Multi-element features of active acid sulfate soils across the Swedish coastal plains

Abstract

Acid sulfate soils are sulfide-rich soils with notable associated environmental risks. The low pH of these soils mobilizes metals from the soil minerals, which will lead to both acidification and metal contamination of the surrounding environment. This paper presents the results of a geochemical study of 66 profiles of acid sulfate soils collected from a total of 22 sites (three profiles for each site) on the Swedish coastal plains, which stretch for some 2000 km along the Baltic Sea and Kattegat. The reduced zone at the bottom of the profiles was characterized by pH frequently >6.0, the transition zone (above the reduced zone) by a steep pH gradient from near-neutral to weakly acidic, and the oxidized zone located above the transition zone by highly acidic conditions with a pH minimum <4.0. Each zone in each profile was sampled for geochemical analyses. The aqua regia extractable S concentrations ranged widely in the reduced zone (0.06–7.5%) and were strongly depleted in the oxidized zone (median decrease 69%) from extensive sulfide oxidation and associated severe acidification and sulfate leaching. In addition to S, there were significant losses from the oxidized zone of eight alkali and alkaline earth metals (Ba, Be, Ca, K, Li, Mg, Na, and Sr), four first row transition metals (Co, Mn, Ni, and Zn), three second row transition metals (Cd, Y, and Zr), the rare earth elements (Ce, La, and Yb), and Al, Th, and B. These elements would therefore be expected to be enriched in nearby surface waters, which was supported by hydrogeochemistry data from elsewhere in the boreal zone. Nineteen other chemical elements were not significantly lost from the oxidized zone relative to the reduced zone (Fe, Ti, W, Se, Sc, V, Cu, Cr, P, In, Te, Ag, Sb, Bi, Sn, Mo, U, As, and Pb) and therefore, would not be expected to occur in elevated concentrations in acid sulfate soil affected surface waters, as was supported by literature data from elsewhere in the boreal zone. A hypothesis of more extensive oxidation, acidification, and leaching of the soils in south Sweden (hemiboreal zone) than north Sweden (boreal zone) due to a warmer climate in the former region was rejected based upon pH and multi-element patterns from 9 field sites from the north and 12 field sites from the south. Consequently, the losses were likely primarily controlled by local factors, such as time since drainage and soil development. However, the oxidized zone was significantly thicker in the north (median: 75 cm) than south (median: 40 cm). This suggested that although the proportional losses (i.e., difference in elemental concentrations between the oxidized and reduced zones) were overall not different between the two regions, the total amount of elements lost from the soils will have been larger in the north. Of the elements with no significant losses, several were extracted with 1 M HCl to a larger extent in the oxidized than the reduced zone (Fe, As, Cr, Cu, Mo, Sc, Ti, U, and V). This indicated retention in the acidic environment following release from weathered and/or oxidized soil minerals. These elements, of which several are particularly toxic (As, Cr, and U), may thus be mobilized and leached if the soils geochemical conditions change either naturally or via human activities.