when modelling the same emissions scenario. Finally, there is enormous
variability across the IPCC’s accepted suite of global greenhouse emissions
scenarios (Nakicenovic and Swart, 2000), and the “realism” of any given scenario is
as yet unknown. These uncertainties are described in more detail by Ward et al.
(2009).
It is because of the uncertainty associated with climate change that the parameters
chosen for this study ( B , R and q
i
) were applied as ranges, representative of
the variability in possible future climate. This is a simple method, but it remains useful
for observing the key parameter sensitivities (such as a greater dependence on
recharge reduction than sea-level rise in driving further seawater intrusion). It is
hoped that future work (see below) will address climate change uncertainty using the
stochastic framework introduced by Naji et al. (1998).
5.4.2 Outstanding Data Gaps
Several data gaps remain that would contribute significantly to any future seawater
intrusion modelling efforts (whether analytical or numerical). Further investigation
and/or research would be beneficial in the following areas:
Knowledge of extent and thickness of the tertiary clay layer near the coast
and its impact on the relevance of the two-part conceptualisation used in this
study.
Improved understanding of vertical gradients between the Quaternary and
Tertiary aquifers.
Further definition of aquifer properties (especially hydraulic conductivity and
recharge) in the coastal fringe area, and the possible influence of
heterogeneity in both properties.
Accounting for dynamic forces such as seasonality of recharge and extraction
(including up-coning), reversibility of intrusion and the time-scales associated
with climate change-induced seawater intrusion. Further work is continuing in
these areas under the fellowship.
Assessment of the aquifer geometry in the coastal fringe, perhaps obtained
through a stratigraphic characterisation of exposed cliff-faces along the Uley
South coastline, for example.
Measurement of pumping bore salinities as ongoing time-series, to assess
the nature and extent of saltwater up-coning, and to allow future studies
insight into how bore salinisation occurs where there is a saltwater wedge
persisting below production bores.
Estimation and field-verification of the coastal hydraulic head representing the
ocean, taking into account tidal over-height effects (e.g. as per the study of
Carey et al., 2009 that found that tidal effects can account for up to 2 metres
of additional hydraulic head at the ocean boundary).
40

6. Conclusions
The Southern Basins on EP are vulnerable to seawater intrusion to varying degrees.
Below average hydraulic heads in the lens aquifers (Coffin Bay A and Lincoln B) in
recent drought years have lead to reduced outflows to the ocean, leading to a
decrease in lens thickness. The deep basements and thick sediment thickness below
sea level in these aquifers means that seawater underlies virtually the entire
freshwater body. A key conclusion from this analysis is that further work is required to
assess the risk of up-coning with regard to bore distribution and penetration and
pumping rates and persistence in Coffin Bay A and Lincoln B lenses, especially if
increased extraction rates are planned in the future.
The Uley South basin, which supplies the majority of groundwater to the EP, has
undergone a decline in hydraulic head over the past two to three decades, on the
order of 1.5 m. The resultant groundwater level is now similar to that in Coffin Bay A
and Lincoln B, implying that seawater intrusion could be a potential threat. However,
the presence of a Tertiary Clay aquitard underlying the relatively thin Quaternary
Limestone (the target aquifer for freshwater extraction) means that the risk of
intrusion into the upper aquifer is probably relatively low in the northwest portion of
the basin under the current conditions. The southeast portion of the basin may be
more vulnerable, as the presence and thickness of the aquitard is uncertain.
Basic vulnerability to climate change (sea-level rise, recharge decline) has been
investigated for the Uley South basin, using steady-state analytical models. While
these models are a simplification of the coastal aquifer system (i.e. relative to its
naturally complex form) they do allow useful insight into coastal aquifer controls, and
some fundamental conclusions can be drawn from the results. Climate changeinduced
seawater intrusion in Uley South appears to be most vulnerable to declines
in recharge (relative to sea-level rise impacts) if water levels are allowed to rise
commensurate with sea-level rise (i.e. assuming a fixed flux inland boundary).
Further investigation is needed to properly analyse the seawater intrusion threat to
the Uley South groundwater system, because the highly heterogeneous nature and
sediments and the regional impacts of pumping haven’t been accounted for in the
current study, amongst other simplifications. It is anticipated that an extension to the
groundwater flow modelling of Werner (2010) will provide the basis for seawater
intrusion analysis of Uley South via the development of 3D seawater intrusion
models.
41

