The traditional notion of defining the sustainable yield of an aquifer based on the average annual recharge rate or some derivative of the recharge component of the water balance is now obsolete. Groundwater technocrats are now aware that ecologically sustainable development of groundwater resources requires the identification of different users including the environment, the determination of the water requirements of each user, and securing the additional groundwater recharge or discharge which is essential to balance the legitimate use by one or more user. Alternatively, the provision of water at rates less than these requirements will mean a loss of environmental value or restricted human use. The Gnangara groundwater Mound located within the Perth Basin is a typical example of an area where a balance is actively being sought to meet the growing demand for groundwater by various users while securing water for the environment.
The Gnangara Mound is a major source of water for market gardens, private and metropolitan water supply, as well as sustaining major pine plantations, native Banksia woodland and numerous wetlands and lakes. Due to increasing consumption by multiple users, water levels are declining at a constant rate. This is an indication that current groundwater discharge is being removed from aquifer storage and from reductions in the aquifer outflow component. It is anticipated that water levels will most likely continue to decline unless preventative measures are taken to increase the recharge or decrease groundwater ouflow to the ocean. The following measures are strongly advocated to abate and reverse current water level trends: (1) the removal of pine plantations to increase recharge; (2) the relocation of high volume abstraction to the discharge margins of the Gnangara Mound; and (3) the injection of wastewater in a line of wells parallel to the ocean west of the supply wells to reduce the outflow component and prevent seawater intrusion.
Applying all these measures will only reduce the environmental impacts and not eliminate them, because the moment an aquifer is developed as a water source, even taking into consideration all the environmental water requirements, several ecologically sensitive areas will be affected. The delicate balance between the groundwater supply and demand by different users cannot be achieved without securing additional recharge for the utilised overdrawn parts of the aquifer or reducing groundwater discharge (aquifer outflow). Nevertheless, it is very difficult to achieve this balance, especially with changing weather patterns, increasing water demands and the relatively low infrastructure cost of groundwater abstraction that encourages further development to satisfy demand during drought periods.
Preliminary analysis of seasonal water level trends indicated that geomorphology and geology control the water level patterns. Each aquifer has a unique water level pattern. Landuse is partly responsible for water level patterns and recharge; however, the hydraulic properties control aquifer response to a greater degree.
Analysis of short and long-term water level data from the southern part of the Gnangara Mound reveals that the water levels prior to 1975 were in a semi steady state. After 1975, a combination of excessive water use by pine plantations, heavy pumping from private boreholes in market gardens, pumping from the shallow wells in private homes and intensive pumping in the Gnangara Mound for the Metropolitan water supply, caused water levels to decline at a constant rate, over and above the decline expected with reduced rainfall since that time. This is an indication that groundwater loss and discharge is taking place from aquifer storage. The decline of water levels is likely to continue unless measures are taken to increase the recharge (removal of pine plantations), reduce the outflow component and prevent seawater intrusion problems through the injection of wastewater in a line of wells parallel to the ocean, and relocate high volume abstraction to the discharge margins of the Gnangara Mound.
The geochemistry and stable isotope signatures of groundwater from three different aquifer systems revealed distinct water types that suggest very little hydraulic connection or mixing of waters between aquifers. Recharge to the shallow aquifer is decreasing due to the decrease in rainfall and the maturing of the pine plantations and recharge to the confined aquifers is negligible and will also decrease; therefore, declining water levels will continue unless strict measures are taken to increase recharge or reduce discharge. At the same time, it is important that the environmental criteria, which control sustainable yield of the aquifers, be sensibly based.