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2 Biuletyn Państwowego Instytutu Geologicznego

2 2010

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Surface water–groundwater interaction in the fractured sandstone aquifer impacted by mining-induced subsidence: 2. Hydrogeochemistry

Jankowski J.

Biuletyn Państwowego Instytutu Geologicznego

2010

nr 441 Hydrogeologia z. 10

43--54

Water quality along the Waratah Rivulet in the Woronora Lake Catchment, New South Wales (NSW), Australia, has been monitored during the last three years by the Sydney Catchment Authority. Water quality data shows changes in chemical composition due to cracking of streambeds and rockbars, and diversion of surface water into subsurface routes in the Hawkesbury Sandstone aquifer. Water quality upstream of the longwall panels is comparable to nearly pristine water in creeks and rivers flowing in similar sandstone bedrock environments and to limited water quality data collected prior to mining. A segment of the Waratah Rivulet, where subsidence and cracking of streambeds and rockbars has occurred, is causing surface water to be redirected into subsurface fracture systems, mix with groundwater already present in the aquifer and partially reappear downstream. This subsurface flow in the shallow fractured sandstone aquifer causes the chemical composition and water quality to change as an effect of water–rock interactions. Salinity, iron, manganese and many cation and anion concentrations increase, whereas oxygen is significantly depleted. Mobilisation of barium and strontium from the rock mass indicates fast chemical dissolution reactions between the subsurface flow and carbonate minerals. Other metals mobilised include zinc, cobalt and nickel. Subsurface water partially discharges from underground receptors downstream of the area impacted by longwall mining. The discharged water is rapidly oxidised by atmospheric oxygen, causing precipitation of iron and manganese oxides / hydroxides out of solution. Hydrogeochemical modelling indicates the dominant iron minerals precipitated out from the water are in this environment goethite, lepidocrocite and ferrihydrite. The paper discusses changes in surface water and groundwater chemistry due to subsurface flow and water–rock interaction, the hydrogeochemical processes responsible for changes in water chemistry, as well as changes in water quality along the rivulet.

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

Jankowski J., Knights P.

Biuletyn Państwowego Instytutu Geologicznego

2010

nr 441 Hydrogeologia z. 10

33--42

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.