Abandoned, flooded, coal mine workings are an artefact of fossil fuel exploitation that can be repurposed as a renewable energy resource. The warm subsurface waters that fill former workings can be developed to provide efficient and low-carbon heat generation using ground source heat pumps. In order to determine the long-term suitability of flooded mine workings as a sustainable thermal resource we have to understand the hydrological components of the system and how they interact in response to exploitation-related disturbance. We investigate pump induced mixing dynamics at the former Caphouse Colliery, which has been pumped since 1996 for regional water level management but only recently started to be explored as a thermal resource. Initial findings from the first 6 months of study show virtually no variation in physiochemical, major ion or stable isotope values. However, placed in context with archived values from 2004 to the present, we see a general pattern of mine water quality recovery punctuated by a doubling of Cl− values (150 mg/l to > 300 mg/l) which may suggest recent ingress of deeper-sourced saline waters. This is supported by O and H isotopic values, which are indicative of ancient, perhaps Late Pleistocene, confined waters. Sulphur isotope values (19.7–23.8‰) are abnormally high as compared to typical values for Carboniferous Coal Measures (0–10‰). There is no simple explanation, so further data collection and investigation are required, though we note that these values are similar to Lower Carboniferous seawater values. The relative stability of recent parameters suggests that Caphouse waters represent a dependable thermal resource. However, much about the hydrogeology of the Caphouse system is still uncertain, so further work is required to check the persistence of recent trends.
- ground source heat pump
- stable isotope