Effectiveness of groundwater nitrate removal in a river riparian area: the importance of hydrogeological conditions

Riparian areas are often considered important ecological ecotones that decrease the nitrate load of groundwater discharging into rivers. In this paper, the effectiveness of nitrogen removal from groundwater in a riparian area has been evaluated. We used a quantitative hydrogeological approach which delimited homogeneous stream tubes based on the water table contour. We constructed water and mass balances and used the results to estimate the mass of nitrogen removed by biological processes. Nitrogen removal effectiveness was calculated as the ratio of nitrogen removed to the total nitrogen mass flowing through the riparian area. This approach requires knowledge of the hydrogeological settings and coverage of an area larger than the study site. The removal we observed was seasonally variable, ranging between 2.2 and 7.6 mg N m^-2 day^-1, and represented up to 95% of the total nitrogen mass entering the riparian area in late summer. Removal effectiveness was only 27-38% in winter, due to the combination of a high nitrogen input and a low plant uptake. Nitrogen removal was highest in spring, but effectiveness was about 60% because the input was as high as in winter. Rainwater infiltrating the riparian area could represent almost the same quantity as groundwater input. Under such conditions, the dilution effect is very important in riparian areas that are not nitrate sources and it is essential to maintain this non-nitrate-emissive use. This study showed that the effectiveness of nitrogen removal in a riparian area is highly dependent on the pathway of water movement through its biologically active layers.