An assessment of the mechanisms for the transfer of lead and mercury from atmospherically contaminated organic soils to lake sediments with particular reference to Scotland, UK

Abstract

While the sediment records of upland lakes are known to be valuable natural archives of depositional changes, recent studies in the UK have shown that full basin records of trace metals are not showing the decline expected as a result of the massive reduction in emissions since the 1970s. As trends in metal deposition across Europe are known to be declining, this ‘additional’ trace metal input to upland lakes can only be the result of mobilisation from catchment storage. We undertook a multiple sediment core, multi-pollutant study at nine lakes across Scotland to test hypotheses that this additional input was a result of (i) a simple time-lag; (ii) catchment soil erosion; (iii) leaching with dissolved organic carbon. We constructed decadally resolved full basin inventories for Pb, Hg and spheroidal carbonaceous particles (SCPs), a particulate component of fly-ash, at lakes with thin soils, lakes with eroded soils and lakes with non-eroded soils in their catchments to assess temporal trends. The use of both trace metals and SCPs allowed a comparison between erosive (SCPs and metals) and leaching (metals only) processes. Our results showed that the full basin inventories for Hg, Pb and SCPs for lakes with thin catchment soils had similar temporal trends to those of atmospheric emissions and deposition. For lakes with significantly eroded catchment soils, increasing trends in inventories continued to the most recent decades while those with non-eroded soils showed an intermediate pattern with some similarity to atmospheric deposition patterns, but with a reduced recent decline. We conclude that catchment soil erosion is the main transfer mechanism for trace metals at these sites and that although the leaching of metals bound to dissolved organic carbon (DOC) may play a role this appears to be less significant. The processes that lead to soil erosion and leaching of DOC from the catchment are exacerbated by increased winter rainfall, prolonged summer drought and increased frequency of high intensity rain events. Hence, predicted climatic changes will increase pollutant transfer from catchment to surface waters. The catchment storage of deposited pollutants is a massive potential reservoir that could keep fluxes of contaminants to surface waters elevated for many decades to come. This would counteract the benefits of reductions in deposition resulting from policy implementations over recent decades and may elevate exposure of aquatic biota to these contaminants.