Tracing denitrification in a deeper aquifer of the Lower Rhine Embayment, Western Germany

Abstract In agricultural regions, near-surface aquifers often show nitrate concentrations exceeding 50 mg/L. Therefore, local water suppliers frequently rely on deeper aquifers for groundwater abstraction. The influx of nitrate-rich groundwater into deeper aquifers occurs through hydraulic windows. Denitrification delays nitrate breakthroughs and simultaneously generates excess-N 2 . Measuring N 2 and Ar in groundwater samples allows quantifying excess-N 2 , determining initial nitrate concentrations, and assessing reaction progress. In the case of chemo-lithotrophic denitrification involving pyrite, the same information can be obtained using SO 4 2− /Cl − ratios. This study employs N 2 and Ar measurements to trace denitrification in a multi-layer aquifer system. For this purpose, groundwater samples were collected along flow lines that originate in areas where confining layers are missing, allowing near-surface groundwater influx. Deeper aquifers were sampled downgradient of these hydraulic windows. High nitrate concentrations up to 145 mg/L and low reaction progress are observed in areas lacking confining aquitards. In contrast, low to absent nitrate concentrations, along with reaction progressions up to 100%, occur downgradient of these hydraulic windows. In regions far away from hydraulic windows, no signs of near-surface groundwater influx are present. In general, N 2 /Ar and SO 4 2− /Cl − ratios develop similarly along the flow lines, indicating that chemo-lithotrophic denitrification is the primary process for reducing nitrate loads. On average, 65% of the overall denitrification is attributed to chemo-lithotrophic denitrification; chemo-organotrophic denitrification involving organic matter contributes 35%. In conclusion, the chosen approach is effective for thoroughly assessing nitrate flux into deeper aquifers and tracing the extent of denitrification beneath confining layers.