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-δ15N and δ18O values up to ca. <InlineEquation ID="IEq1"><EquationSource Format="TEX">$+$</EquationSource><EquationSource Format="MATHML"><math><mo>+</mo></math></EquationSource></InlineEquation>12‰ and <InlineEquation ID="IEq2"><EquationSource Format="TEX">$+$</EquationSource><EquationSource Format="MATHML"><math><mo>+</mo></math></EquationSource></InlineEquation>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-δ15N and δ18O values up to <InlineEquation ID="IEq3"><EquationSource Format="TEX">$+$</EquationSource><EquationSource Format="MATHML"><math><mo>+</mo></math></EquationSource></InlineEquation>40‰ and <InlineEquation ID="IEq4"><EquationSource Format="TEX">$+$</EquationSource><EquationSource Format="MATHML"><math><mo>+</mo></math></EquationSource></InlineEquation>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-δ34S values (ca. <InlineEquation ID="IEq5"><EquationSource Format="TEX">$-$</EquationSource><EquationSource Format="MATHML"><math><mo>-</mo></math></EquationSource></InlineEquation>18‰) compared with terrace groundwater (ca. <InlineEquation ID="IEq6"><EquationSource Format="TEX">$-$</EquationSource><EquationSource Format="MATHML"><math><mo>-</mo></math></EquationSource></InlineEquation>10‰), and high sulfate-δ18O values (up to <InlineEquation ID="IEq7"><EquationSource Format="TEX">$+$</EquationSource><EquationSource Format="MATHML"><math><mo>+</mo></math></EquationSource></InlineEquation>6‰) compared to ambient riparian groundwater (water-δ18O: <InlineEquation ID="IEq8"><EquationSource Format="TEX">$-$</EquationSource><EquationSource Format="MATHML"><math><mo>-</mo></math></EquationSource></InlineEquation>20 to <InlineEquation ID="IEq9"><EquationSource Format="TEX">$-$</EquationSource><EquationSource Format="MATHML"><math><mo>-</mo></math></EquationSource></InlineEquation>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.

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

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2026年6月15日

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Caitlin M. Mayernik,
Stephanie A. Ewing,
Robert A. Payn

摘要

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-δ¹⁵N and δ¹⁸O 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-δ¹⁵N and δ¹⁸O 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-δ³⁴S values (ca. $-$ $-$ 18‰) compared with terrace groundwater (ca. $-$ $-$ 10‰), and high sulfate-δ¹⁸O values (up to $+$ $+$ 6‰) compared to ambient riparian groundwater (water-δ¹⁸O: $-$ $-$ 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.