Reconnaissance hydrogeochemical evaluation of an abandoned Pb-Zn orefield,Nent Valley, Cumbria, UK

The Nent Valley is underlain by cyclic successions of Carboniferous rocks u nconformably resting upon Lower Palaeozoic basement rocks intruded by Lower Devonian granites. Mineralization of this sequence created a large, once p roductive orefield. Two centuries of intense lead and zinc ore extraction h ave created over 90 adits in the Nent Valley, five of which presently disch arge metal-rich water to the River Nent with serious consequences for the f lora and fauna. High aquatic zinc concentrations are toxic to fish and caus e a depletion in invertebrate food supply, consequently decreasing fish pop ulations. Mine waters are rich in several ecotoxic metals (lead, zinc and c admium). By computer modelling the speciation of these elements and the pha ses in which they occur it is hoped that a mechanism for the removal of met als as mineral phases can be devised. The effect of passing mine waters thr ough an aerobic system such as a constructed wetland was modelled and no ch ange in species distribution or mineral phase saturation was observed. Mode lling showed that a small pH rise would remove zinc from the mine waters as the carbonate (smithsonite). If active treatment were considered, the mine waters could be diverted and treated together by making full use of the Ne nt Force Level, a feature that was created during the mining era.