Hydrogeochemistry of coal mine drainage and other ferruginous waters in north Derbyshire and south Yorkshire, UK

Fifteen coal mine drainage waters from the Millstone Grit and Coal Measures of Derbyshire and Yorkshire have been compared hydrochemically with other iron-rich waters from spoil tips, natural springs and lead mine soughs in the area. The coal mine waters typically contained several tens of mg/l iron, with a range from 0.1 to 101 mg/l, and sulphate from 60 to over 1000 mg/l. The waters are undersaturated with respect to siderite. There is a strong linear correlation between iron and sulphate indicating that pyrite weathering is the dominant source of these parameters, although iron appears to be preferentially removed by precipitation or adsorption within the mines. The pH values of coal mine water are typically c. 6, although one water has a pH of 3.6, contains some 17 mg/l aluminium and lacks the saturation relative to gibbsite and kaolinite present in the other waters. The coal mine waters may be net acidic or net alkaline: there is some evidence to suggest that the former are typically derived from unsaturated, under-drained (i.e. drained from underneath the workings) shallow workings and the latter from saturated, overflowing workings. The spoil tip waters are neutral to alkaline and saline. A possible explanation for this is the leaching of residual pore-water brines from the deep (several hundred metres) mined strata. The spoil tip leachates are poor in iron, which may be retained as iron hydroxides, sulphates or even siderite within the spoil.