Hydrogeology of the Pilliga sandstone aquifer in the western Coonamble embayment and its implications for water resource management

Water resources in Australia are undergoing the greatest changes since Federation. The Council of Australian Governments' (COAG) 1995 Water Reforms had two control themes, the need for pricing mechanisms to reflect the true costs of supplying water and the need to better balance water allocation between consumers and the environment. These, coupled with the Murray-Darling Basin Ministerial Council's 1995 Cap on diversions of surface water in the Basin at 1994 levels have placed pressures on water resources, particularly groundwater. In the New South Wales (NSW) part of the Basin, water has been over-allocated by 150%. The pressures on water are exemplified in the lower catchments of the Bogan-Macquarie and Castlereagh River where highly profitable cotton irrigators compete with graziers and the internationally important RAMSAR wetland, the Macquarie Marshes. In this area, which forms the Coonamble Embayment of the Great Artesian Basin (GAB), groundwater is also used for both irrigation and town water supplies. Within the Embayment, the Pilliga Sandstone aquifer is the important artesian freshwater aquifer. This study seeks to characterise the Pilliga Sandstone aquifer. Recent work has produced a general model of the GAB that in the Coonamble Embayment suggests recharge of the Pilliga Sandstone in the south and eastern portions of the Embayment with groundwater movement towards the north and north-west, to discharge in an area of extinct mound springs on the western edge of the Embayment. The overall aims of this work for the western Coonarnble Embayment are: I. to collect, collate and analyse existing groundwater databases, 2. to determine the structural features and broad regional stratigraphy, 3. to define the potentiometry and hydrodynamics, 4. to examine the hydrogeochemistry of the Pilliga Sandstone aquifer, 5. to understand regional groundwater and surface water interactions, 6. to suggest a management plan for the Pilliga Sandstone aquifer. In this work bore logs, pressure tests, temperatures, and bore water chemistry are analysed to determine the overall stratigraphy of the Embayment, the structure of the Pilliga Sandstone aquifer, the aquifer's hydraulic head gradient, the transmissivity, hydraulic conductivity, and the distribution of groundwater chemistry. Using this data a subsurface geologic model is developed in the Pilliga Sandstone aquifer in the western Coonamble Embayment in which: • The aquifer is within an alluvial fan deposit splaying outwards from the Warrumbungles into coalescing lobate fans in the west; • Recharge is from the easterly portion of this alluvial fan; • The flow regime appears isolated from the rest of the GAB but may have preferred pathways within the Coonamble Embayment; and • Discharge appears to take place into the overlying aquifers at the west of the Embayment. Radke et a/. (2000) adds that a stagnant groundwater area in the north-north-east is chemically different from the rest of the Pilliga Sandstone aquifer. Generating a groundwater sub-regime is significant contribution from the shallow groundwater in the south and surface waters (Radke et a/. 2000). Flow directions are primarily to the north and north-west with discharge in the vicinity of extinct mound springs in the north-west portion of the Macquarie catchment (Radke et a/. 2000). Potentiometric head and major ion spatial and temporal distributions data support the alluvial fan deposition model and its relative isolation from the rest of the GAB. Discharge may not result in surface phenomena and flow is towards the north-west with primary recharge from the Warrumbungles in the east. Anomalies in the basin's chemical evolutionary sequence suggest possible mixing in the Pilliga Sandstone aquifer due to extraction since the late 1890s. Analysis to obtain regional hydrochemistry signatures reveals distinct chemical features of the Macquarie Marshes' shallow groundwater, the Warrumbungles, and major rivers of the Castlereagh and Macquarie catchments for comparison to the Pilliga Sandstone. Leakage of the Pilliga Sandstone appears in the shallow groundwater of the Macquarie Marshes. The widely fluctuating yearly rainfall is the biggest modifier in the region's hydrologic cycle affecting the Pilliga Sandstone recharge. The water balance of the aquifer estimates show a recharge less than 9740 ML/y or a log mean of 6 mm/y. The leakage is estimated at 3500 ML/y, extraction between 5800-7900 ML/y and a change of storage of266 ML/y. The model is used to recommend local management strategies for the Pilliga Sandstone resource. The effectiveness of a proposed monitoring plan is increased by including data quality issues revealed from this work. Also, from the conceptualisation high leverage areas, that is areas with the greatest return for monitoring effort, are evaluated