Nitrate concentrations in the Gulp catchment: some spatial and temporal considerations

in a chaik catchrnent in the Belgium-Dutch boundary region the median N03- concentration was 30 mg/l in 1991. Groundwater in wells, which are mostly located close to the villages, is more polluted than spring water of surface water from the Gulp brook. Median N03- concentrations were 39, 22 and 17 mgll, respectively. Since 1980 time- series of N03- from two gauging stations in the Gulp brook have showed a distinct seasonal pattern. in wet penods (winter), when the discharge is higher, the nitrate concentration also is higher (30-35 mg N03-A), whereas in dry penods the opposite occurs; i.e. lower discharge and nitrate concentrations (10-15 mg N03-A). The positive correlation between the discharge and the N03- concentrations cannot be explained by the contribution of overland flow and interflow (quick flow components) probably having higher N03- contents. Saturated groundwater modelling shows that water following different flow paths might be an explanation. In periods with high groundwater recharge, young groundwater in the upper chaik layer can bypass medium- aged and old groundwater in the lower chaik and greensands. Young groundwater is likely to have higher N03- concentrations than old groundwater. So, in penods with high groundwater recharge the stream flow of the Gulp brook might consist of more young groundwater with higher NO3- concentrations, whereas in periods with low recharge mainly old groundwater with lower NO3- concentrations feeds the Gulp