Investigation of the hyporheic zone as a potential significant source of diffuse metal pollution to the River Teign, Devon, UK

Past metal mining has left a legacy of elemental contamination and is a major cause of surface and groundwater pollution in former mining districts in the UK and worldwide. It is a potential barrier to achieving good status water bodies, which is a requirement of the EU Water Framework Directive. There has been a concerted effort in the UK to address the scientific and practical challenges in remediating mine water pollution. The focus however, has been in remediating point source pollution, arising from abandoned adits and mine shafts for example. The behaviour of mine water at the surface water-groundwater interface ("hyporheic zone") has in the past received less attention, but is now recognised as an important diffuse component answering to the large proportion of unaccounted for, dissolved and particulate metal contamination to water bodies. The proposed project investigated the hyporheic zone as a potential significant source of diffuse input of Pb, Zn and Cd contamination to the River Teign, as a result of past mining activities at Wheal Exmouth mine. Further sources of metal pollution to the River Teign have been highlighted that include surface water, groundwater and sediments. The hyporheic zone multilevel sampler was used in this study for obtaining pore water within the riverbed of the River Teign. Elevated levels of Pb, Zn and Cu were discovered in hyporheic zone pore waters at two sites (average Pb concentrations between 10- 40cm depth 38.3 - 488 µg/I; Zn 6496- 13842 µg/I; Cu 3.92- 160 µg/I). Zn, Cd and Cu in groundwater were highest of all water courses measured (average Zn 48120 µg/I; Cd 206 µg/I; Cu 724 µg/I). Groundwater was found to significantly influence element concentration in the hyporheic zone. Surface water concentrations at Wheal Exmouth mine were (maximum Zn 3095 µg/I; Cd 29.7 µg/I; Pb 2114; Cu 114 µg/I). Sediment concentrations of the metals were found to be directly related to pH and inversely related to the concentrations of metals in the water. Metal concentrations in the hyporheic zone and groundwater alone represent complex processes and relationships, pose a threat to ecosystem functioning and represent a significant barrier to the target of good ecological status. The investigation of Wheal Exmouth mine seeks to inform subsequent remediation planning as part of the 'identification and prioritisation of abandoned non coal mines' project