Integrated Hydrological and Geophysical Characterisation of Surface and Subsurface Water Contamination at Abandoned Metal Mines.

The mining and processing of metal ores in the UK has left a legacy of environmental degradation, and abandoned metal mines still pose a significant threat to terrestrial and fluvial environments. Flow gauging, water quality and geophysics were combined in an integrated assessment of surface and subsurface hydrological contamination at Esgair Mwyn, an abandoned mine in Ceredigion, Wales. Heavy metals discharged from the site are polluting downstream watercourses, leading to widespread Environmental Quality Standards (EQS) compliance failures. Through salt water dilution gauging and water quality sampling, a daily efflux of 876 g of heavy metals was calculated, with contaminant mobilisation occurring mainly in two primary surface streams draining an exposed tailings heap. Electrical resistivity tomography subsurface imaging found a seepage plane within the tailings lagoon wall, whilst the main tailings heap became increasingly saturated with depth. A large adjacent field also had a high potential to convey pollutants in solution, yet its morphological characteristics have limited transmission, as the area acts as a passive treatment type system. With remediation of already polluted water both difficult and expensive, this approach provides a cost-effective way to identify the origins and pathways of contaminants, informing mitigation strategies focussed on containment. Esgair Mwyn is not an isolated case, as abandoned metal mines release at least 860 t of heavy metals annually into UK water bodies. These techniques could reduce or prevent abandoned metal mine hydrological pollution for decades to come, and enable associated UK water bodies to comply with future water quality standards.