Hydrogeochemistry of minewaters flowing from abandoned coal workings in County Durham

Although all deep mining has now ceased in the Durham coalfield of northern England, dewatering still continues (in June 1995) in the central area of the Coal Measures outcrop. Beyond the radius of influence of the dewatering pumps, the water table has already recovered, and several uncontrolled minewater discharges are found. These waters are moderately mineralized (total dissolved solids in the range 600 to 2500 mg/l) and may have appreciable alkalinity. Iron loadings are typically high and give rise to thick ochra deposits on streambeds. White deposits are associated with high aluminium concentrations. Sulphate occurs in the range 130-1300 mg/l, but in the receiving waters downstream from discharge points reductions in sulphate concentration are observed. Preliminary hydrochemical modelling results suggest that these observed reductions are explicable by precipitation of iron sulphates (e.g. jarosite). Existing hydrochemical classification schemes inadequately characterize these waters; therefore a new scheme is proposed which should be applicable in many areas with similar geology. Immediate practical use of the results presented here lies in predicting possible future river pollution if dewatering ends, and planning for abatement of such pollution using constructed wetland technology.