How alternating gaining and losing conditions along a low order agricultural stream govern the behavior of nitrogen species

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Abstract
Perennial low order streams are normally well connected to shallow groundwater and therefore, they are among the first receptors of agricultural effluents. Understanding the processes governing the water quality in agricultural areas requires identifying sources of potential pollutants (such as nitrate), hotspots of biogeochemical reactivity and defining the different hydrologic flowpaths connecting groundwater and surface water. To this end, we have equipped an agricultural drainage system (Schönbrunnen) in south-western Germany with 3 stream gauging stations along a test segment of approximately 550 m and 33 piezometers in the adjacent shallow aquifer. Hydrological, hydrochemical, isotopic and microbiological variables have been monitored between August 2017 and December 2019 to spatially and temporally identify the controls of nitrogen cycling dynamics in our stream.The Schönbrunnen generally loses water in its mid-segment and it gains in the lower part of the catchment, although this behavior showed strong seasonal variations, considering winter and summer as the two main annual seasons. The groundwater-streamwater (GW-SW) exchange flux, and the replacement of streamwater lost to the aquifer over a reach by shallow groundwater, defined as hydrologic turnover, was found to influence streamwater chemistry. The main groundwater flow directions were determined based on hydraulic head contour maps. We used them to characterize the nitrogen (N) species’ behavior along the flowpaths under two different hydrologic conditions: losing and gaining. Even though the losing condition at the midstream provides more favorable condition for N-species reduction at the GW-SW interface, reduction occurred also along gaining reaches. The isotope analyses of nitrate yielded data points plotting along the denitrification trend (slope of 0.5) in a dual isotope plot (15N-NO3- versus 18O-NO3-) for some of the sampling locations within the losing area. Comprehensive molecular approaches suggested a hotspot for denitrifying microbial communities in sediments of the losing stream reach. Along the GW flow path to the gaining area a depletion of nitrate was identified in concert with increasing sulfate and declining sulfide (H2S) concentrations. Sulfide-driven nitrate reduction was likely to occur under anoxic conditions in this part of the aquifer. In summary, the findings demonstrate, that hydrologic turnover does not only mean hydrological exchanges, but also triggers variations in water composition along the transition zone between groundwater and streamwater by linking both, mixing and reactive processes.