Groundwater acidification at Birkenes, southern Norway: Comparing trends of chemical composition of precipitation, throughfall, soilwater and groundwater

Chemical time trends for precipitation, throughfall, and soilwater (1986–1992), and groundwater (1980–1993) at Birkenes, southern Norway, are compared to gain insights into possible causes for the recent increase in groundwater acidification there. Precipitation and throughfall trends do not show evidence for an increase in anthropogenic acids (e.g. sulphate), but seasalt deposition (e.g. chloride) has been marginally greater in 1990–1992 than in most previous years on record. Soilwater composition partly indicates increasing acidification in recent years (pH, Al and ANC), but hardness and sulphate content are decreasing. Soilwater ANC became negative in 1989, revealing a lasting deficit in its potential to buffer acidity. Groundwater shows clear signs of intensifying acidification (pH, Al, ANC, hardness and sulphate), and this may result partly from climatic conditions (mild winters, ‘seasalt episodes’) and partly from the deterioration of an acid buffering system within the soil cover. Acidification via sulphate deposition certainly is not a direct cause. The declining hardness of soilwater suggests that the ion-exchange buffer in the soil may have ceased to function properly. The necessity for obtaining long-term time-series of water chemistry is underscored by this study.