ACIDIFICATION, BUFFERING, AND SALT EFFECTS IN THE UNSATURATED ZONE OFA SANDY AQUIFER, KLOSTERHEDE, DENMARK

Acidification of groundwater in a noncalcareous sandy aquifer,at Klost erhede, Denmark, is the result of acid rain deposition. In the 4- to 5 -m-thick unsaturated zone the pH ranges from 4.2 to 4.9 with Al concen trations of up to 0.8 mmol L(-1). The groundwater at the top of the sa turated zone still has a pH below 5. Deposition of sea salt affects th e solute profiles, and its importance varies both spatially from the f orest margin to the inner part of the forest and temporally through se asonal variations in infiltration and dry deposition. As a result, pul ses of high solute concentrations travel downward through the unsatura ted zone. The cation exchange capacity (CEC) of the sediments ranges b etween 0.2 and 1 meg 100 g(-1), and in the acidified zone, base satura tion is around 17%. The pore waters are close to equilibrium with gibb site, supersaturated for kaolinite, and strongly undersaturated for ot her silicate minerals. Mass balance calculations on increases in disso lved silica over depth suggest that the buffering effect of silicate w eathering is small. Buffering processes and solute transport were mode led with the code PHREEQM. Simulation of pre-acid rain weathering indi cates that this process operates on a timescale of thousands of years, yielding minimum pH values near 5.2 and a base saturation of greater than 70%. The present leaching of Al3+ rich acid water from the soil y ields acidification rates of 7 and 10 cm yr(-1) for weathering of a na turally weathered and a pristine profile, respectively. Simulation of infiltration of sea-salt pulses indicates that the cation distribution quickly becomes attenuated by the exchanger composition. However, due to coupling of gibbsite equilibrium with ion exchange processes, down ward traveling pulses with high solute concentrations will cause pH va riations throughout the unsaturated zone by precipitation and dissolut ion of gibbsite. Accordingly, the general acidification pattern at Klo sterhede is overprinted by salts effects in a complicated fashion.