Stream riparian zones can be important buffers to reduce the influence of diffuse nitrate pollution to surface water. The distribution of hydrological flow paths and solutes moving through the riparian lowland are controlled by local heterogeneity, which to a large degree influences the N-removal efficiency and the occurrence of denitrification “hot spots”. The overall objective of this PhD project was to investigate and quantify the heterogeneity in water fluxes to riparian zones and streams, and how the relative importance of groundwater-fed surface flow to subsurface flow affects nitrate delivery to a stream in agricultural managed riparian stream corridor. Hydrological, hydrogeological and water chemistry data was collected through comprehensive field investigations conducted at the field site in Jutland, western Denmark.
In the aquifer, on both sides of the stream, nitrate was effectively removed by autotrophic and heterotrophic denitrification. To the Northeast of the stream, only a small amount of nitrate was found to discharge directly into the stream from a subsurface drainpipe system. The riparian lowland received low water and N-inputs from the adjacent small forested upland and despite the ongoing cultivation denitrification was likely limited by a low nitrate availability. On the opposite side of the stream, a riparian wetland received large water and N-fluxes from a large permeable upland mainly comprised of arable land. The large water fluxes resulted in groundwater-fed surface flow to the stream mainly caused by man-made preferential flow paths; comprised of ditches, subsurface drainage system, and a pond, and via two natural springs. This allowed nitrate-rich water to bypass the reduction capabilities of the subsurface. However, re-infiltration of nitrate-rich surface water to a peat layer could be a very important factor in reducing the N-load to the stream.
In summary, human alteration of extant flow patterns, affected residence times and the occurrence of denitrification “hot spots” in the riparian lowland, and hence the nitrate load to the stream. The Water Framework Directive prescribes that water management to be carried out at the catchment scale. Thus transferring knowledge obtained from plot/field scale studies to catchment scale application is necessary. Anthropogenic changes are common for many riparian lowlands situated in agricultural lowland catchments. Future investigations to detect areas of high groundwater discharge and occurrence of drainage systems using time- and cost efficient methods would be valuable for planning target investigations and the designation of Nitrate Vulnerable Zones.