The production and use of nitrate and phosphate in agriculture and their consequences on regional groundwater quality

The one-dimensional quantitative model MANRUU has been developed for the purpose of physical planning on a national level to calculate the overproduction of nitrogen and phosphorus in animal manure per municipality. MANRUU calculates total manure production per municipality from input data on animal-specific manure production (kg N, P2O5 per year), fertilizer use and the number of animals of various categories. Data on crop-specific fertilizer demand and on acreage of all crops grown are used to calculate the maximum amount of manure per municipality applicable from an environmental point of view. The two-dimensional model WATRUU calculates mean concentrations of nitrate and ortho-phosphate in shallow groundwater for catchment areas. It uses the output of the model MANRUU and realistic assumptions on conversion processes of nitrate and phosphate. By using watersheds, an indication can be given of the flow direction and spatial effects of the polluted shallow groundwater. The results of the calculations are displayed as maps, based on input data for 1983. Computed mean nitrate concentrations are up to 16 times in excess of the EC-standards for drinking water. Phosphate saturation down to the groundwater level is expected to occur in the soil of 145,000 ha of farming land within 25 years when no preventative measures are taken. The model results were compared with data from a detailed study of the quality of the top groundwater layer in the entire Province of Utrecht. The comparison was found to be reliable at a confidence level of 95%