Investigation of the source of acidification in an aquifer in Northern Germany

Shallow groundwater in the Nethen well field, NW Germany, is affected by acidification, accompanied by elevated ferrous iron and sulphate concentrations. The acidity results in the mobilization of aluminum to groundwater which is subsequently re-precipitated in deeper, less acidic layers. This, in turn, caused the clogging of a water well by aluminum hydroxide phosphate. Analyses of the regional geological framework and both groundwater and sediment samples revealed that acid sulphate soils, although not uncommon in the region, and acid rain are not the cause of the acidification. The unconfined aquifer contains pyrite, which is exposed to oxygen during the partial dewatering of the aquifer during pumping, especially in the cone of depression. Nitrate, stemming from agricultural activities, may contribute to the pyrite oxidation but dewatering is the main cause. Both the oxidation of pyrite and the subsequent acid release influence the mobilization of trace elements.