Kevin began his talk by describing the geological history of the Tertiary Otway Basin that covers most of the South East of South Australia. It began when Australia began to drift away from Antarctica 150 million years ago and sediments were deposited in the rift valley. 40 million years ago sea inundated the area and limestone was deposited. Over the past 10 million years the sea has regressed leaving a series of stranded beach lines that run from northwest to southeast. These are locally known as “ranges” but are actually low elevation limestone ridges and sand dunes.
There is little surface water in the northern part of the area. A few creeks drain eastwards from Victoria into lake and lagoons. To the south there were large areas of surface water when the first settlers arrived in the mid 1800’s. These drained northwards along the flats between the beach strand lines. Drainage schemes began in the 1863 when channels were cut through the ranges to provide dry land for agriculture. By 1965, 667 kilometres of drains had been cut. Now it is believed that the drains are taking away too much water and works are being done to regulate the flow along drains.
The main water source for domestic, agricultural and industrial use is ground water. There are two main aquifers: an unconfined aquifer from the surface to an impermeable clay layer and a deeper confined aquifer between the clay layer and impermeable base rock.
The unconfined aquifer flows through limestone usually from 20 centimetres to over 100 metres below the surface but reaches depths of 300 metres around Mt Gambier. Blue Lake at Mt Gambier is fed from this aquifer. The aquifer is recharged from rainfall at a rate of 10-250 millimetres per year but is heavily used. Pollution risk is high and high iron content can be a problem in urban water supplies. Early irrigation was by the inefficient flooding. Efficiency has been improved by laser levelling, travelling and centre pivot irrigators and drip or micro sprinklers for vines.
The confined aquifer flows through sand and is under pressure. Bores may be artesian and were once allowed to flow freely to form wetlands. The aquifer is slowly recharges from distant areas such as the Dundas Plateau i n Victoria. Water use from this aquifer is restricted to town supplies and stock and domestic use.
Water levels have varied in past times. Henty Brothers farmed the floor of Valley Lake at Mt Gambier when the area was first settled. Water in Blue Lake is now 12 metres above sea level; the same as it was in the 1800’s, but it has fallen from higher levels during the recent dry years.
Prior to 1972 there was little control over the amount of ground water used. However concern over the pollution hazard of dumping rubbish in sink holes and discharge of industrial waste into aquifers resulted in legislation to ensure sustainable use. Licences now limit the volume of ground water used, recharge rates are being measured and irrigation methods changing to ensure more efficient water use. More research is necessary to determine the level of water allocation required for the environment.