Assessing necessary nutrient reduction for measurement planning in groundwater bodies.

For the Federal State of Lower Saxony, Germany, nitrogen management options are developed and implemented in three pilot areas using new participation approaches and technologies suitable for programs of measures to reduce diffuse pollution from agriculture. As a target value for water protection measures a nitrate concentration in percolation water of 50 mg/l as an average for a larger area defined by the groundwater bodies and their hydrogeological subdivisions has been defined. An integrative emission model is used to simulate the interactions between agricultural practice, nitrogen surpluses and the nitrogen flow through the soil and aquifer to the outflow into surface waters. The actual nitrate concentrations in percolation water are calculated for the entire Federal State of Lower Saxony considering site-characteristics, N-surpluses, water balance and denitrification in the soil. The tolerable N-surpluses needed to meet the environmental target are quantified as averages for each of the hydrogeological subdivisions by "backward" calculation using this model system. The required reduction of N-surpluses was estimated by comparing the tolerable N-surpluses with the actual state of nitrogen emission. For the evaluation of the amount and efficiency of water protection measures, the required reduction of N-surpluses to accomplish the environmental target is quantified, using the current status as a reference.