A nation-wide investigation of nitrate concentrations in groundwater in Denmark has been carried out. The investigation is based on analyses of groundwater samples from app. 11,000 wells and drinking water from 2,800 groundwaterworks spread equally all over the country, 43,000 km2. Furthermore the trends in nitrate concentrations have been investigated on the basis of time-series of nitrate concentrations for 184 waterworks.

The investigation shows that:

- the overall mean level of nitrate concentrations in the groundwaters has trebled within the last 20-30 years,

- there are pronounced regional differences in the nitrate concentrations of the groundwaters and in the trends of the nitrate concentrations,

- in the western parts of the country the groundwaters are more seriously affected than in the eastern parts and rising trends are significantly more pronounced,

- 36% of the investigated time-series of nitrate concentrations in drinking water from 184 groundwaterworks show a rising trend of 1 to 6 mg NO3/1 per year with a mean value of 3.3, and there is no sign of levelling off,

- drinking water from 8% of the waterworks exceed the EEC maximum admissible concentration for nitrate of 50 mg/1 and 18% EEC guide level of 25 mg/1.

The causes of the general rising trends in the nitrate concentrations in the groundwaters and the regional differences of the trends are discussed. In particular, the impact of agricultural activities and the role of the geological conditions are considered.