7. Future Work
7.1 Ongoing Work
7.1.1 Monitoring of Saltwater Interface in Coastal Wells
In Uley South there are currently two long-screened monitoring wells (ULE205 and
SLE069), both approximately 500m from the coast. The wells allow the direct
measurement of a saline interface by salinity profiling (sonding). At the time of this
report, two additional wells were under construction.
Previous results of sonding have established what could be considered baseline
information for the position of the saline interface. However, to date insufficient
temporal information has been collected to evaluate any changes in salinity profiles
and other coastal parameters over time (see Section 3.1). A new study is being
planned by EPRNM (to commence in 2010) to characterise the dynamic nature of the
seawater interface which may potentially vary with water level fluctuations, tidal
cycles and even with local atmospheric pressure variations. The study will aim to
provide advice regarding the sonding method as an ongoing monitoring technique,
that is, to inform its reliability and how regularly monitoring should occur. Any
analysis of this nature should be closely coupled to ongoing modeling efforts to allow
for a comparison between simulated and measured saltwater wedge changes.
Modelling is expected to offer important insights into future monitoring efforts, and
vise versa. As a preliminary measure, it is expected that 2 – 4 wells would be sonded
every month for 1 year. The program should observe a seasonal water level
maximum and minimum, accompanied by transient interface behaviour on the
seasonal timescale. Monitoring dates and times can be compared against the known
tidal movement in order to assess tidal impacts on the interface position. Once the
“normal” variations of the saline interface have been identified, ongoing monitoring
will be able to observe any net changes to the system and provide an early warning
monitoring system for signaling seawater intrusion that may threaten pumping wells.
The newly constructed wells that are sited inland of the wedge and constructed to the
aquifer basement will also offer an early-warning measure for any impending
seawater intrusion.
Due to the presence of vertical gradients, one must be cautious when attempting to
directly relate the depth and fluctuation of a saline interface observed in a well via
sonding (particularly long screened wells) to the depth of the interface in the aquifer.
Despite this, it is anticipated that frequent monitoring will be more useful than
sporadic monitoring, and will improve the understanding of relative changes in the
interface depth over time.
7.1.2 Scenario Testing Using a Numerical Model
Under Milestone 10 of the GAPM project, a numerical model of seawater intrusion in
Uley South is expected to be developed through the Research Fellowship. Key
benefits of the numerical modelling framework include an ability to consider the
aquifer geometry in three dimensions, and the transient nature of the numerical
42

scheme allows prediction of time-dependent impacts on the seawater interface. This
will therefore complement the proposed new monitoring regime. The primary purpose
of the model is to build a tool that can be used to assess the possible risks
associated with future management scenarios, although the predictability of seawater
intrusion models at the regional scale is debatable and requires further research.
Nonetheless, regional-scale seawater intrusion models are considered the optimal
methodology for supporting management decision-making for key coastal aquifers.
The current management approach to groundwater allocation is based on estimates
of historical and recent recharge volumes (flux-based approach; Werner et al., In
prep). “Trigger-level management” presents as an alternative strategy that allows for
adaptive management of groundwater abstraction, based on the condition of the
aquifer at any given time (Werner et al., In prep.). Trigger-level management involves
defining a measurable threshold or series of thresholds, designed to protect
important aspects or features of a system. Allocations of groundwater may be
reduced or ceased altogether as the defined thresholds are approached or
exceeded.
The principles of trigger-level management were applied by Alcoe (2009), using a
water balance model that considered seawater intrusion (in Uley South) to be an
unacceptable risk. One of the major limitations of the work was the application of a
steady-state seawater interface to a changing (monthly) water-level step. This
limitation can be overcome once the transient numerical model is available, allowing
flux-based and trigger-level management approaches to be rigorously compared.
7.2 Suggested Further Work
The following ideas have been tentatively discussed between academics at Flinders
University, EPNRM, DWLBC and SA Water. At the time of writing this report, these
ideas had not necessarily transformed into definite projects and should be read here
as a suite of possible future research directions that – if pursued – would feed into an
improved understanding of seawater intrusion processes on EP.
7.2.1 Investigation of Heterogeneity
An extension to the numerical model (developed under Milestone 10) is currently
being discussed, in which 2D “slices” (perpendicular to the coast) are simulated using
a high grid resolution, in order to investigate the impacts of small-scale densitydependent
processes that may not be captured within the large-cell regional model.
By “condensing” the model from 3D to 2D, an extra level of detail can be achieved
without increasing computational demands. This enables different scenarios of
heterogeneity to be investigated in a fundamental way. Such investigations would
help determine whether, for instance, the regional model is likely to be over- or
under-predicting the extent of seawater intrusion in Uley South, and whether such
effects are significant.
43

