Lower‐order streams (first‐ and second‐order) define the initial, landscape‐related, chemical signature of stream water in catchments. To date, first‐order streams have been perceived as predominantly draining systems, which collect water and solutes from the surrounding groundwater and surface runoff and simply mirror the chemical composition of the inputs. In this study, the impact of stream‐groundwater exchange fluxes on water chemistry of a first order agricultural stream (Schönbrunnen) and its connected groundwater in south‐western Germany was assessed combining 222Rn, dissolved ions (chloride, sulphate, nitrate), and salt tracer tests with investigations of stream discharge and groundwater hydraulic gradients. The findings suggest that stream‐water chemistry in lower‐order streams is governed by an intricate interplay between dynamic, bidirectional water and solute exchange between groundwater and the stream leading to a pronounced hydrologic turnover along the studied reaches. High nitrate concentrations (up to 79 mg/L as NO3−) in stream water were attenuated in downstream direction (a mean value of 39 mg/L) without an increase in discharge, suggesting that redox processes occurring during sediment passage in sequential infiltration and exfiltration zones affect stream water chemistry. Nitrate in stream water infiltrating into the aquifer at distinct losing spots was subject to denitrification within the first few decimetres of the streambed, while concurrent exfiltration of low‐nitrate groundwater into the stream at gaining spots compensated for flow losses and in turn diluted instream nitrate concentrations. In summary the findings imply that (1) instream mixing resulting from the bidirectional exchange of water between groundwater and the stream (hydrologic turnover) affects instream nitrate concentrations, (2) denitrification in the streambed of losing reaches and the near‐stream aquifer significantly contributes to reactive nitrate turnover and elimination, and (3) oxidation of ammonium could be a secondary source of nitrate inputs into the stream.
<p>Low order streams drain a big proportion of river catchments. They are not only fed by groundwater, but may also lose water to their connected aquifers and, in turn, can contribute to a substantial fraction of groundwater recharge. In such cases, these streams are typically characterized by the coexistence of gaining and losing stream reaches. Along the bidirectional exchange flow paths large biogeochemical gradients can evolve so that the exchange zones can function as hotspots for biogeochemical processes (such as the important (de)nitrification processes in croplands), which can substantially change under these two conditions. An agricultural first order stream (Sch&#246;nbrunnen) in south-western Germany was equipped with stream gauging stations and piezometers were installed in the adjacent shallow aquifer, in order to find out how these biogeochemical processes change under losing versus gaining conditions. Hydrological and hydrochemical variables within the immediate vicinity of the stream, as well as stable nitrogen isotopes have been monitored between August 2017 and May 2020 to spatially and temporally identify the controls of nitrogen cycling dynamics in the Sch&#246;nbrunnen.</p><p>Gaining and losing conditions at the Sch&#246;nbrunnen were determined by salt tracer experiments and the flow direction (upwelling groundwater or downwelling streamwater) of the exchanging fluxes was determined based on hydraulic head contour maps.</p><p>Hydrochemical data suggests that nitrate reduction occurs within the first 20 cm of the streambed in the losing reaches. In these reaches, isotopic data depicts that nitrate is reduced along the flow path between stream and groundwater. Ammonium and organic electron donors (DOC) were found at greater depths in these reaches. By contrast, increasing nitrite and nitrate concentrations were observed also along the last 20 cm of the upwelling flow paths (gaining reaches). In summary, assuming that the transition zone between groundwater and streams is only a hotspot for denitrification might not always be true, as our field data suggests that redox conditions in the streambed, and in turn, the resulting biogeochemical processes differed substantially between losing and gaining reaches.</p>
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