Aquaculture Groundwater Resource Atlas - Southwest Agricultural Region

Overview

To the east of the Darling Fault, the South west Agricultural Region (SAR) is underlain principally by crystalline basement rocks of the Archaean (over 2.5 billion years old) Yilgarn Craton and the Proterozoic (around 1 billion years old) Albany Fraser Orogen. Proterozoic sediments of the Moora Group and Stirling Range Formation, and Eocene (around 45 million year old) sediments of the Bremer Basin, also lie within the region. A description of the geology, physiography and soils is available at www.avonicm.org.au/avonicm/wbeltvalleys/page2.html

In general, the crystalline rocks (which are mainly granite and gneiss) and the regolith (weathered zone) are not easily prospective for groundwater; bore yields are generally low and highly variable, and the groundwater salinity is high. Locally, larger supplies of low-salinity groundwater occur in favourable rock types and elevated positions. The comparatively thin sediment cover in some areas provides local groundwater supplies. Larger supplies of saline or hypersaline groundwater are present in sediments underlying some of the major valleys.

Groundwater has been explored for throughout the region, principally for stock watering, and a large amount of information has also been gained from salinity management investigations. The density of bores is greatest in the north (Perenjori), whereas in the east (Merredin) and south (Ravensthorpe) dams are the preferred source of stock water. Potable groundwater for town water supplies has been obtained locally in the west (Calingiri, Brookton) and south of the region (Albany, Bremer Bay, Esperance).

Bore yields vary widely throughout the region. Most exploration has been for stock water, and a supply of 4.5 kL/ day, suitable for equipping with a windpump, is considered adequate for stock. Consequently, there is little information on larger yields. Anecdotal evidence of high bore yields has subsequently been shown to be erroneous. Caution should therefore be applied in accepting bore yields based on short term airlift testing, as such yields may subsequently not be maintained.

Groundwater salinity varies widely, both laterally and with depth. Bores separated by only a few hundred metres may have widely differing groundwater salinity. Consequently, it is not possible to map groundwater salinity in many areas, even at the broad scale, with current data.  Groundwater salinity information is generally poor. Older records in the Water and Rivers Commission's database distinguish only between fresh, brackish, stock, saline and too saline, and actual numerical values are not available in many areas. Groundwater salinity exceeds that of seawater under the main valleys, and close to salt lakes it may be several times the salinity of seawater (35 ppt). The Department of Agriculture's AgBores database also contains 7300 bore records which have accurate water level and salinity data.

An extensive program of drought relief drilling was carried out in 1969. Success rates (defined as a bore with more than 4.5 kL/day and a salinity less than 11 ppt) ranged from 25% in the North Stirlings to less than 1% in the South Yilgarn (Lord, 1971). Some 17% of the bores drilled were classified as ‘wet-saline supply'. However, a followup survey (Davidson, 1977) found that less than half of the successful bores drilled had been equipped and utilised.

Hydrogeological maps using groundwater salinity data from existing bores have been compiled for the Perenjori 1:250 000 map sheet in the north, and for the Collie, Pemberton-Irwin Inlet,  Mount Barker-Albany, Dumbleyung, Newdegate, Bremer Bay, Ravensthorpe and Esperance 1:250 000 map sheets along the south coast.

Position in the landscape is the main determinant of groundwater conditions in areas underlain by crystalline rocks. Areas of sandplain, commonly found on drainage divides, are considered favourable for low salinity groundwater, based on better recharge conditions.

There is no comprehensive account of groundwater conditions in the SAR. Groundwater occurrence in the wheatbelt has been described by O' Driscoll (1977) and Bestow (1985), and in an overview of the region by Allen and others (1999). More detailed aspects are covered in studies by George (1990, 1991,1992), George and Frantom (1990, 1991), Salama and others (1993,1994), Clarke and others (1999) and George and Coleman (2001).

