Applicability of overbank sediment for environmental assessment according to wide-spaced sampling in Fennoscandia

IGCP Project 259, International Geochemical Mapping, and its successor IGCP Project 360, Global Geochemical Baselines, aim at a global geochemical map, which is becoming increasingly an environmental concern. As a pilot survey for Project 259 overbank sediment, till, humus and river water were sampled from 49 large catchment basins in Fennoscandia. The deeper layers of overbank sediment were deposited hundreds of years ago and reflect natural pre-industrial geochemical background. In this survey the geochemistry of one sample of overbank sediment was found to represent catchment basins up to 7000 km2 in size, and to correspond to that of composites of 6–20 random till samples taken from the same basin. The surface layers were deposited recently and reflect pollution in the basin. Distinct pollution was detected in some basins with mining activities and industries. A large province with high Pb contents in the southern parts of Sweden and Norway is caused by atmospheric deposition. In some basins metal contents from both anthropogenic and natural sources and exceeding values considered (phyto)toxic were measured. According to other surveys 30% of the river plains in Europe are severely polluted, and many of these plains have toxic contents of heavy metals. Therefore, and because population is concentrated to river valleys, it is of great importance to determine the degree of, and future threats from this pollution. Overbank sediment is a most suitable medium for this purpose, because it can be used in basins ranging from a few up to thousands of km2 to determine the geochemical background (deep layers), pollution history (profile sample), degree of pollution (deep vs surface sample), and for monitoring change.