Abstract
Historic sulphidic Pb–Zn mining catchments at Leadhills and Wanlockhead, in the south of Scotland, UK have a legacy of mining of PbS (galena) and ZnS (sphalerite) from the twelfth century to the 1930s. The mining activities created tailing piles, ponds, adits and contaminated soils that contribute leaching and surface runoff of potentially toxic elements, particularly lead (Pb), which impact on the surface water and groundwater and are rapidly diluted in the wider catchment area. Studies by environmental regulators have shown that Pb, Cd and Zn in water can locally exceed the Environmental Quality Standards (EQS), particularly at Leadhills. To evaluate geochemical controls on release, 20 water sources (adits, surface water and near-surface groundwater) were sampled over four seasons (spring, summer, autumn and winter) over a 1-year period and characterized. Samples were circum-neutral pH from 6.3 to 7.9 (with average total dissolved solids < 55.0 mg/L), with no characteristics of acid mine drainage. The concentrations of PTEs in the water exceed UK EQS and WHO standards (and non-compliance on the Water Framework Directives). Geochemical modelling (GWB and PHREEQCv2) predicted mineral control on solubility which identified PbSO4 (anglesite), Fe2O3 ferric oxide (haematite), Fe3O4 (magnetite), FeCO3 (siderite), CaMg(CO3)2 (dolomite), CaCO3 (calcite) and Ca(Fe·Mg)(CO3)2 (ankerite) to be important. These were confirmed in solid phases analysed from tailings and sediments in contact with the hydrological cycle at the sites. Multivariate statistical analysis (PCA) of water samples associated with leaching through mine tailings showed strong seasonal variation with some elements (Fe, Cu, Cd, Pb, Na, Ca and Zn) with higher variance. The strong negative association of pH with soluble Zn, Cu, As, Cd and Pb highlights typical sulfide oxidation processes are taking place and supported by a positive correlation with temperature. Dissolution processes of mineral phases indicated by positive association of TDS and EC with Na, Ca and Zn. The data from PCA suggest contributions with potential for active generation of acid mine drainage and dissolution of solid phases influencing the release of PTEs into surface waters.
Highlights
The Leadhills and Wanlockhead mining district, which is in the Southern Uplands region in the southern part of Scotland, was an important mining centre for lead for many centuries and is one of the most elevated human settlements in the United Kingdom
The stability of Eh–pH and speciation among trace elements, cations and anions was calculated by Geochemist’s Workbench (GWB), and PHREEQC computer software which predicts the possible reactivity of the subsurface mineralogy controlling temporally release of potentially toxic elements (PTEs) was determined
At sampling site SW1 and SW/G 2, the pH of the shallow groundwater flowing through the bottom of mine waste spoil heaps into the streams and river ranges between 6.42 and 7.94, with an average of 7.06 and 7.47, respectively
Summary
The Leadhills and Wanlockhead mining district, which is in the Southern Uplands region in the southern part of Scotland, was an important mining centre for lead for many centuries and is one of the most elevated human settlements in the United Kingdom. The primary ore-minerals that were exploited in the area include galena (lead sulfide) and sphalerite (zinc sulfide), together with alluvial gold in local. Mines that contain sulfide minerals produce acid mine drainage (AMD) due to the oxidation of sulfide minerals (e.g., pyrite) loaded with associated potentially toxic elements (PTEs), such as lead, zinc, arsenic, copper, cadmium and antimony, dependent on the precise mineralization (Banks et al 1997; Lottermoser 2010a, b; Nordstrom 2011). The case at the abandoned Leadhills and Wanlockhead mines is more complex where the pH of drainage water is buffered at circum-neutral pH values due to the presence of carbonate minerals in the host rock (SEPA 2011b). The bioavailability of PTEs, such as arsenic and antimony, in soils from an abandoned local antimony mining area has previously been described (Gal et al 2007)
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