South East groundwater
Groundwater is the South East region's main source of water. It supports internationally significant ecosystems and recharges Blue Lake in Mount Gambier.
The groundwater of the South East is in two aquifers: an upper unconfined aquifer and a lower confined aquifer. Over most of the South East these two aquifers yield high quality water that is used extensively over the region.
About the aquifers
The unconfined aquifer underlies much of the South East and is the main water supply to most towns in the region. Groundwater may come to the surface in some areas on the plains, and be as deep as 60 m in the Tintinara area. More detailed information on the depth of the watertable can be obtained from the WaterConnect.
The water in the confined aquifer comes from Victoria and is estimated to be more than 25,000 years old by the time it reaches parts of South Australia. It is separated from the unconfined aquifer by an impermeable layer.
The confined aquifer is not present under the whole of the South East and is thin or absent in some parts such as Padthaway. The depth of the confined aquifer varies from 35 m to 300 m. The confined aquifer is under pressure in many places, which can cause water temperatures to be as high as 30°C.
What are the concerns about the aquifers?
Salinity of the unconfined aquifer is one of the major factors that limits water use in the South East. The salinity increases significantly towards the north and west of the region. It varies from 400 mg/L, which is suitable for drinking, in the lower South East to 35,000 mg/L, or about the same as seawater, in the Tintinara area.
Groundwater contamination of the unconfined aquifer is the result of either point source contamination (eg spills from copper-chrome-arsenate wood preservation industries) or diffuse source contamination. Point source contamination is due to the historical disposal of wastes into the aquifer by industries such as dairying, timber mills, gasworks, cheese factories, abattoirs and septic disposal. The Environment Protection Act 1993 now prohibits disposal of waste into the aquifer.
Inappropriate pesticide use has resulted in groundwater contamination, to the extent that organochlorine and triazine pesticides have been detected in groundwater in parts of the region.
Diffuse source contamination of the unconfined aquifer is due to agricultural activity and forestry. Diffuse nitrate pollution has had the largest effect on the aquifer with increasing nitrate levels being observed in groundwater south of Mount Gambier and in the Coonawarra area. Pesticides have also been detected in limited testing of groundwater in the lower South East. The problem of diffuse source contamination is being addressed by better land management practices.
The water quality of the confined aquifer is very good because the aquifer is protected from contamination not only by its depth, but also by the presence of the confining layer that separates the two aquifers.
What is being done to improve the condition of groundwater in the South East?
The EPA has a regional office in the South East. One of its roles is to manage and regulate potential pollution of the aquifers. Key EPA strategies for managing pollution include:
- groundwater monitoring programs as core licence conditions for licensees who could pollute groundwater
- a risk assessment of groundwater contamination associated with historical and current copper-chrome-arsenate timber treatment plants
- a risk assessment of pollution threats to Blue Lake is currently being made as a joint government and CSIRO research project. This project is intended to link aquifer hydraulics with surface water pollution threats to provide planning and management tools. The risk assessment will provide the basis for future groundwater monitoring in the Mount Gambier region
- presence on regional management boards, engaging the community and industry and providing guidance
- development of a code of practice for regional stormwater disposal
- development of a concept model of Blue Lake identifying information gaps
- monitoring programs targeting key aquifer units and Blue Lake
- commissioning work to investigate long-term trends in nitrates.
There are close links between groundwater quantity, quality and supply in the South East. To this end, the Department of Environment, Water and Natural Resources is investigating the flow patterns around and into Blue Lake. This work will feed into a risk assessment of the lake.
Groundwater monitoring in the South East
The EPA monitors groundwater across the South East for a wide range of analytes. Many industrial sites, ranging from timber mills to landfills, are required to monitor the groundwater in the vicinity of their site to ensure that they are not affecting the groundwater.
The EPA groundwater monitoring program in the South East is grouped into programs centred on the main towns and areas of agricultural activity including Keith, Millicent, Coonawarra and Padthaway.
The area in between these groups (referred to as the Greater Mount Gambier Program) is also monitored to ensure groundwater quality throughout the South East region is covered.
The aquifer system that feeds Blue Lake in Mount Gambier (the Blue Lake Capture Zone) is also separately monitored because of its importance as a water supply.