Acidification effects on groundwater - prognosis of the risks for the future

The natural acidification of groundwater in some types of environment has accelerated by acid atmospheric emissions and cultivation during the last 200 years. The direct effects are seen in changes of the groundwater chemistry in four stages, (1) seasonal depression of pH, alkalinity and some cations in very shallow groundwater, (2) long-term increase of calcium, magnesium, nitrate and sulphate in shallow groundwater from areas with acid rock and soil and high acid load, (3) reduction of alkalinity and/or pH over time in shallow groundwater in non-calcareous sandy aquifers and high total acid deposition, (4) zero alkalinity, pH below 5, high contents of aluminium, nitrate and sulphate as well as notable contents of heavy metals. Indirect positive effects are known when groundwater with high alkalinity neutralizes the input of acid rain or meltwater in streams and lakes. Indirect negative effects on surface water are the discharge of very acid groundwater containing high content of aluminium, iron and manganese with strongly negative, sometimes toxic, impact on biota. Another negative effect is the corrosion of water pipes, which leads to leakage and high contents of heavy metals in drinking water. The prognosis for the future by modelling is very alarming: 85% reduction of sulphur and 60% of nitrogen are required to halt and reverse the ongoing acidification of shallow groundwater in sensitive areas in Sweden. PROBLEMS AND OBJECTIVES Acidification of soil and water is an on-going natural, internal process, known for a long time in some types of environments. But acidification can also be due to anthropogenic causes linked to the industrialization. This external acidification has an imperceptible course during a long initial phase, which is why the effects were not recognized until long afterwards: on surface water in the 1950s and 1960s and on groundwater and soil not until twenty years later. The scientific problem is still how to distinguish between natu-ral and anthropogenic causes of acidification. The objectives of this paper are to descrribe the two types of acidification and then to sort out the effects on groundwater of anthropogenic origin as well as to make an assessment of the total acidification effects and to present a tentative prognosis of the risks for the future. NAMJRAL ACIDIFICATION Natural acidification of soil and water is a very slow, biogeochemical process, caused