Nitrate in sources of drinking water? Dutch drinking water companies aim at prevention

In the Netherlands 65% of the drinking water is extracted from groundwater. A quarter of the groundwater wells, in particular those on the sandy soils in the east and the south of the country, face rising levels of nitrate, mainly of agricultural origin. In 1998 the Dutch Government launched a policy (MINAS) which aims at a drastic reduction in nitrate leaching by 2010. However, this program will not achieve a sufficient reduction in leaching on the dry sandy soils in the east and south of the country. As a result, drinking water companies are facing purification costs of US$ 35–70 million/year, by the next century. So far, the Government has failed to introduce additional policies. Therefore drinking water companies have extended their own preventive approach, aimed at reducing the total nitrogen load in a catchment area to a sustainable level. In this approach extra measures are to be taken in those parts of the area where they are most effective and cost-effective. This can compensate for higher nitrate leaching in other parts of the area, as long as the average nitrate concentration is safe below the norm of 50 mg NO 3/l. That opens the door to a wide range of measures: e.g. taking land out of cultivation, supporting farmers to change to organic farming, contracting farmers to grow low-leaching crops, and so on. Different measures can supplement each other. The most cost-effective set of measures for each specific catchment area is generated by means of a linear programming model.