Hydrogeochemistry of Durham Coalfield minewater pumped to the River Wear, N.E. England

During recent mining of the Durham Coalfield, an extensive inland de-watering programme was needed to prevent down-dip flow towards the deep coastal collier- ies (Easington, Murton, Dawdon, Seaham, Vane Tempest, Wearmouth and Westoe). In the late 1980s, for example, 67 Mld − 1 was pumped at the coast and 109 Mld − 1 from shafts at abandoned inland collieries (Harrison et al. , 1989). Inland pumping has continued, despite final closure of all deep mines, to prevent acid groundwater rebound and the surface discharges which are expected to follow (Younger, 1993). At present up to 71 Mld − 1 of minewater is pumped from 8 sites between Bishop Auckland and Chester-le-Street directly into the River Wear or its tributaries. The mean average flow of the River Wear above Durham is 970 Mld − 1 and minewater can provide up to 50% of flow passing the Lumley drinking water abstraction point in dry spells (Younger, 1993). A further northern pumping station Kibblesworth discharges 26 Mld − 1 to a tributary of the Tyne in the Team Valley. Pumping costs the Coal Authority c £1 million per annum (Younger, 1994) but if discontinued it is anticipated that uncontrolled seeps of low pH minewaters rich in metals will cause considerable environmental damage and reduce water quality in the River Wear (Younger, 1994). Furthermore, the future acidification of minewaters might remobilise those metals present in ochreous deposits and stream sediments at the discharge sites (Lord et al ., 1999). This article describes sampling of the minewa- ter discharges during overnight pumping, in order to fully determine their current hydrogeochemistry and to investigate seasonal variations therein.