Two open-loop, minewater-based, ground source heating schemes have been operating since 1999–2000 in Scotland (UK), at Shettleston (Glasgow) and near Cowdenbeath (Fife). In both cases, ferruginous water is pumped from flooded coal mine workings via a borehole, circulated through the evaporator of the heat pump and reinjected via a shallower borehole to Carboniferous strata. The heat is delivered to a water-based thermal store providing central heating for apartment complexes and contributing to domestic hot water. It is demonstrated, via hydrochemical analysis and speciation modelling of the Fife minewater, that the success of the schemes is due to lack of contact between minewater and the atmosphere (thus limiting degassing of CO2 and absorption of O2). Indeed, recent difficulties with one of the schemes are ascribed to vandalism of the recharge main allowing access to oxygen, causing precipitation of ferric oxyhydroxide and clogging of the recharge borehole.