Precipitation may carry significant amounts of chemical substances of natural as well as anthropogenic origin, and these substances may influence the fate of other chemicals already present in the surface soil. In Norway the chemical composition of precipitation shows substantial variation among different areas for two main reasons: At sites close to the coast the atmospheric supply of marine cations and anions is many-fold greater than in regions shielded from marine influence by mountains. The southernmost part of the country has been, and still is, substantially affected by soil acidification due to long-range atmospheric transport of acidifying substances from areas elsewhere in Europe. This means that not only is the precipitation a significant source of chemical substances, but these airborne substances may appreciably affect the fate of other substances already present in the surface soil and their bioavailability.
This lecture will give some examples on the influence of precipitation chemistry on the mobility of trace substances in natural surface soils in Norway, with emphasis on ¹³⁷Cs from the Chernobyl accident. Mobility and plant uptake of Cs in soils is generally limited by the presence of clay minerals in the soil. Repeated measurements of ¹³⁷Cs in surface soils all over Norway however show that its mobility in the soil – plant system over the period 1986-2005 varies considerably with the precipitation chemistry. The greatest transfer of ¹³⁷Cs to natural vegetation is apparent in the areas subject to soil acidification, followed by those near the west coast receiving high amounts of marine cations and anions. The specific uptake of ¹³⁷Cs varies considerably among different natural plant species. The specific uptake of ¹³⁷Cs in herbivorous animals appears to be much greater in the south where its mobility in the soil-plant system is strongly affected by the greater soil acidity.