Arseniks löslighet i grundvattenakviferen i Hjältevad : Utvärdering med geokemisk modellering

Sammanfattning: Excessive concentrations of arsenic in soil- and groundwater constitute a global issue. The spread of arsenic is due to both natural and anthropogenic effects in the environment. Historically, the anthropogenic emissions have originated from several different industrial sectors e.g. wood impregnation. In Sweden, there are approximately 85 000 contaminated sites. Most of them are contaminated due to industrial activities. The emissions of arsenic in Sweden are mainly due to the wood impregnating agent CCA (copper, chromium and arsenic). Arsenic is a toxic metalloid that predominantly occurs as the inorganic compounds arsenite As[III] and arsenate As[V] in soil and groundwater. The mobilization of the arsenic compounds in soil water is affected by the redox conditions and by other conditions in the field, e.g. pH. Arsenate adsorbs stronger to iron- and aluminum hydroxides. Arsenite is the most toxic, mobile and soluble of the two compounds.   In a previously remediated impregnation plant in Hjältevad, Småland, arsenic has started to spread in the soil and groundwater. High dissolved concentrations of arsenic have been measured in the area. A hypothesis about the recent mobilization of arsenic is that pollutants below the groundwater table which were left behind after remediation started to dissolve due to changed redox conditions. This report aims to evaluate how the mobilization and adsorption of arsenic in Hjältevad is effected by pH and redox potential. This was addressed by leaching tests and geochemical modeling in Visual MINTEQ. Soil samples were collected during autumn 2017. Soil samples from seven different sampling points, taken from different depths were collected. Leaching tests were carried out for both dry and humid soil samples. Oxalate extractable arsenic was used together with measured dissolved concentrations of cat- and anions as input in Visual MINTEQ. The input data were used to evaluate the mobilization and adsorption of arsenite, arsenate and total arsenic due to pH, redox potential and the specific surface area of ferrihydrite. The modeling showed that the mobilization and adsorption of arsenic is dependent on pH, redox potential and reactive surfaces in the soil solution. Arsenate was adsorbed more strongly between pH 4.5 and 6.5 while arsenite was adsorbed at pH values greater than 7. The conclusion is that the mobilization and adsorption of arsenic were affected to some extent by the pH value. However the redox potential and the specific surface area of ferrihydrite were more influential. To verify the trends seen in the report, additional modeling is required. The report shows that most likely, arsenic started to mobilize in Hjältevad due to the changed redox conditions.