Groundwater extraction is proposed to fulfil the partial water supply requirements for the proposed gold-platinum-palladium mine at Coronation Hill, Northern Territory.
The aquifers in the Conservation Zone surrounding the mine are fractured Proterozoic dolomite and volcanic rocks. These aquifers are recharged partly by direct infiltration of rainfall, and partly by inflow from upstream reaches of the South Alligator River. Theaquifers discharge to downstream reaches of the river. The environmental impact review of the mine proposal has raised two hydrogeological issues: whether seepage of cyanide-bearing process water will pollute the aquifer and river; and whether aquifer drawdown will affect the flow and ecology of the river.
Available information indicates that some cyanide-bearing process water will seep into the aquifer, but that the pollution plume will be attenuated to some extent by chemical reaction and degradation. Available river stage data have been used to estimate the effects of drawdown due to proposed extraction of groundwater for mine water supply, and also mine pit groundwater inflow. The long term groundwater extraction and pit inflow is probably about 25 L/s. River flow at Coronation Hill has fallen below 250 L/s on 84 days in 8 out of a documented 51 months, and the long term groundwater extraction and pit inflow is equivalent to about 10 % of river flow in these low flow months. The wellfield should be designed for minimal drawdown, and management criteria should be developed to avoid ecological damage in low flow conditions.
Remaining doubts about the river/aquifer interaction need to be resolved by further investigations.