7.2.2 Investigation of Transient Behaviour and Reversibility
There is ongoing research at Flinders University to investigate the timing of seawater
intrusion, in particular addressing the question of how long a given system will
transition from one steady state to another (e.g. in response to climate change). This
work will greatly enhance the practical applicability of steady-state analytical
solutions such as the one presented in this report. One outcome of this research may
be that seawater intrusion responds more slowly to one process (e.g. sea-level rise)
than another (e.g. recharge change). This research is being undertaken in a general
way and is thought to be of international significance, and of course will be relevant
to EP’s aquifers.
7.2.3 Investigation into Saltwater Up-coning
Given that (a) seawater apparently underlies freshwater in Coffin Bay A lens, Lincoln
B and possibly in the southeast portion of Uley South (if the aquitard is absent), and
(b) saline water has already been detected in extracted water from the former two of
these, it is recommended that a saltwater up-coning study be conducted to determine
safe extraction rates and/or appropriate trigger-levels to guide appropriate
groundwater resource management. This would help to inform appropriate extraction
locations and rates in high-risk areas, and ensure that groundwater users are
adopting the best management methods to prevent this complicated contamination
process.
7.2.4 Uncertainty Analysis of Seawater Intrusion Approaches
Using contemporary uncertainty analysis methods, such as those described by
Doherty (2004), it is possible to evaluate the predictive uncertainty of various
approaches to seawater intrusion analysis. Application of these methods would allow
a comparison between sharp-interface and diffuse-interface approaches (i.e. the
current analytical, sharp-interface analysis compared to 3D numerical models) to
afford future investigators of the Uley South aquifers insight into the best approach to
coastal aquifer and seawater intrusion analysis.
7.2.5 Assessment of Sources of Salinity
It is clear that in some parts of the EP, groundwater salinity may have different
origins, even where groundwater occurs near the coast. Possible sources include
discharge from saline (e.g. low-recharge) aquifers, basement dissolution, relic
seawater, and of course, seawater intrusion. It is proposed that a thorough hydrochemical
analysis be undertaken to ensure that the sources of salinity are well
distinguished as these influence the management reactions to bore salinisation, and
also assist in shaping future operational management strategies and pumping
allocations.
44

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47

Appendix - Data and
interpretation provided by SA
Water
-
Amendment to the Report on Saltwater Intrusion in Southern Eyre
Peninsula regarding the presence of Uley Formation aquitard in the
southern part of the basin
The previous understanding of the aquifer systems in this area suggested that the Uley
Formation aquitard was absent and the Quaternary and Tertiary aquifers were directly
hydraulically connected.
During the latest drilling programs, 5 monitoring wells were constructed in three different
locations, Figure 1. New data obtained from these wells and review of the down-hole
geophysical Gamma log for the monitoring well ULE 205 suggest that the Quaternary and
Tertiary aquifers are separated by the Uley Formation stratigraphic clay layer which can be
seen from the following:
Nested monitoring site ULE 156/ULE 209
Drilling
In the lithological log of the ULE 156 it is indicated that 3 m (123 – 126 m) of the Uley
Formation (clayey sand and clay) is present (R Baird – SA Geodata),
In the stratigraphic log, this 3 m interval is identified as a sandy clay (R Baird – SA
Geodata),
In the Hydrostratigraphic log, the same interval is identified as a weak clay (R Baird – SA
Geodata),
Groundwater encountered in the bottom part of the Quaternary Limestone Aquifer (114 m
and 120 m) during drilling was evidently of lower salinity (~ 1000 uS/cm) than the
groundwater salinity encountered in the Tertiary Sand Aquifer at 132 m (~2100 uS/cm),
Salinity profiling
The salinity profile of the Quaternary Limestone Aquifer (March 2010) in the monitoring
well ULE 209 indicates no salinity stratification present through the production zone (top
part of the aquifer 88 – 94 m) where salinity ranges between 1016 uS/cm and 1063
uS/cm and it is in good positive correlation with the groundwater salinities at the bottom
part of the Quaternary Limestone Aquifer, obtained during drilling ULE 156, ~ 1000 uS/cm
(Figure 2),
The salinity profile of the Tertiary Sand Aquifer (March 2010) in the monitoring well ULE
156 indicates the presence of the salinity stratification through the production zone where
salinity has first jumped from ~2080 uS/cm to ~2360 uS/cm and then steadily increased to
~3170 uS/cm. The groundwater of lower salinity obtained during drilling could be due to
mixing and dilution from the overlying Quaternary Aquifer.
These salinity contrasts suggest that the Quaternary and Tertiary aquifers are separated by
an aquitard.
Groundwater level monitoring
The Groundwater level elevation of the Tertiary Sand Aquifer is always 5 – 7 cm higher than
in the Quaternary Limestone Aquifer.
48

