SIMULATION OF NITRATE TRANSPORT IN GROUNDWATER WITH A 3-DIMENSIONAL GROUNDWATER MODEL RUN AS A SUBROUTINE IN AN AGROECOSYSTEM MODEL

Abstract

The agroecosystem model, developed by the system research group of the University of Osnabruck (1990), describes the impact of intensive land use on the groundwater. The main substance in this model is nitrogen: the nitrogen flux through the soil down into the groundwater in the form of nitrate is simulated as a function of agriculture policy over farm management and field processes. This complex model is split into several submodels. One of these submodels describes the nitrate distribution and transport in the saturated zone of groundwater and is presented in this paper. A model is used in which a transport program is added to a groundwater flow program. It is assumed that the density of the transporting ground water is not significantly changed by the dissolved nitrate, which allows the calculation of the ground water flow and the calculation of the nitrate concentration to be separated, as proposed for the ideal tracer problem by Bear (1979). The groundwater flow is calculated from the three-dimensional groundwater flow model of McDonald and Harbaugh (1984). That model has been enlarged by a transport module for substances carried in dissolved form by groundwater. This requires the modelling of the mechanisms of convection and hydrodynamic dispersion. For the latter the standard description in porous media was used. The applicability of the model was tested for an area, about 200 km2 in size, surrounding the village of Holdorf in South Oldenburg. The consistency of the model was tested, and a comparison with an analytical solution is presented.