Seasonal variability of groundwater—surface exchange and its implications for riparian groundwater nitrate retention at the Havel River

Abstract

The IWAN (Integrated Water Balance and Nutrient Dynamic) model was used to quantify the spatially and temporally variable impact of groundwater—surface water interactions on the groundwater nitrate dynamics in a riparian floodplain of the Havel River, Germany. The model coupled simulation of the floodplain water balance and groundwater nitrate dynamics to quantify the exchange fluxes and the nitrate transport between the river and the riparian groundwater. Investigations of the riparian water balance indicated extensive heterogeneity of process interactions between the groundwater and surface water, which have a temporally and spatially variable impact on the floodplain water balance, groundwater dynamics and river discharge. The simulated transport of nitrate across the groundwater—surface water interface was transient, varying both in intensity and direction. The floodplain acted as a nitrate sink or source with a seasonal periodicity. Riparian groundwater contributed with seasonally variable nitrate loads of the Havel River. The simulated annual groundwater nitrate contributions of the research area represented on average 1 % of the observed nitrate loads of the Havel River. However, in summer the groundwater nitrate contributions were significantly higher, representing more than 20% of the river loads. The simulation results indicated that 93% of the nitrate input into the riparian zone was attenuated within the groundwater, highlighting the role of the riparian floodplain as an important retention filter for diffuse nutrient pollution.

Keywords:

• Nitrate
• attenuation
• riparian zone
• groundwater—surface water interactions
• coupled model
• denitrification