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Agriculture is the largest contributor of nitrogen pollution to groundwater. Nitrogen from excess fertiliser percolates through the soil and is detectable as elevated nitrate concentrations under aerobic conditions and as elevated ammonium concentrations under anaerobic conditions. The rate of percolation is often slow, and excess nitrogen concentrations may be the effects of pollution on the surface up to 40 years ago depending on the hydrogeological conditions.

The Nitrates Directive (91/676/EEC) aims to control nitrogen pollution and requires Member States to identify groundwaters that contain more than 50 mg/l nitrate or could contain more than 50 mg/l nitrate if preventative measures are not taken. In addition, the Drinking Water Directive (98/ 83/EC) sets a maximum allowable concentration for nitrate of 50 mg/l. It has been shown that consuming drinking water in excess of the nitrate limit can result in adverse health effects, especially in infants less than two months of age. Groundwater is a very important source of drinking water in many countries and it is often used untreated particularly from private wells.

Mean nitrate concentrations in groundwaters in Europe are above background concentrations (<10 mg/l (as NO3) (EEA, 2000)) but do not exceed 50 mg/l as NO3 (Figure 3.2). Elevated mean nitrate concentrations in 1996 and 1997 are mostly caused by single very high values. However, the annual mean nitrate concentration in at least one sampling site in about one third of the groundwater bodies (included in Eurowaternet) exceeds 50 mg/l nitrate (Figure 3.3). Concentrations higher than 50 mg NO3/l were detected frequently or very frequently in 39 groundwater bodies (14 %).

According to the latest European Commission report (EC 2002), 20 % of EU stations had concentrations in excess of the maximum allowable concentration and 40 % were in excess of the guide value in the drinking water directive (25 mg/l as NO3) in 1996-98. Countries showing an overall increase in nitrate concentrations in groundwater are France and Sweden.