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EGC-01 General contributions to environmental geochemistry - Part 2

Trace metals, sulphate and sulphur isotopes in small boreal streams: The importance of landscape type

Louise Björkvald, Stockholm University (Sweden)
Hjalmar Laudon, Umeå University (Sweden)
Borg Hans, Stockholm University (Sweden)
Carl-Magnus Mörth, Stockholm University (Sweden)
 

In a world of global change wetlands are playing a major role in the biogeochemical cycling of e.g. C, N, and S. However, little attention has been on the cycling of trace metals (TM) within peat systems and how different landscape elements influence the transport of metals from land to sea. We studied temporal and spatial variations of trace metal (TM) concentrations (As, Cd, Co, Cr, Cu, Ge, La, Ni, Pb, Rb, Sc, and Y) in stream water and their correlation with catchment properties (i.e. coverage of wetland and forest), but also with Fe and dissolved organic carbon (DOC). Stream water samples (0.4 μm filtered) collected from 10 streams (0.13 to 67 km²) in a boreal stream network in northern Sweden, were analyzed for metals using ICP-OES and ICP-MS. Also, the sulphate (SO₄^2-) concentrations were determined by ion chromatography and the isotopic composition of stream water SO₄^2- (δ³⁴S_SO4) was analyzed on precipitates of BaSO₄, using IRMS. The results from two consecutive years of sampling reveal that landscape type is significant for river chemistry. In this region, characterized by peat wetlands and coniferous forests, streams with different catchment characteristics responded differently to hydrological episodes (i.e. spring flood). In forested streams (2% wetland area) concentrations of TM (e.g. As, Cd, Co, Fe, Pb), Fe and DOC increased at spring flood. Conversely, in wetland influenced catchments (30% wetland area) the opposite seasonal variation was observed, i.e. concentrations of all TM decreased by a factor of 2 to 3. The seasonal variation of Fe and DOC showed a similar pattern to many TM, due to the association of TM to Fe oxyhydroxides and organic compounds. Furthermore, the only metals showing positive correlation with wetland coverage were Fe and Pb (r²=0.54 and r²=0.80, respectively) indicating that wetlands may act as a source for these elements but as a sink for all other TM. Alternatively, there is a source limitation in wetlands and that increased concentrations during base flow are due to mineral groundwater influence. Moreover, stream water SO₄^2- showed a significant negative correlation with wetland coverage (r²=0.96) indicating that bacterial SO₄^2- reduction is important within the wetlands. This result was confirmed by the higher δ³⁴S_SO4 values in stream water of wetland influenced catchments. This study emphasises the importance of understanding stream water chemistry from a landscape perspective.