Modelling long-term acidification trends of forested sandstone catchments in the Black Forest (SW Germany)

Abstract

An extensive database from eight Triassic (Bunter) sandstone catchments in the Black Forest, SW Germany, was used to apply the MAGIC model and simulate long-term acidification trends. Using the ion ratio (Ca+Mg)/(SO₄+NO₃) as criterium (values< 1.5 indicate an acidified state), hindcast simulations showed that the brooks of three catchments have reached values <1.5, three catchments are approaching 1.5, and two catchments (carbonate bearing upper Bunter) are still well above this limit. The different acidification state of the catchments is mainly caused by the amount of acidic deposition and bedrock geology. Other differences (shallow or deep groundwater circulation, sulphate sorption and soil parameters) are less significant. To simulate the future evolution, three scenarios were tested: a pessimistic, an optimistic and a most probable case. The latter leads to a still progressing but decelerated acidifcation in the next 100 years. In the pessimistic case, acidification rates will be accelerated in two of the catchments. Even in the optimistic case, the initial state, prior to acidification, cannot be restored up to the year 2130. However, the forecasting of the future evolution is still markedly hampered by the significant uncertainty in the evaluation of nitrogen-driven acidification, a process which today already predominates in parts of the Black Forest.