Denitrification rates in the saturated zone are of large importance for the maintainance of acceptable nitrate concentrations in ground water aquifers and its use as potable water. This paper presents an approach to estimate nitrate leaching for a medium-scale river basin in South-West Germany based on the spatially distributed simulation of nitrate leaching with the soil and land information system SLISYS-Neckar. The data domain of SLISYS-Neckar is constituted of information layers and databases on climate, soils, geology and land cover/land use. In addition it contains a spatially distributed model for the estimation of nitrate leaching with high spatial (< 1km2) and temporal (1 day) resolution based on the EPIC model (USDA 1990). The calculated leachate concentrations were related to the mean measured nitrate concentrations of the ground water monitoring points in each water body. Mean nitrate concentrations in the leachate correlate closely with the proportion of arable land within the water bodies. With the exception of five water bodies, the comparision between estimated mean nitrate concentrations in the leachate and measured nitrate concentrations in the ground water shows a significant correlation. Apparent dinitrification factors (ADF) were calculated and ranged between 1 and 2.5 with its maximum frequency between 1 and 1.5. This is in agreement with findings from ground water aquifers in river sediments in Great Britain where between 32 and 48% of oxidized nitrogen input were concerted into dinitrogen (Dong et al 2000). However, the level of the apparent denitrifcation factors varied with the dominant geological strata in the water bodies. Water bodies which are dominantly or totally covered by upper triassic formations (Keuper) showed significantly lower ADF as the water bodies located on rocks of the lower and middle jurassic (Lias and Dogger). Through a regional modelling approach it has been demonstrated that the nitrate concentration in the percolating water is a function of land use intensity. The comparison of simulated leachate concentrations with measured ground water concentrations yielded spatially distributed denitrification factors, indicating significant differences between geological formations in the Nechar catchment.