Modelling the long-term water balance of a pit lake with recreational end uses

Abstract

Pit lakes can be used for recreation when water quality is suitable. However, when lake levels are too low, pit lakes may pose access and slope instability risks to recreational users. An essential tool to assist in managing these risks is a good understanding of the lake water balance. A groundwater model was developed to investigate whether impending cessation of mine water discharge into historic Lake Stockton (Collie, Western Australia) would result in reduced recreational values, e.g. for water skiing. Currently, the lake water level is kept reasonably constantly elevated with the addition of mine water from nearby dewatering at Ewington mine (Lanco, Griffin Coal) operations. Lake Stockton has a maximum depth of 30 m and groundwater flows through historic underground mine workings contribute largely to the overall pit inflow. A steady state groundwater model was used to emulate the regional groundwater flow in the Collie Coal Basin. The model gave estimates of groundwater fluxes and predictions of the future lake water level if mine discharges were to stop. Transient simulations forecast a 3.6-3.9 m decline in the long-term lake water level without mine water input, with non-mine groundwater inflow rates increasing from commensurately steeper hydraulic gradient from a declining lake level in steady state. As a result, although the water level of Lake Stockton would still be suitable for public recreation, the decline in the water level would expose steep pit walls and reduce the regulation recreational area by 17%. Consequently, rehabilitation planning for this lake should account for safe access if recreation is to be permitted when the predicted decline in water levels takes place. The study highlights the critical importance of understanding the hydrogeological setting and the controls over lake levels in-pit voids for maintenance of recreational end use values.