ABSTRACT - DELIVERABLE 181
Report on Scottish sediments and soil cores
Recent studies (Yang et al. 2002a) 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 other waterbodies 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 (trace metals and POPs) with potential to be released to freshwaters is large. Yang et al. (2002a) 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 (Yang et al. 2002b) 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 may explain the lack of decline in sediment metals. First, a simple timelag, i.e. it takes decades for metals deposited onto the catchment to percolate through to the lake (or stream) water. Second, the increase in DOC observed in many waterbodies across Europe (including Lochnagar; Monteith et al. 2007) over recent decades is bringing in metals and POPs for which it has a high affinity. This increase may be a response to the recovery from acidification or a result of increased decomposition of catchment organic matter in warmer summers and elevated leaching in wetter winters. Third, climate change is increasing catchment soil erosion elevating the input of previously deposited metals bound to soils. Climate enhanced soil erosion is discussed more fully in Helliwell et al. (2007). Fourth, longer ice–free periods as a result of climate warming allows longer for algae to scavenge metals from the water column (nutrients permitting). 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 EUROLIMPACS Workpackage 5 aims to assess the roles of these mechanisms in transferring trace metals and POPs from soils to sediments. The study areas, sampling design and methodologies were described in a previous report (Deliverable 68; Rose 2005). This current report presents the final dataset for this study and is the basis for the final 18 months of work within this Task – that of interpreting the data and thereby evaluating the roles of the various mechanisms. A final report will be produced at the end of the Euro–limpacs project.
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