『Abstract
　A hydro-economic modelling framework is developed for determining optimal management of groundwater nitrate pollution from agriculture. A holistic optimization model determines the spatial and temporal fertilizer application rate that maximizes the net benefits in agriculture constrained by the quality requirements in groundwater at various control sites. Since emissions (nitrogen loading rates) are what can be controlled, but the concentrations are the policy targets, we need to relate both. Agronomic simulations are used to obtain the nitrate leached, while numerical groundwater flow and solute transport simulation models were used to develop unit source solutions that were assembled into a pollutant concentration response matrix. The integration of the response matrix in the constraints of the management model allows simulating by superposition the evolution of groundwater nitrate concentration over time at different points of interest throughout the aquifer resulting from multiple pollutant sources distributed over time and space. In this way, the modelling framework relates the fertilizer loads with the nitrate concentration at the control sites. The benefits in agriculture were determined through crop prices and crop production functions. This research aims to contribute to the ongoing policy process in the Europe Union (the Water Framework Directive) providing a tool for analyzing the opportunity cost of measures for reducing nitrogen loadings and assessing their effectiveness for maintaining groundwater nitrate concentration within the target levels. The management model was applied to a hypothetical groundwater system. Optimal solutions of fertilizer use to problems with different initial conditions, planning horizons, and recovery times were determined. he illustrative example shows the importance of the location of the pollution sources in relation to the control sites, and how both the selected planning horizon and the target recovery time can strongly influence the limitation of fertilizer use and the economic opportunity cost for meeting the environmental standards. There is clearly a trade-off between the time horizon to reach the standards (recovery time) and the economic losses from nitrogen use reductions.

Keywords: Nitrogen management; Diffuse groundwater pollution; Hydro-economic modelling; Optimization; Water Framework Directive』

Introduction
Nitrate groundwater pollution
Method
　Management model
　Nitrate fate and transport and groundwater flow
　Pollutant concentration response matrix
　Agronomic simulation
Application of the modelling framework
　Illustrative example
　Pollutant concentration response matrix and breakthrough curves
　Scenarios and results
　Scenario 0. No nitrate standard
　Scenario 1. Variable fertilizer application
　Scenario 2. Constant fertilizer application
　Scenario 3. Recovery from pollution
　Scenario 4. Constant fertilizer application with initial pollution
Conclusions
Acknowledgments
References