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

The Nent Valley is underlain by cyclic successions of Carboniferous rocks unconformably resting upon Lower Palaeozoic basement rocks intruded by Lower Devonian granites. Mineralization of this sequence created a large, once productive orefield. Two centuries of intense lead and zinc ore extraction have created over 90 adits in the Nent Valley, five of which presently discharge metal-rich water to the River Nent with serious consequences for the flora and fauna. High aquatic zinc concentrations are toxic to fish and cause a depletion in invertebrate food supply, consequently decreasing fish populations. Mine waters are rich in several ecotoxic metals (lead, zinc and cadmium). By computer modelling the speciation of these elements and the phases in which they occur it is hoped that a mechanism for the removal of metals as mineral phases can be devised. The effect of passing mine waters through an aerobic system such as a constructed wetland was modelled and no change in species distribution or mineral phase saturation was observed. Modelling 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 Nent Force Level, a feature that was created during the mining era.