This report describes the hydrochemistry of the Frongoch mine site, mid Wales. The study characterises the spatial and temporal variations of groundwater and surface waters, and assesses dominant controlling processes.\ud The Frongoch mine site [SN 72 74] is situated in mid Wales. It was once mined for Pb, and to a lesser extent, Zn. The mine was abandoned in the early 20th century, leaving potentially reactive spoil and tailings which have a detrimental effect on the local groundwater and surface water.\ud Previous work has described the location and stratigraphy of the tailings area at Frongoch. The tailings layer is approximately 1 metre thick deposit discontinuously underlain by peat. The peat layer, of variable thickness, is underlain by glacial till and occupied the valley prior to the tailings deposition.\ud Groundwater was samples from three trial pits excavated to depths ranging from 1.65 to 3.35 metres below ground level and from three boreholes installed in the waste heaps to a depth ranging from 2 to 4 metres below ground level. The groundwater pH varied between 4.8 and 6.5. All waters were clearly dominated by SO4 and characterised by very low alkalinity. This dominance is caused by the oxidative weathering, and subsequent dissolution of sulphide minerals. The scarcity of pyrite accounts for the pH values closer to neutrality. Zinc is the main contaminant of concern, with concentrations reaching 1735 mg/L in the groundwater. Other potentially harmful elements including Fe, Al, Ni, Pb, Co, Cu, Cr, and Cd, were highly concentrated.\ud The highest concentration of contaminants was generally found in the groundwater intercepting the tailings and underlying peat. Water collected from the clayey till underlying the tailings and peat units, although relatively less enriched in Zn, SO4, Pb and Cd, has similar chemistry. This suggests that the contaminant plume extends down the underlying tailings through the peat layers to the glacial till.\ud Groundwater redox measurements reflect the redox potential dominated by the Fe (II/III) redox couple in the system. These measurements ranged from 210 to 470 mV.\ud Anglesite (PbSO4) is at equilibrium or close to equilibrium at all the sites, except the adit and the mill pond, and it is likely to be the main mineral phase controlling the Pb concentration in solution. Modelled saturation indices for Zn minerals suggest that no obvious mineral exerts solubility control for Zn concentrations in solution.\ud The impact of the discharge of the tailings groundwater and surface runoff to the receiving Frongoch stream is indicated by very high concentrations of Zn (up to 97 mg/L), Fe, Al, Ni, Pb, Co, Cu, Cr, and Cd, which are present in waters discharging from the site in concentrations that could cause failure of water quality standards.\ud Water chemistry seasonal variations were indistinct in many element distributions (e.g. Cu, Cr, Pb, Ba, Sr), although Co, Ni, Zn and SO4 had a strong seasonal relationship, with concentrations being higher in summer months
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