On the recognition of areas with problematic sulphidic sediments using multi-disciplinary data

Abstract

Increased concentrations of several trace elements have been observed in water draining areas with sulphidic sediments along the Baltic Sea coast of Sweden and Finland. Some of these elements are harmful to the water environment. The increased leaching of trace elements is often caused by antrophogenic activities, such as ditching, which cause oxidation of the sulphides and thereby acid soil conditions. It is therefore important to recognise areas with sulphidic sediments and, if possible, avoid land use, which causes sulphide oxidation. The aim of this study was to find time effective methods, which can be used to delineate areas with sulphidic sediments. Three areas in Sweden with known occurrences of sulphidic sediments were studied. Data obtained from the mapping program at the Geological Survey of Sweden (SGU) were first studied. Selected sites were thereafter visited and geoelectrical measurements were carried out by the use of several instruments. The results imply that areas with sulphidic sediments are reflected as areas with high trace element contents on SGUs biogeochemical maps, which are based on analyses of trace elements in aquatic plants. The same deposits are often shown as gyttja clay on the maps of Quaternary deposits. Furthermore models derived from airborne electromagnetic measurements show that areas with sulphidic sediments often have a lower electric resistivity than surrounding sediments. Also the results from ground geoelectrical measurements suggest that the sulphidic sediments have a lower resistivity than surrounding fine-grained deposits. That interpretation was verified after stratigraphical studies and analyses of the sulphur contents of the sediments. It is therefore suggested that data from ground geophysical measurements can be used for a detailed delineation of sulphidic sediments, whereas data obtained from airborne geophysical and geological maps give an overview over potential areas with these sediments. The absolute resistivity of the sulphidic sediments varies, however, between the three investigated areas. Complementary corings and stratigraphical descriptions are therefore always necessary to verify the occurrence of these sediments.