Geology

The geological map below shows the broad distribution of rock types within the Southwest Agricultural Region. The region is mostly underlain by granites, which range from fine grained up to coarse grained and porphyritic (containing large felspar crystals). The other dominant rock types are gneisses, which range from banded fine-grained granitic rocks, to metamorphosed sedimentary rocks ranging from schists (foliated, micaceous rocks) to chert (silica) and banded iron (massive ferruginous silica). Small areas of greenstone belt, dominantly basalt, fine-grained metamorphosed sediments, and chert, occur near Southern Cross, Ravensthorpe, Wongan Hills, Perenjori, and Boddington.

Geological map of the Southwest Agricultural Region

Proterozoic sediments (aged around 1000 million years), mainly sandstones or quartzites, occur in the Stirling Range and Mount Barren to the east. Sediments of similar age, the Moora Group, occur along the northwest margin of the Yilgarn Craton, adjacent to the Perth Basin. These include sandstone, siltstone, basalt and tuff, dolomite and chert.

A detailed description of the geology is given in Memoir 3 of the Geological Survey (1990). A regional map can be accessed on the Department of Mineral and Petroleum Resources website (www.dme.wa.gov.au). Detailed mapping is available on the 1:250 000 geological series maps.

Aquifers

The characteristics of the aquifers are known mainly from drilling for stock water supply and land salinisation investigations. While these investigations have provided details on the weathered profile and the groundwater salinity, few bores have been tested adequately for yield, and there has been very little drilling reported in the crystalline bedrock.

The granite bedrock is commonly weathered to form a saprolite profile. This consists of an upper zone of kaolinite (white clay) in which the quartz grains are dissolved or partially dissloved, and the felspars altered to clay, overlying a gritty zone of quartz grains which may pass downwards into a weathered rock (saprock) where the quartz and felspar grains are separated. These ‘saprolite grits' are the main aquifer and, where developed, may yield as much as 100 kL/day (George and others, 1992). Reference to the 1:250 000 series geological maps published by the Geological Survey may assist in delineating coarser grained, more prospective granite bodies.

The gneissic bedrock is less prospective for high yields, as the grain size is finer and the weathered profile more clayey.

Metamorphic rocks within the general areas of gneiss in the Darling Range have a variety of rock types, of which quartzites may be generally prospective for significant supplies. These are best developed in the Toodyay area. Quartz veins may also be highly prospective, though they are not common. Dolerite dykes are not likely to yield useful groundwater supplies, although the weathered margins may yield water in the saprock and fractured zones.

Fracture zones in the unweathered bedrock, especially the prominent fault zones in the southwest part of the region may have significant potential. Fracturing may also occur along geological contacts.

Sedimentary rocks of the Moora Group along the western margin of the region between Moora and Coorow have significant potential. Groundwater from the Noondine Chert is used for town water supply, and the rocks are known to be fissured, with large caverns developed, and with potentially high yields.

The Stirling Range Formation and Kundip Quartzite (near Ravensthorpe) may have local potential where fractured.

Palaeochannel sands (representing former river beds) are the most prospective aquifers in the region, though the groundwater salinity is high. These sands are located at the base of buried palaeovalleys within the broad wheatbelt valleys (Commander and others, 2001; George and Coleman, 2001).

Sandplain seeps locally discharge low-salinity groundwater, though the storage is low, and the seepage dependent on annual rainfall. Such supplies range from 5 to 20 kL/day.

The Southwest Agricultural Region has been subdivided into soil–landscape zones (Short and McConnell, 2001), and these are used here to describe the groundwater resources. These zones principally reflect rainfall, landforms and soil type.

Summary of best prospects for high yields in the Southwest Agricultural Region – fresh-brackish-saline

• Saprolite grits on the edges of major valleys
• Fracture zones, fractured quartzite, and chert
• Palaeochannel sands
• Sandplain (NW wheatbelt)
• Collie, Wilga and Boyup Basins
• Bremer Basin sediments (south coast)
• Coastal dune limestone (south coast)

Soil-landscape zones in the Southwest Agricultural Region
(excluding the Perth Basin)