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

Article history: Received 18 March 2016 Received in revised form 19 August 2016 Accepted 22 August 2016 Available online 24 August 2016 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. 6000mg/l chloride, andwas highly reducing. Avoiding contactwith oxygenwas 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 δS exhibits values typical for British Coal Measures (c. +5‰), whilst δO/δH 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 minewater in the shaft. Furthermonitoring ofminewater chemistry and isotopic composition is required to better assess the sustainability of theMarkhamheat pump scheme and advise on optimal management of this mine water resource. © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).