Effects of groundwater extraction from crystalline hard rock on water chemistry in an acid forested catchment at Gårdsjön, Sweden

Atmospheric deposition of S in Sweden has decreased by some 80% over the last 15 a, resulting in a general reduction of SO4 concentrations in ground and surface water. This project, however, shows that artificial hydrological alteration in an acid wetland can reverse this trend and increase acidity and SO4 concentrations. The experiment involved the monitoring of two catchments in relatively virgin conditions. In one of the catchments, an experiment with intensive groundwater extraction from the bedrock was carried out. During the experiment, the runoff from the catchment decreased by 50%. Furthermore, the extraction of groundwater resulted in increased seasonal aeration of the centrally located wetland, leading to oxidation of reduced S bound to the soil layers of the wetland. The S changed to solute SO4, with a subsequent SO4 surge. Thus, the experiment resulted in an induced acidification of the wetland and runoff waters. The extraction of groundwater significantly increased the recharge of water from the overburden, glacial till and organic soils to groundwater in the bedrock, which in turn reduced the retention time in the bedrock aquifer. These changes resulted in the chemical signature of the groundwater in the bedrock becoming similar to those of the wetland. The findings revealed deterioration in the water quality in the bedrock due to increased concentrations of dissolved organic C and SO4, as well as a decrease in pH.