WATER QUALITY BENEFITS OF MINE WATER REMEDIATION QUANTIFIED BY SYNCHRONOUS WATER CHEMISTRY AND FLOW-RATE MEASUREMENTS ACROSS A RANGE OF HYDROLOGICAL CONDITIONS

Repeated monitoring campaigns across a watershed affected by an abandoned barite-lead mine, in the Lake District national park of the UK, have been undertaken under a variety of hydrological conditions (Q86 to Q4). The hydrology and water quality of the 10 km² watershed is deeply influenced by the mine, with zinc pollution the main concern. There are a number of point sources of pollution, but a single portal discharge is the most important (1.73 – 4.66 mg/L Zn; 8.5 – 24.4 L/s flow-rate). The flux of metal from this discharge is rather constant irrespective of flow conditions across the watershed (2.64 – 4.03 kg/d Zn). Repeated synchronous monitoring of flow and chemistry of the receiving watercourse at multiple locations down the watershed indicates that under base-flow conditions this single discharge dominates the zinc flux, with downstream attenuation of zinc evident under such conditions. At increasing flow-rates other sources of zinc become important, resulting in zinc fluxes in the receiving stream, below the mine site, as high as 14.13 kg/d. These increases in flux are ascribed to direct inflows of groundwater from the vicinity of the mine site and surface runoff from exposed waste rock. However, remobilisation of zinc from stream bed sediments may also be partly responsible, since stream pH decreases under higher flow conditions, due to increasing importance of peat runoff during rainfall events.

Besides the hydrogeochemical insights gained, the utility of these data is that they have enabled quantitative predictions to be made of the potential benefits that will accrue from treatment of the main portal discharge (an initiative is currently underway to install a passive treatment system at the site). Specifically, given the hydrology of the region and predictions about the performance of the treatment system, it is possible to use mass balance modelling to forecast that the treatment system will result in measurable benefits to the water quality of the lower reaches of the watershed 75% of the time during a typical year. During higher flows (~ Q25 and higher flows) benefits will likely be seen in the receiving stream immediately downstream of the main portal discharge, but there are diminishing returns with distance downstream, since diffuse sources dominate the pollution signal under such conditions.