Acid sulphate soils and its influence on metal concentrations in adjacent water bodies. - A case study from Halland, SW Sweden

Sammanfattning: Acid sulphate soils have been referred to as being among the most harmful soils in the world due to their ability to severely decrease the pH of the water, consequently enabling an increased metal load to recipient water streams, lakes, groundwater reservoirs and estuaries. The presence of acid sulphate soils has triggered several fish kill episodes in northern Scandinavia and the soils are today threatening many of the national environmental goals of Sweden. Their widespread coverage in northern Sweden and western Finland have endorsed these regions to be the focal point for previous investigations. However, in early 2019, acid sulphate soils were discovered in Halland, SW Sweden and this study therefore aimed to investigate the impact from these non-investigated soils on the surface water chemistry of an affected area. This was conducted through in situ surface water sampling from two areas, in order to cover both smaller ditches (area 1) and a larger canal (area 2), located adjacent to a confirmed acid sulphate soil. Sampling was conducted during three separate occasions; autumn, winter and spring with the purpose of investigating differences in discharge depending on season. Water pH and electric conductivity was determined in situ, and the total metal compositions of Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn was analysed with an ICP-MS instrument. Additionally, groundwater data was collected to determine potential impact on the drinking water of the region. The results provide strong indications of leakage from the acid sulphate soils into the surface water, visible through elevated metal concentrations along with pH values between 3.8 and 6.6. Additionally, high electric conductivity values were measured in the water (32-129 mS/m) implying high SO4 2- composition in the water. Higher metal concentrations and lower pH values were measured in area 1 compared to area 2, hence assumed to be more influenced by the adjacent sulphuric soil layer. The metal concentrations in the water decreased from autumn to spring, likely reflecting the high temperatures and prolonged precipitation in Halland 2019/2020, further implying the importance of considering climatic parameters when investigating acid sulphate soil leaching. No indications of contaminated groundwater were seen in the results; however, no definite conclusions could be drawn due to limitations in the dataset. The study site was further compared to other sites in Sweden and Finland, where indications of slightly lower discharge were seen in Halland.