GROUNDWATER ACIDIFICATION IN TRIASSIC SANDSTONES - PREDICTION WITH MAGIC MODELING

Acidification of groundwater lags behind acid deposition due to the re latively long water residence time in conjunction with various bufferi ng processes in the soil zone and deeper aquifer (chemical weathering, cation exchange, sulfate sorption, and N uptake by the biomass). Exte nsive field data from eight forested catchments in the Bunter Sandston e of the Black Forest, including results from water budget studies and hydrochemical analysis of stream and spring waters, were used to simu late the future evolution of ground-water acidification with the MAGIC model. The present acid deposition exceeds the ''critical load'' (her e meaning buffering due to chemical weathering and protonation of orga nic acids) in six of eight catchments. Two catchments are well buffere d because they contain carbonate-bearing layers in the Upper Bunter sa ndstone. Transient buffering (i.e., cation exchange, N uptake, the sul fate sorption) thus far prevents worse acidification, but this effect will decline in the future. For one of the poorly buffered catchments (Seebach), a two-layer simulation was carried out, based on extensive data from 10 years of measurements. Validation of the long-term simula tions by hydrochemical and soil data was hampered by strong annual var iations but generally supported by paleolimnological studies. In the f uture, reductions in the S deposition by 20% and the N deposition by 1 0% up to the year 2030 are assumed as the most probable scenario. N up take through soil and vegetation will come to an end as suggested by d ecreasing C/N ratios of the organic matter. This process is arbitraril y included in the simulations. In the periglacial soil layer, acidific ation will decrease until the year 2030 and then approach a steady-sta te condition. In the fractured aquifer, acidification will also procee d at a decreasing rate; however, sulfate desorption up to the year 213 0, the end of simulated period, will prevent earlier remediation. Desp ite a significant reduction in S deposition since the mid-1980s, furth er efforts are necessary to reduce the emission of acidifying substanc es. Liming in the recharge area is partially effective to ameliorate ' 'shallow'' groundwater but largely fails to ameliorate ''deeper'' grou ndwater in the sandstone aquifer.