Surface water–groundwater interaction in the fractured sandstone aquifer impacted by mining-induced subsidence: 1. Hydrology and hydrogeology

Mining-induced subsidence under surface waterways enhances surface water–groundwater interaction due to the enlargement of existing fractures, development of new fractures and the separation of bedding planes. Fracturing of streambeds and rockbars causes surface flow to divert to subsurface routes. The surface water–groundwater interaction in an undermined stream in the Southern Coalfield of New South Wales, Australia, has been assessed by analysing hydrological data including flow measurements upstream and downstream of the longwall panels. The data suggests leakage of surface water to the subsurface through fractured streambeds and rockbars. Mining-induced fracturing across the catchment is likely to have caused increased rainfall infiltration, reduced runoff, and reduced baseflow discharge, resulting in streamflow reduction and possibly loss, particularly during low flow conditions affecting the catchment’s water balance. During medium and high flow conditions, the streamflow loss is relatively small in comparison to the total volume of flow in the stream, as the capacity of the subsurface system limits the volume of water that can enter subsurface routes. Streamflow reduction in mining-impacted catchments is likely to be an effect of the spatial distribution and density of fracture networks, changes in porosity and permeability of the subsurface rock mass, changes in groundwater storage capacity, modification to baseflow discharge and alteration of the hydraulic gradient near streams.