ABSTRACT - DELIVERABLE 68
Report describing the main transfer mechanisms between POPs, metals and Hg from soils to sediments
Recent studies at Lochnagar, a remote mountain lake in the UK, have shown that whole lake sediment basin fluxes of toxic metals (Pb and Hg) are not declining as might be expected as a result of major reductions in emissions since the 1970s. This suggests that emission reductions policy is not protecting this site and presumably many others in the UK and across Europe. Although it is uncertain how widespread this problem is, it seems reasonably clear that if deposition levels are declining, then this “additional” trace metal must be derived from previous deposition to the catchment.
As catchment areas are generally far bigger than lake surface areas, deposition and thus storage in catchment soils is likely to be significantly higher than that deposited directly to the lake surface. Hence, the store of toxic pollutants (POPs and trace metals) with potential to be released to freshwaters is large. It has been proposed that Pb released from this store at Lochnagar is now a significant contributor to the total flux to the loch and showed that the catchment soils, although sparse, contain the equivalent of more than 400 years of Pb deposition at 2000 levels. A similar situation has also been observed for Hg at Lochnagar although the full basin flux of Hg continues to increase through time, and more importantly across the period of Hg emission reduction, rather than level off in recent decades as is observed for Pb.
There are four hypotheses which might explain the lack of decline in sediment metals;
1. a simple time−lag, i.e. it takes decades for metals deposited onto the catchment to percolate through to the lake (or stream) water,
2. warmer summers as a result of climate change lead to increased decomposition of catchment organic matter which is leached to freshwaters as DOC. Trace metals are known to have a high affinity for DOC and hence they pass to the freshwaters along with it. It is known that DOC concentrations are increasing at many sites,
3. climate change is increasing catchment soil erosion elevating the input of previously deposited metals bound to soils and
4. longer ice−free periods as a result of climate warming allows longer for algae to scavenge metals from the water column.
The situation for persistent organic pollutants (POPs) is unknown but these processes are thought to equally apply to these contaminants. Three of these hypotheses implicate climate and hence there is concern over the continued inputs of toxic pollutants to freshwaters from catchments into the future. Task 4.1 in Euro−limpacs Workpackage 5 aims to assess the roles of these mechanisms in transferring trace metals and POPs from soils to sediments. This report describes the sites and methodologies involved in this study and the preliminary data available at the 18 month stage. A final report will be produced at the end of the project.
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