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Groundwater under industrial sites is characterised by heterogeneous chemical mixtures, making it difficult to assess the fate and transport of individual contaminants. Quantifying the in-situ biological removal (attenuation) of nitrogen (N) is particularly difficult due to its reactivity and ubiquity. Here a multi-isotope approach is developed to distinguish N sources and sinks within groundwater affected by complex industrial pollution. Samples were collected from 70 wells across the two aquifers underlying a historic industrial area in Belgium. Below the industrial site the groundwater contained up to 1000 mg N l⁻¹ ammonium (NH₄⁺) and 300 mg N l⁻¹ nitrate (NO₃⁻), while downgradient concentrations decreased to ∼1 mg l⁻¹ DIN ([DIN] = [NH₄⁺single bondN] + [NO₃⁻single bondN] + [NO₂⁻single bondN]). Mean δ¹⁵N-DIN increased from ∼2‰ to +20‰ over this flow path, broadly confirming that biological N attenuation …