Investigation of contaminant metal dispersal from a disused mine site at Tyndrum, Scotland, using concentration gradients and stable Pb isotope ratios

Abstract

Results are presented for a study of Pb and Zn concentrations and stable Pb isotope ratios for mining wastes, river sediments and 210Pb-dated peat cores from the vicinity of a derelict Pb/Zn mine at Tyndrum in central Scotland. Mining was carried out at Tyndrum between 1741 and 1862 and the original waste dumps were reworked between 1916 and 1925. Little remains of the original buildings and workings and the contemporary distribution and dispersion pathways of the residual waste are poorly defined. The mine and ore processing waste dumps were readily identified as highly polluted, barren areas, with concentrations of up to 21 and 3.4% Pb and Zn, respectively. The Pb in the waste was characterised by a 206Pb/ 207Pb atom ratio of 1.146±0.004 and a 208Pb/ 207Pb ratio of 2.432±0.007. Significant quantities of waste were also found along riverbanks and in river channel sediments in the vicinity of the mine, with concentrations of up to 5.22% Pb and 2.35% Zn. Concentrations of Pb and Zn in river channel sediments decreased markedly with increasing distance downriver from the mining area. However, the decrease in Pb concentrations in the <53 μm fraction was less pronounced, with a relatively high concentration of 0.076% being observed at a distance of 6.5 km from the mine, suggesting significant fluvial transport of this size fraction of waste. The stable Pb isotopic characteristics of the river sediments were consistent with the mine waste being the dominant source, along with minor inputs from local bedrock and vehicle exhaust emissions. The peat core data revealed high levels of Pb deposition throughout the period of the mining operation and very high levels of input in the early 20th century, almost certainly as a result of the reworking of the former waste dumps. As with the river sediments, the isotopic characteristics of the Pb in the peat cores were consistent with a dominant input from the mine waste and minor contributions from bedrock and vehicle exhaust emissions. The results suggest that waste from the mining operation has been a significant source of contaminant heavy metals for several hundred years and continues to be so. The apparent fluvial transport of Pb from the mine site is consistent with previous work suggesting that Tyndrum mine waste is the probable origin of anomalously high levels of Pb deposited in the sediments of Loch Tay, some 25 km to the east. The study highlights the utility of stable Pb isotope analyses in the investigation of sources and environmental dispersion of contaminant Pb.