Groundwater-Surface Water Exchange Fluxes in a Pleistocene Lowland and the Impacts on Riparian Zone Water Bala-nce and Nitrate Conditions

Water balance and groundwater nitrate dynamics within the riparian flood plain of the Havel River in northeast Germany are investigated by experimental analyses as well as by model simulations, using a coupled soil water balance and groundwater model approach. The model simulations indicate a substantial impact of exchange fluxes across the groundwater - surface water interface on the riparian water balance, the river discharge, as well as on the nitrate dynamics within the groundwater and river. Experimental and model-based results give evidence that the impact of groundwater contributions and associated nitrate fluxes from the riparian flood plain are temporarily very variable. Substantial seasonal variations in the riparian groundwater-borne nitrate contributions to river discharge are observed. Hence, nitrate contributions from the groundwater represent a significant proportion of the observed loads in the Havel River during summer, when discharge and nutrient condtions in the surface waters are crucial for the ecological health of the lowland river.