Monitoring site ULE 210 (Tertiary Sand Aquifer)
Drilling
Downhole geophysical logging data reported a response in the Gamma supporting the
assessment made from drill bit sample (drill cutting, clay, brought to the surface when
retrieving the drill bit) that the Uley Formation may form an aquitard at this location, Evans
et al (2009),
The downhole geophysical logging results for the induction log are not conclusive in
identifying the presence of a seawater interface, Evans et al (2009),
Salinity profiling
The salinity profile of the Tertiary Sand Aquifer (March 2010) in the monitoring well ULE
210 indicates that there is no salinity stratification in the production zone interval where
salinity ranges between 2098 uS/cm and 2114 uS/cm. However, the bottom 3 metres of
the production zone do not have salinity readings, Figure 3.
Nested monitoring site ULE 211/ULE 212
Drilling
Loss of returns from the coring (ULE 211) made it difficult to determine if the Uley
Formation in this location is a significant clay layer that will effectively separate the
Quaternary and Tertiary aquifers, Evans et al 2009.
Initial salinity results indicate that the EC is considerably lower in the Quaternary Aquifer
than in the underlying Tertiary aquifer, 1100 uS/cm and 1530 uS/cm respectively.
Difficulty in downhole geophysical logging equipment prevented using the induction tool to
interpret whether a saline interface is present within the Tertiary aquifer. However,
anecdotal evidence obtained during development suggests that the presence of an
interface is likely, Evans et al 2009.
The groundwater salinity recorded during development of the Quaternary Limestone
Aquifer monitoring well ULE 212 was in the range of ~950 uS/cm to ~1070 uS/cm.
The groundwater salinity recorded during development of the Tertiary Sand Aquifer
monitoring well ULE 211 was in the range of ~7600 uS/cm at the early stage of airlifting to
~1500 uS/cm at the end of development.
Salinity profiling
The salinity profile of the Quaternary Limestone Aquifer monitoring well ULE 212
indicates no salinity stratification present through the production zone, where salinity
ranges between ~1170 uS/cm at the top to ~1150 uS/cm at the bottom, which is in good
positive correlation with the groundwater salinity obtained during development (1100
uS/cm), Figure 3. In the sump interval salinity is in the range of 1150 uS/cm to 3490
uS/cm. This zone should not be considered as representative of the aquifer salinity as this
column of water is stagnant and was most likely entrapped during the well completion,
Figure 4. However, it is not clear where groundwater of 3500 uS/cm salinity is coming
from, since during development no salinity of this range was recorded. To confirm
groundwater salinity from the sump section and better understand its origin/salinity this
groundwater from the sump should be pumped out prior to the next round of salinity
profiling.
The salinity profile of the Tertiary Sand Aquifer monitoring well ULE 211 indicates that
there is salinity stratification in the production zone interval where salinity ranges between
~1750 uS/cm (top seven metres of the production zone) and up to ~3500 uS/cm at the
bottom of the production zone. In the sump interval salinity has increased further and is in
the range of ~3500 uS/cm to ~6100 uS/cm. This zone should not be considered as
representative of the aquifer salinity as this column of water is stagnant and most likely
was entrapped during the well completion. At the start of the well development initial
49

groundwater salinity readings, measured at field were at range of ~6000uS/cm, which has
decreased over time to 1530 uS/cm. To confirm groundwater salinity from the sump
section and better understand its origin this groundwater should be pumped out prior to
the next round of salinity profiling.
Monitoring site ULE 205 (Quaternary Limestone and Tertiary Sand
Aquifers)
Drilling
In the lithological log of the ULE 205 it was not indicated that the Uley Formation (clayey
sand/ sandy clay) is present,
Salinity profiling
The review of downhole geophysical logging data (Gamma log) suggests that the Uley
Formation may be present at this location (~1 – 1.5 m thick) and to an unknown degree
separates the Quaternary Aquifer from the underlying Tertiary Aquifer, as marked in
Figure 5.
A scale of gradual increase in the groundwater salinity with depth through this zone 90.3
– 91.3 m (-23.23 to -24.23 mAHD) is higher than in the overlying Quaternary Aquifer, but
lower than in the underlying Tertiary Aquifer, as marked in Figure 6.
50

ULE 210
ULE
211
ULE
212
ULE 209
ULE 156
Appendix Figure 1. Modified from Fitzpatrick at al 2009
51

Appendix Figure 2. Quaternary and Tertiary aquifers salinity profiles
52

Appendix Figure 3. ULE 210 - Tertiary aquifer salinity profiles
53

Appendix Figure 4. ULE 211 & ULE 212 - Tertiary and Quaternary aquifer
salinity profiles
54

Appendix Figure 5. ULE 205 Composite water well log
55

Appendix Figure 6. Quaternary and Tertiary aquifers salinity profiles
56