Dealing with salinity in Wheatbelt Valleys - Department of Water

(1997) later refined the relationship between aquifers
and salinity into ten models. They concluded that
local-scaled regolith or saprock aquifers were
dominant, although intermediate-scaled sedimentary
systems were common in valleys. Clarke et al.
(2000) showed that the K sat of the saprock aquifer
and intensely weathered aquitard was similar in
western areas (Woolbelt). Clarke also found that in
palaeo-valleys in the western areas, the lower sandrich
palaeochannel aquifer has a K sat ~300 times
higher (~3 m/d) than the lacustrine aquiclude
(Clarke et al, 2000). These are similar to those
found by Salama (1997; 3–6 m/d) in the Avon
palaeochannel and George & Dogramaci (2000;
5 m/d) in the Toolibin palaeochannel.
Groundwater velocities are typically very low. Given
typical hydraulic conductivities (0.5–5 m/day) and
gradients (1:1000–1:10,000) for wheatbelt valley
sediments, flow velocities in the deeper saprolite and
sedimentary aquifers of about 0.1 m per year are
considered likely. Under exceptional circumstances,
in surface sediments, where there is a high
permeability (> 1.0 m/day), and gradient (0.005),
velocities may increase to 5 m/yr. For velocities of
greater than this either the permeability must be
very high (soil macropores) or the gradient increased
by intervention (drainage or pumping).
Hydrology and salinity of wheatbelt valleys
The magnitude of recharge to wheatbelt valleys is
not known with precision although several authors
have attempted to estimate it. McFarlane et al.
(1989) used hydrograph analysis to determine that
recharge in valley sediments was at least as significant
as that on hillslopes, recording rates of 35 mm/yr at
Toolibin. George et al. (1991) in reviewing recharge
processes from a range of wheatbelt sites agreed,
and concluded that recharge occurred everywhere
that discharge did not and that even in these areas, it
could alternate between seasons. They further
showed that episodic recharge is critical, a subject
Lewis (2001) has comprehensively reviewed.
A range of techniques (chloride, water balance and
hydrograph) was used to estimate the relative
magnitude of recharge between catchments and
landforms in wheatbelt valleys (George 1992b). He
concluded that recharge from uncleared catchments
was typically of the order of 0.05 mm/yr, while for
the same catchments, recharge increased to be
between 6 and 10 mm/yr. after clearing. Recharge
was highest in upland (e.g. Danberrin and Ulva soil
associations) and sandy surface valley soils (e.g.
– 3 –
George and Coleman
Colgar, Belka) and least in clay-rich soils (e.g.
Merredin and Booraan surfaces).
Recharge to valley sediments also takes place
laterally, either directly from connected fluvial
sequences of sediments or from depth, where
groundwater from adjoining saprock and potentially
fractured rock systems enters sedimentary aquifers
(Salama et al. 1993; Salama 1997). The degree of
interconnectedness has not been assessed in detail;
however modelling at Toolibin (George & Dogramaci
2000) suggests saprock and palaeochannel aquifers,
and at Merredin (Matta 2000) saprock and overlying
sediments are both well connected and respond to
pumping.
Groundwater discharge from valley sediments has
also never been measured directly. In only one
experiment known to the authors, Greenwood &
Beresford (1980) used ventilated chambers to
document the 'water use' (discharge) of bare soil
near Kellerberrin. They measured rates of the order
of 0.4 mm/d in summer (equated to 80 mm/yr) at a
site where saltbush was growing. George (1992a)
calculated potential discharge rates from
piezometers (calculated vertical flux), concluding
rates of between 50 and 230 mm/yr. Using a water
balance approach, George (1992b) indicated that if
the effective discharge rates were 80 mm/yr (~3%
of pan evaporation rates), the entire recharge to the
Wallatin Creek catchment (i.e. water added annually
to the groundwater) could be lost from 12%
(2880 ha) of the catchment.
Furthermore, from first principles, at equilibrium
recharge will equal discharge and as catchment
average recharge is about 20 mm/yr, and the
potential discharge area is 20–30% of the landscape,
average discharge rates will be no more than
100 mm, a number similar to that estimated above.
This low flux rate has significant implications for
salinity management, as we will discuss later.
The effect of the imbalance between recharge and
discharge can be measured by monitoring trends in
groundwater levels. In wheatbelt valleys, landholder
groups and government, typically using shallow
piezometers or observation wells, are monitoring
trends in water levels. Short & McConnell (2001)
documented trends in deep bores across the entire
agricultural area. They concluded that where the
watertable was within 2 m of the soil surface, there
was often no trend (water levels neither rose nor fell
from their 1–2 m position). However, where
watertables were greater than 2–5 m below the soil