Nitrate and sulfate isotopic compositions reveal patterns of production and loss in stream corridors draining agricultural landscapes

Stream corridors play a critical role in reducing contaminant export, yet limited understanding of controls on riparian biogeochemical processes hinders effective water quality management. To infer outcomes from riparian biogeochemical processes, we analyzed nitrate and sulfate abundance and isotopic composition in water samples from upland groundwater, riparian groundwater, and stream water across three ca. 0.7-km reaches draining an extensively cultivated terrace landform. Nitrate showed net loss from upland groundwaters to stream water, with stream samples having nitrate-δ 15 N and δ 18 O values up to ca. $$+$$ + 12‰ and $$+$$ + 2‰, respectively, and lower nitrate concentrations (ca. 3 mg L −1 ) than terrace groundwater inflows (ca. 20 mg N L −1 ). Riparian groundwater samples had nitrate-δ 15 N and δ 18 O values up to $$+$$ + 40‰ and $$+$$ + 15‰, respectively, with low concentrations near 1 mg N L −1 , indicating loss along riparian flow paths. Sulfate showed net gains in concentration, with stream water having low sulfate-δ 34 S values (ca. $$-$$ - 18‰) compared with terrace groundwater (ca. $$-$$ - 10‰), and high sulfate-δ 18 O values (up to $$+$$ + 6‰) compared to ambient riparian groundwater (water-δ 18 O: $$-$$ - 20 to $$-$$ - 14‰). These results suggest that sulfide oxidation during marine shale weathering is cycled through redox transformations under fluctuating saturation conditions in riparian systems. We use relationships in abundance and isotopic composition from uplands to streams to constrain the potential magnitude of gross gains and losses influencing observed net sulfate gains and nitrate losses. Our findings highlight how losses, gains, and mixing processes influence water quality through solute loss to gaseous phases, solute production in the riparian system, and redox cycling in stream corridors.