Metal transport in the boreal landscape : the role of wetlands and the affinity for organic matter

Stream water concentrations of 13 major and trace elements (Al, Ba, Ca, Cr, Cu, La, Mg, Na,Ni, Si, Sr, U, Y) were used to estimate fluxes from 15 boreal catchments. All elementsdisplayed a significant negative correlation to the wetland coverage in each catchment, butthe influence of wetlands was stronger for organophilic metals. 73% of the spatialdifferences in the normalized element fluxes could be explained based only on the wetlandcoverage and the affinity for organic matter, which was quantified using thermodynamicmodeling. When restraining the analysis to the smaller streams (<10 km2) the explanatorypower increased to 88%. The results suggest that wetlands may decrease the fluxes ofmetals from boreal forests to downstream recipients by up to 40%. We suggest that thedecrease in element fluxes is caused by a combination of low weathering in peat soils andaccumulation of organophilic metals in peat. The model could not explain the spatial pattersfor some metals with low affinity for organic matter, some redox-sensitive metals and somemetals with exceptionally high atmospheric deposition, but the results still demonstrate thatwetlands play a crucial role for the biogeochemical cycling of metals in the boreal landscape.
Originally published in dissertation in manuscript form.