NEUTRALIZATION OF ATMOSPHERIC ACID INPUTS IN SMALL SPRING CATCHMENTS IN THE FRANKENWALD MOUNTAINS, GERMANY

Investigations on soil and freshwater acidification are usually focuse d on well-aerated systems. This study deals with the role of reductive processes for the neutralization of acid soil solution within helocre ne springs. Two toposequences consisting each of three profiles (fores t soil, margin of fen, fen) were established to study the chemistry of the solid phase (soil pH, CEC, pedogenic Fe-and Al-oxides) and the so il solution in two small spring catchments on three dates during 1991 and 1992. Despite high acid inputs and acidified forest soils the pH o f the spring outflow is near neutral, and the soil solid phases of the spring fens are not acidified. The results support the following hypo thesis: Aluminum with its corresponding anion sulfate is leached with the soil solution into the water-saturated fens. Dissimilatory iron an d sulfate reduction take place within the fen and generate alkalinity. Reduced iron either reacts with sulfide to form pyrite or migrates wi thin the fen profile and precipitates in the uppermost, oxic horizons, consuming part of the generated alkalinity. Due to the higher pH valu es in the fens the incoming aluminum precipitates releasing acidity. T he alkalinity generated exceeds the amount of acidity released by oxid ation and precipitation of iron and the precipitation of aluminum. A b alance of alkalinity consuming and alkalinity generating processes bas ed on solid phases showed that iron and sulfate reduction can account for at least 67% of the neutralization of acidity entering the fen of one of the catchments. Due to shorter water retention times and higher discharge these processes are of minor importance in the other catchm ent.