The hydrochemistry of coal mine drainage and other ochre waters in North Derbyshire and South Yorkshire, UK
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Fifteen coal mine drainage waters from the Millstone Grit and Coal Measures of Derbyshire and Yorkshire have been hydrochemically compared 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 mgl-1 iron, with a range from 0.1 to 101 mgl-1, and sulphate from 60 to over 1000 mgl-1. 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 prefer- entially removed by precipitation or adsorption within the mines. Coal mine water pH values are typically c.6, although one water has a pH of 3.6, contains some 17 mgl-1 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, underdrained (i.e. drained from underneath the workings) shallow workings and the latter from saturated, overflowing workings. The spoil tip waters are nautral to alkaline and saline. A possible explanation for this is the leaching of residual pore-water brines from the deep (several hundred m) mined strata. The spoil tip leachates are poor in iron, which may be retained as iron hydroxides, sulphates or even siderite within the spoil.