Effects of longwall mining in the Selby Coalfield on the piezometry and aquifer properties of the overlying Sherwood Sandstone

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Abstract In the UK, the first longwall faces at Wistow Mine in the Selby Coalfield, with only 80 m depth of cover to the base of the Permian, experienced several inrushes of groundwater derived from the overlying Lower Magnesian Limestone, causing serious disruption to coal production. Subsequent decrease in panel width coupled with increased depth of cover to the Permian reduced the incidence of water problems. However, there has never been any quantitative investigation to determine the effects of mining on the hydraulic properties of the stratigraphically higher Permo-Triassic age Sherwood Sandstone, a major aquifer of regional importance. This opportunistic study fills that gap. Precautionary observation boreholes had been drilled above and around the margins of two proposed longwall panels prior to working the 2.5-m thick Barnsley seam at a depth of 550–600 m. Data loggers permitted continuous monitoring of the Sherwood Sandstone and Drift piezometric levels over a 2-year period. Widespread drawdown and recovery effects due to intermittent groundwater abstraction from a nearby factory were observed. Standard aquifer pumping test analyses of the hydrographs allowed transmissivity and storativity to be determined before, during and after mining. The results showed apparently permanent post-mining transmissivity increases of up to 149% around the margins of the panels, and up to 234% directly over the first panel. Post-mining storativity remained mostly unchanged. However, the greatest effects were noted during the closest approach by the second longwall panel, which also caused some additional subsidence over the first panel, when peak transmissivity increases of 1979% and storativity increases of 625% occurred. Anomalous, intra-cycle, recovery-drawdown events were also observed during this phase and interpreted as indicating rapid mining-induced dilation and compression of fractures within the aquifer fabric. The results are consistent with similar investigations carried out at relatively shallow depths (<220 m) in USA coalfields. However, the Selby study shows that mining at much greater depths still has a significant impact on shallow aquifers, with implications for enhanced aquifer recharge, abstraction well yield and possible increased contaminant transport rates.