THE FIRST ACTIVE ACID SULFATE SOIL FOUND OUTSIDE THE BALTIC BASIN, IN VISKA VALLEY WEST COAST OF SWEDEN

Abstract: Acid sulfate (AS) soils are naturally formed but are responsible for environmental problem due to its low pH. Drainage of active AS soils can lead to mobilization of harmful metals and a severe pH drop in nearby aquatic environments, which in turn may harm the ecological conditions. Most AS soil in Sweden has been found mainly along the northeast coast, and this area has received much research attention. A few discoveries have also been made in Mälardalen and Skåne. Recently, during a geological survey by the Geological Survey of Sweden (SGU) in the Viska Valley in Halland, old drilling profiles were found that showed sulfide-rich layers that hinted about the possible existence of AS soil. Focusing on the Viska Valley, this project aims to investigate the existence of AS soil in southwestern Sweden and find clues about its formational environment. Electrical Resistivity Tomography’s (ERT) suitability was evaluated as a method for locating AS soil on the west coast, and the ERT data was compared to the old drilling profiles. This study together with another master thesis study in Falkenberg was carried out during September 2019 to June 2020 in collaboration with SGU. Sampling sites were determined based on the old drilling data and SGU’s soil map. Samples were collected by using an Edelmann drill and brought to the laboratory for oxidation and further analyses, including grain-size distribution, metal-, sulfur- and organic content. Two ERT-measurements were made along one of the drilling profiles. This study is the first that proves the existence of AS soil outside the Baltic basin. In two areas of the valley, AS soil was found, one which was active and one potential, both in post-glacial gyttja clay covered by floodplain sediments. The gyttja clay is believed to have formed during the Tapes transgression when the valley consisted of a fjord-like bay, protected from wind and currents with saltwater input from Kattegat. The data from the ERT measurements displayed anomalies where the AS soil was found, but the information is not sufficient to determine with certainty whether the layer of AS soil is present. However, the study made simultaneously in Falkenberg showed more optimistic results with the ERT, which suggests that it may be an appropriate method with different electrode spacing and a larger number of measurements. This study only covers a small area of the Viska Valley, and most likely it is possible to find more AS-soil in the area as well on the entire west coast in areas that were below the highest coastline.