Water flow and nitrate transport to a groundwater-fed stream in the Belgian-Dutch chalk region

Human activities, such as high fertilizer applications, groundwater abstractions and land use changes, might have a negative impact on drinking water supply, environment and nature reserves of the chalk region in the Dutch-Belgian boundary area. In this area, the groundwater and surface water systems of the Noor experimental catchment have been monitored intensively and modelled to investigate groundwater flow and the transport of nitrate to groundwater-dominated streams. Groundwater flow towards the riparian area, the springs and Noor brook is strongly controlled by long-term variation of the groundwater recharge. Relatively small, but permanent changes in the recharge have a higher impact on spring flow and stream flow than current groundwater abstraction in this chalk catchment. Land use, groundwater flow patterns and the characteristics of the geological formations result in distinctly different groundwater types and associated nitrate concentrations. High concentrations were found under the plateau (median 55 mg l(-1)) and extremely low in the riparian area (median 2 mg l(-1)). The springs and thereby the Noor brook are mainly fed by groundwater directly cowing from the plateau with a low probability of denitrification, This results in median NO3- concentrations which are clearly above the drinking water standard of 50 mg l(-1) (i.e. 68 and 58 mg l(-1)). Monitoring and modelling show that the north springs have higher NO3- concentrations than the south springs due to different probabilities of denitrification. Nitrate in the groundwater-dominated Noor brook is hardly correlated with the discharge, but the major spring shows a clear upward trend in terms of nitrate concentrations, i.e. an increase by about 30 mg l(-1) from 1980 onwards. A policy of reduced nitrogen application will result in lower nitrate concentrations in the surface water system not earlier than in 10 or 20 years, because of the long travel times of water particles in the unsaturated and saturated zone. The north springs will react earlier than the south springs. Copyright (C) 1999 John Wiley & Sons, Ltd.