Hydrochemistry and stable isotopes as tools for understanding the sustainability of minewater geothermal energy production from a 'standing column' heat pump system: Markham Colliery, Bolsover, Derbyshire, UK

The abandoned workings of the former Markham Colliery are still in the process of flooding. They are being exploited, via a 'standing column' heat pump arrangement in Markham No. 3 shaft, to produce thermal energy. From 2012, water was pumped from 235 m below ground level in the shaft at c. 15 °C and 2-3 l/s, through shell and tube heat exchangers coupled to a 20 kW heat pump, to supply space heating to commercial offices. The thermally spent (cool) water was returned to the same shaft at c. 250 m bgl at around 12 °C. The mine water contained iron, c. 6000 mg/l chloride, and was highly reducing. Avoiding contact with oxygen was effective in preventing problems with ochre scaling. In January 2015, taking advantage of rising water levels, the pump was repositioned at 170 m bgl, and the reinjection diffuser at 153 m bgl. Since then, both iron concentration and salinity have fallen significantly, suggesting stratification in the shaft. Stable isotope data from sampling in 2015 generally show little variation. Sulphate δ34S exhibits values typical for British Coal Measures (c. + 5‰), whilst δ18O/δ2H indicate influence of fresh meteoric waters. Chloride and sodium concentrations have gradually increased since May 2015, possibly indicating a renewed influence of deeper, more saline, waters and reflecting the gradual rise of mine water in the shaft. Further monitoring of mine water chemistry and isotopic composition is required to better assess the sustainability of the Markham heat pump scheme and advise on optimal management of this mine water resource.