Eutrophication of Surface Waters and the Contribution of Agriculture

Keywords: Eutrophication, Phosphorus, Nitrogen, Manure, Fertilisers, Runoff.

Eutrophication has become one of the most serious water quality problems we face today. In the Netherlands, blooms of obnoxious cyanobacteria turn lakes into turbid systems, practically devoid of the original biota. In ditches, floating aquatic weeds choke the aquatic community underneath and in the North Sea the frequency of blooms of scum-forming algae has increased.

The Netherlands follow a two-track policy with respect to eutrophication control. General emission-oriented measures are emphasised as an indispensable first track. The resulting emission reduction and a similar reduction of transfrontier pollution, resulting from the Ministerial Declarations on the North Sea, the Rhine Action Plan and future Community legislation, will lay the foundation for the restoration of affected waters. The restoration process will be reinforced and! or accelerated by the second policy track, regional effect-oriented measures.

The intensive use of manure and fertilisers in the Netherlands results in considerable nitrogen and phosphorus loads on surface waters through runoff. According to the results of calculations, presented in the paper, the nitrogen load on surface waters from agriculture amounts to more than 150 million kg nitrogen per year. This emission represents less than 15% of the nitrogen input into the Dutch agriculture, but more than 50% of the total nitrogen load on surface waters from domestic sources. The agricultural phosphorus load on surface waters is concentrated in sandy areas. Here, the manure surplus has resulted in considerable accumulation and even saturation of phosphorus in the soil. In these areas, phosphorus concentrations were established in the drainage water that are in the order of 10 times higher than the natural concentrations.

Present policies with respect to manure and fertiliser handling fail to reduce nitrogen loads sufficiently, while phosphorus loads are not even affected significantly. Additional measures aim at meeting the drinking water standard of 50 mg nitrate per litre in the upper groundwater and at a balance between input and crop uptake of phosphorus. The effects of the different measures on the nutrient runoff, as calculated, are discussed.

E J B Uunk, Institute for Inland Water Management and Waste WaterTreatment, PO Box 17, AA Lelystad, Netherlands.

55 pages, 20 figures, 10 tables, 45 refs.