Underground water is McLaren Vale largest water resource. It is also the most heavily used.
The McLaren Vale Prescribed Wells Area (PWA, or wells area), formerly known as the Willunga Basin, is the term used to describe the water that is held underground. It is estimated that the safe yield of underground water for the McLaren Vale PWA is 6,560 megalitres (ML) per year.
Since water usage records started in 1992/93, the annual metered groundwater use in the McLaren Vale PWA has ranged from 3,713ML (1992/93) to 8,924ML (1994/95).
Since 1997/98 there has been a steady decrease in the amount of groundwater used in the McLaren Vale wells area. In recent years, groundwater usage has averaged around 4,500ML per year.
All of the water resources of the McLaren Vale wine region are part of a larger water system and they interact with one another. The table below shows the major flow transfers in the Onkaparinga Catchment, based on current water use. It highlights how these major water resources interact with each other and how an increase or decrease in one can affect another.
Underground water within the McLaren Vale can be drawn in varying quality and quantity from four aquifers – Quaternary sediments, Port Willunga Formation, Maslin Sands and Fractured basement rock. In many locations all four of these aquifers can be accessed at different depths, with the aquifers being separated vertically by different layers of rock. The schematic diagram of McLaren Vale Prescribed Well Area groundwater system (below) shows this.
Quaternary- This aquifer occurs mainly south of Pedler Creek, and as smaller perched aquifers in gullies and along the edges of the basin where bedrock outcrops. Quaternary aquifer is generally not used for vineyard irrigation. It is shallow and of poor quality.
The Port Willunga Formation- is the most utilised underground water resource in McLaren Vale, which is estimated to account for around 64% of all metered water usage in the McLaren Vale wells area. It can be accessed throughout the floor of the Willunga Basin. Salinity ranges from 350 mg/l at the northeastern extent of the formation and adjacent to the Willunga Fault to approximately 1000 mg/l around South Road and then increases to greater than 2500 mg/l at the coast.
The Maslin Sands aquifer is estimated to account for around 20% of all metered underground water usage in the McLaren Vale wells area. Like the Port Willunga formation aquifer, recharge is via direct rainfall infiltration, recharge from streams and inflow from surrounding fractured rocks.
Underground water salinity in the Maslin Sands aquifer varies from less than 500 mg/l to more than 50000 mg/l at the coast.
Groundwater from the Fractured basement rock aquifer is believed to account for around 16% of all metered underground water usage in McLaren Vale. This water source is accessed by the deepest bores in the district, generally sunk over 100 meters into the ground.