Model Based Assessment of Nitrate Pollution of Water Resources on a Federal State Level for the Dimensioning of Agro-environmental Reduction Strategies

The main goal of the project was to assess nitrogen pollution of surface waters and groundwater in the Federal State of North Rhine Westphalia (NRW), Germany. For this purpose the hydro(geo-)logical models GROWA-DENUZ/WEKU were coupled to the agro-economic model RAUMIS in order to assess the diffuse nitrogen loads and to approaches to determine the nitrogen loads from point sources. In this way the complex socio-economic interrelations and hydrological/hydrogeological interdependencies were simultaneously. The model network was applied consistently across the whole territory of NRW. At first the actual N inputs into groundwater and surface waters resulting from diffuse sources and point sources were assessed. For the relevant diffuse input pathways (groundwater runoff, drainage runoff and natural interflow) this was done in a spatial resolution of 100 m ∙ 100 m. In the case of point source inputs information from municipal waste water treatment plants, industrial effluents, rainwater sewers and combined sewer overflows has been considered. For NRW an actual total N input into surface waters of ca. 117.000 t ∙ a⁻¹ N has been quantified. As the inputs via natural interflow (ca. 30 %), groundwater runoff (ca. 26 %) and drainage systems (ca. 18 %) hold the largest portion, it is evident that measures to control nitrate pollution have to focus on the inputs from diffuse sources. For this purpose, initially the development of the agrarian sector according to the Common Agricultural Policy, CAP until 2015 including supplementary measures and other impact factors has been analysed. The impact of this so-called baseline scenario 2015 was predicted for both, the diffuse N surpluses and the N pollution of groundwater and surface waters. It could be shown that the baseline projections for the agricultural sector through 2015 may lead to decrease of the diffuse N inputs into groundwater by ca. 13.500 t ∙ a⁻¹ N and an overall decrease of the diffuse N inputs into surface waters by ca. 25.000 kg ∙ ha⁻¹ ∙ a⁻¹ N. Based on the baseline scenario 2015 the additional N reduction to guarantee nitrate concentrations in groundwater below the EU-threshold value of 50 mg ∙ l⁻¹ NO₃ was determined by means of a backward model calculation. This was done using the predicted nitrate concentrations in the leachate 2015 for the individual 100 m ∙ 100 m grids as starting points. In this way for the whole territory of NRW an additional N reduction beyond the baseline scenario 2015 of ca. 12.000 t ∙ a⁻¹ N has been assessed. Model results indicate that additional N reduction measures don’t have to be implemented area-covering in order to be efficient, but in certain subareas only. It is suggested that in these subareas the available financial resources for the implementation of N reduction measures shall be used for individual, i.e. regionally adapted nitrate reduction measures.