Dealing with salinity in Wheatbelt Valleys - Department of Water

In 1999 the Department of Agriculture’s Rural Towns Program chose Merredin as a site to test the
application of a groundwater model to assist the section of management options. A more
comprehensive pump test was conducted along with a further drill program to obtain aquifer properties
for calibrating the model.
Three scenarios were then modeled: tree planting, groundwater abstraction through pumping wells and
a “do nothing differently” scenario. The model found that pumping would be effective at lowering the
watertable within a radius of 200 m, tree planting would be effective at reducing recharge in the
immediate vicinity of the plantation and doing nothing would result in extended areas of land within the
town with a watertable within 2 m of the surface in 10 years time.
In 2000 the Merredin Council secured a State Salinity Council Grant for the “Merredin Townsite pumping
and evaporation basin project”. This project aims to look at the longer term potential of lowering
groundwater levels in the town, the technical feasibility of desalination and the environmental
implications of a 2 ha evaporation basin.
PHYSICAL ENVIRONMENT OF THE MERREDIN CATCHMENT
Topography and Climate
The Merredin catchment is located 265 km east of Perth on the Great Eastern Highway. The total area
is approximately 400 km 2 . The catchment is characterised by a flat topography low relief (109 m).
The climate of the area is semi-arid with hot dry summers and cool, wet winters. Average annual rainfall
(1901-1998) measured at Merredin Shire is 328 mm, with 70% falling between May and October. Ten
km west of Merredin at Nangeenan the average annual rainfall is 305 mm. The estimated potential pan
evaporation is about 2,630 mm/yr.
Vegetation
Approximately 10% of the Merredin catchment retains a cover of remnant vegetation. Bettenay and
Hingston (1964) described the soil-landform characteristics of the Merredin district. The areal extent of
each unit and its predominant vegetation was as follows: Danberrin (York gum) 9%, Ulva (tamma) 20%,
Norpa (grevillea, wodjil) 15%, Booran (white gum) 20%, Collgar (mallee) 5%, Merredin (salmon) 20%
and Nangeenan (morrell) 11%.
Surface Water Hydrology
The catchment is located in the Swan-Avon drainage basin. Surface drainage is from east to west via a
series of intermittent creeks. The catchment discharges into the salt lake chains at Hines Hill, which
form part of the Yilgarn River palaeodrainage system.
In 1984, 110 km of absorption banks were constructed throughout the Merredin catchment to protect the
townsite from extreme flood events and reduce catchment erosion. The town had previously constructed
a network of storm water drains to deal with street runoff within its perimeter.
Geology
The Merredin catchment is underlain by highly weathered Archaen age granite and gneissic bedrock.
Major lineaments (faults, fractures in bedrock) trend in NNE and ESE directions. Dolerite scree from
Proteozoic intrusions and quartz veins are present in the upper, more dissected area of the catchment
(George and Frantom 1990).
Above the bedrock there is up to 50 m of clay-rich unconsolidated material derived either from in situ
weathering of the underlying bedrock or from transported material. The insitu-weathered profile consists
of a poorly weathered saprock zone underlying a more intensely weathered pallid zone.
Above these units can be found sequences of colluvial, alluvial and aeolian sediments derived from
erosion of the upper slopes. A graphical representation of the relationship between the sedimentary and
weathered zones in the upper valley area is presented in Figure 1.