Groundwater‐surface water exchange as key control for instream and groundwater nitrate concentrations along a first‐order agricultural stream

Lower-order streams (first- and second-order) define the initial, landscape-related, chemical signature of stream water in catchments. To date, first-order have been perceived as predominantly draining systems, which collect solutes from surrounding groundwater surface runoff simply mirror composition inputs. In this study, impact stream-groundwater exchange fluxes on chemistry a first order agricultural (Schönbrunnen) its connected south-western Germany was assessed combining 222Rn, dissolved ions (chloride, sulphate, nitrate), salt tracer tests with investigations discharge hydraulic gradients. The findings suggest that stream-water lower-order is governed by an intricate interplay between dynamic, bidirectional solute leading to pronounced hydrologic turnover along studied reaches. High nitrate concentrations (up 79 mg/L NO3−) were attenuated downstream direction (a mean value 39 mg/L) without increase discharge, suggesting redox processes occurring during sediment passage sequential infiltration exfiltration zones affect chemistry. Nitrate infiltrating into aquifer at distinct losing spots subject denitrification within few decimetres streambed, while concurrent low-nitrate gaining compensated for flow losses turn diluted instream concentrations. summary imply (1) mixing resulting (hydrologic turnover) affects concentrations, (2) streambed reaches near-stream significantly contributes reactive elimination, (3) oxidation ammonium could be secondary source inputs stream.