Abstract

The efficiency of biogeochemical mapping for identifying acid sulphate soils (AS soils) was studied by sampling and analysing water, transplanted aquatic moss (Fontinalis antipyretica) and leaves of the vascular plant Calamagrostis purpurea at two sites in a stream in northern Sweden with AS soils in its drainage area. One upstream sampling station (Mårtsmarken) was situated in an area dominated by till, and a downstream station (Persraningen) was situated in an area where AS soils are common. Metal contents in F. antipyretica and C. purpurea at these sites were compared to those in water (both unfiltered samples, and samples subjected to 0.22 µm membrane filtration and 1 kDa ultrafiltration to obtain data on metal speciation and its effects on uptake). Oxidation of sulphides with associated increases in acidity and release of metals were clearly reflected by differences in the water samples. At Persraningen the pH was lower than at Mårtsmarken, and the unfiltered concentrations of Al, Ca, Cd, Co, Cu, K, Mg, Mn, Ni, P, S, Sr, Y and Zn were higher. At the upstream station, Mårtsmarken, the suspended fraction was an important carrier of Fe and P, and the colloidal concentrations were higher than or similar to the dissolved concentrations for all determined elements except Na and K. At Persraningen the low pH resulted in changes of speciation, so that the dissolved concentrations were higher than the colloidal concentrations for most elements. For Al, As, Cr, Cu, Pb and Y, the dissolved and colloidal concentrations were similar. For Fe, the colloidal concentration was usually higher than the dissolved concentration, and suspended Fe was detected. Aluminium, Cr, Cu, Fe and Y concentrations were significantly higher in Fontinalis antipyretica at Persraningen than at Mårtsmarken. In addition, concentrations of Al and Fe in C. purpurea were significantly higher at Persraningen. Our results highlight the potential of biogeochemical mapping for predicting the occurrence of AS soils. A limitation is the strong pH dependence of the uptake of metals in the aquatic mosses resulting in that only a few of the elements typically occurring at high concentrations in waters draining AS soils are enriched in the macrophytes. A combination of maps showing both absolute concentrations and elemental ratios would be useful for this, including not only ratios of previously suggested utility (e.g. Y:Pb and Ni:Pb), but also others, such as Y:Ca, Al:Ca, Cu:Ca, Y:Mg, Al:Mg and Cu:Mg.

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