Contaminant hydrogeochemistry and aquatic ecosystem health at abandoned metal mines: the Afon Twymyn, central Wales

2010-05-10T16:03:09Z (GMT) by Patrick Byrne
Following the decline of the UK metal mining industry by the 1920s, over 3,000 abandoned metal mines exist in England and Wales. Contaminated drainage from this historical industry causes approximately 20% of all water quality failures in England and Wales. In this thesis, a holistic geographical approach, incorporating aspects of hydrology, hydrogeochemistry and freshwater ecology, is employed to investigate the hydrological, sedimentological and ecological impacts of the abandoned Dylife lead/zinc mine on the Afon Twymyn (central Wales). Examination of river sediment quality highlights the need for measurements of the quality of this component of the river ecosystem and, in particular, measurement of bioavailable as opposed to total metals. The majority of heavy metals in bed sediments of the Afon Twymyn exist in highly mobile geochemical phases, potentially posing serious threats to ecological integrity. Significant metal flushing occurred during flood events at Dylife mine and a distinct seasonal pattern was observed with greater levels of flushing occurring during flood events in the summer months. It is suggested that investigations of contaminant/ecosystem relations and potential remediation strategies should include high-resolution temporal sampling of river water chemistry under conditions of flood flow. Paradoxically, a range of biological indices failed to identify significant negative impacts of metal mine contaminants on macroinvertebrate communities, suggesting there is little contamination of the river ecosystem. However, Canonical Correspondence Analysis did identify significant differences in community structure between polluted and unpolluted river stretches, suggesting that standard unimetric biological indices might only be successful in identifying impacts at the most severely polluted mine sites. It is suggested that the ecological approach of the European Union Water Framework Directive to the assessment of river ecosystem status may not yield an accurate representation of contamination in rivers such as the Afon Twymyn where contamination by mining is moderate, circum-neutral and the mining operation is long abandoned.