How do you quantify and predict surface water losses from catchments subsided by underground mining? This is a key question for Sydney drinking water supplies, managed by WaterNSW, and many other catchments around the world. The emerging answers are complex and multi-disciplinary.
Current practice in NSW is to predict surface water impacts using groundwater models, but these are limited to estimates of changes in baseflow to, or changes in leakage from, streams and storages. Recently, the Independent Expert Panel on Mining in the Catchment (IEPMC) used a new method analogous to baseflow separation analysis to estimate the volumes of mine inflows which can be correlated to more rapid travel of surface water to the mine void. This method is robust but doesn’t account for the total surface water diversions.
Two alternative methods are currently being evaluated by WaterNSW. The “volume conservation” method combines volumes of mine voids, strata subsidence and volumes of groundwater pumped during mine operation to deduce the total volume of water that will ultimately be diverted from the surface. Estimating the distribution and rate of diversion can only be inferred indirectly however, e.g. from streamflow response.
The other alternative method recently investigated by the authors is to analyse the changes in pre- and post-mining measured streamflows and to develop a relationship between these losses to the area of catchments undermined. This relationship can then be used to estimate the total diversion as a function of time. The two methods are complementary but independent, and the consistency found between results suggests the methods are useful for estimating the total diversion as a function of time. A preliminary study (Tammetta, 2018) presents an overview of the reliability and potential limitations of these methods for several mined catchments in NSW, and suggests how future work might continue to refine and improve water loss estimates.