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Abstract Groundwater nitrate‐N isotopes (δ15N‐NO3−) have been used to infer the effects of natural and anthropogenic change on N cycle processes in the environment. Here we report unexpected changes in groundwater δ15N‐NO3− for riparian zones affected by relict milldams and road salt salinization. Contrary to natural, undammed conditions, groundwater δ15N‐NO3− values declined from the upland edge through the riparian zone and were lowest near the stream. Groundwater δ15N‐NO3− values increased for low electron donor (dissolved organic carbon) to acceptor NO3− ratios but decreased beyond a change point in ratios. Groundwater δ15N‐NO3− values were particularly low for the riparian milldam site subjected to road‐salt salinization. We attributed these N isotopic trends to suppression of denitrification, occurrence of dissimilatory nitrate reduction to ammonium (DNRA), and/or effects of road salt salinization. Groundwater δ15N‐NO3− can provide valuable insights into process mechanisms and can serve as “imprints” of anthropogenic activities and legacies.