Dealing with salinity in Wheatbelt Valleys - Department of Water

George and Coleman
two to four fold increase in flood volumes and
changes in peak flow are predicted (Bowman &
Ruprecht 2000).
Finally, the South-West of Western Australia is
recognised as one of the ten megadiverse biological
regions on the planet, containing over 12,000 native
species of flora. In the Wheatbelt where native flora
and fauna is only represented in small reserves,
especially within valleys, as many as 450 extinctions
may result from increased groundwater levels and
salinity.
Minimising the current and potential menace posed
by continued salinisation is dependent on our ability
to develop both successful and economic
management options and by finding productive uses
for saline groundwater and land. This paper
discusses the hydrologic factors responsible for the
menace, reviews how wheatbelt valleys worked
before clearing and how they have changed. It also
explores some of the salinity predictions in the
Central and Eastern Wheatbelt. Finally, it points
towards some of the opportunities we have for the
use of saline groundwater which may assist
wheatbelt communities capitalise on the large scale
environmental, social and political change which is
forecast.
Hydrogeology of wheatbelt valleys
Archaean rocks of granitic composition intruded by
Proterozoic mafic dykes underlie wheatbelt valleys.
The crystalline basement has been deeply weathered
and chemically leached during a period of relative
stability (~100 Ma), resulting in a deeply weathered
mantle which comprises the preserved saprolite (up
to 50 m) and overlying sediments (usually less than
50 m). The saprolite consists of in situ and partially
weathered material above the basement (saprock),
then intensely weathered sandy clays (pallid colored),
which are in turn overlain by mottled materials and
the sandy soil layer. The saprolite is thickest on
hillsides and within the lowerslopes.
In valleys, sedimentary sequences dominate the soil
profile (George 1992a). At the base, particularly in
association with trunk or palaeo-valleys, but also in
some tributaries, occur sequences of coarse alluvial
sediments which fine upwards into clay-rich materials
(Salama 1997). In many palaeodrainages (typically
< 270 m AHD in western areas) very fine-grained
lacustrine (Eocene; lake or swamp) sediments occur
(George et al. 1994; Clarke et al. 2000), and may in
turn be overlain by indurated sands. Recently,
drainage-inspired excavations have revealed iron and
– 2 –
silica cemented horizons in the upper 3 m of the soil
profile. These sequences are highly variable in
vertical and spatial distribution.
Beard (1999) recently deliberated on the history and
flow direction of wheatbelt palaeodrainages. Building
on earlier work and together with experimental data
obtained from drilling at many places (e.g. George
1992a; George et al. 1994; Salama 1997, George &
Dogramaci 2000), a picture of the genesis and
structure of wheatbelt valleys has been formed (see
Commander et al., this volume).
Mapping the location of the deep, sandy
palaeochannel sediments has not been undertaken in
the Wheatbelt, except where drilling for minerals or
airborne electromagnetic surveys have been
undertaken (George 1999). However landformmapping
undertaken in the 1960's (Malcolm 1983),
may provide a guide as to their general location.
Bettenay and others recognised three principal valley
forms in the Eastern Wheatbelt: the lower 'Baandee
and Nangeenan' surfaces, which correspond to the
playa and primary saline areas; the 'Belka surface'
(sandy surface comprising braided streams); and the
'Merredin surface' (dominated by sandy clay loams,
'red' soils). The former was considered to overlay
palaeodrainages, the second, is a more modern,
fluvial depositional environment, while the latter was
derived by sheet flow at lower velocities, hence
depositing only fine-grained transported material
(Bettenay et al. 1964). In this unit, colluvial
sediments were expected and salinity risk was
believed to be low.
Wheatbelt aquifers and their properties
In 1924, W.E. Wood identified the essential
components of the Wheatbelt's groundwater
hydrology, concluding that water movement was
greatest in the lower layers of the weathered profile.
This conclusion was modified by work by Bettenay et
al. (1964) and Smith (1962) and later, by Peck et al.
(1980) and Nulsen & Henschke (1981), although to
this date, little reference had been made to
sedimentary aquifers.
George (1992a) described in more detail the
hydraulic properties of aquifers in the Wheatbelt.
Three dominant types were recognised: a sandplain
perched aquifer, a saprolite 'grit' aquifer (saprock)
and a valley sedimentary aquifer (~20 m). George
showed that the saturated hydraulic conductivity
(K sat) of the saprolite and sedimentary aquifers
(~0.6 m/d) were about an order of magnitude higher
than that of the pallid zone aquitard. George et al.