Arsenic in field-collected soil solutions and extracts of contaminated soils and its implication to soil standards

Large concentrations of arsenic in soils, sediments, and freshwaters require risk assessment across the Central Alps and other regions. We measured arsenic concentrations in soil samples collected from 38 sites located in the Austrian Central Alps that had been contaminated due to mining and smelter activities and geogenic mineralization. Medians and ranges of arsenic concentrations (in mg kg^—1) in the soil solid phase were: 77.1 (1—3000) for the total (As<INF>t</INF>), 19.2 (0—726) for (NH<INF>4</INF>)<INF>2</INF>C<INF>2</INF>O<INF>4</INF>-extractable (As<INF>o</INF>), 2.35 (0—169) for (NH<INF>4</INF>)<INF>2</INF>HPO<INF>4</INF>-extractable (As<INF>p</INF>), and 0.143 (0—11.1) for (NH<INF>4</INF>)<INF>2</INF>SO<INF>4</INF>-extractable (As<INF>s</INF>) arsenic. Arsenic concentrations in soil solutions (As<INF>sol</INF>) collected from organic surface layers and mineral horizons at five selected sites using suction cups fitted with nylon membranes ranged from 0 to 171 μg l^—1. Typically, the prevailing species of As in the soil solution was As(V). As<INF>sol</INF> was correlated with As<INF>s</INF> (As<INF>sol</INF> = 0.279 + 15.6 As<INF>s</INF>; r² = 0.938; n = 17) and As<INF>t</INF> (As<INF>sol</INF> = 1.272 + 0.043 As<INF>t</INF>; r² = 0.833; n = 17). Using these empirical models, As<INF>sol</INF> can be predicted quite accurately based on extraction with 0.05 M (NH<INF>4</INF>)<INF>2</INF>SO<INF>4</INF> or total arsenic concentrations in the soil. Linking these models to drinking water standards (DWS) we propose soil standards for freshwater protection that vary for As<INF>s</INF> (mg kg^—1) between 0.62 (for DWS = 10 μg l^—1 WHO) and 3.19 (for DWS = 50 μg l^—1). Corresponding standards for As<INF>t</INF> (mg kg^—1) are 203 (DWS = 10 μg l^—1) and 1133 (DWS = 50 μg l^—1). These considerations demonstrate that changes in legislation on DWS may have dramatic impact on As concentrations in soil that are acceptable for groundwater protection.