title: Multi-species measurements of nitrogen isotopic composition reveal the spatial constraints and biological drivers of ammonium attenuation across a highly contaminated groundwater system.
abstract: 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(+)N] + [NO3(-)N] + [NO2(-)N]). Mean δ(15)N-DIN increased from ∼2‰ to +20‰ over this flow path, broadly confirming that biological N attenuation drove the measured concentration decrease. Multi-variate analysis of water chemistry identified two distinct NH4(+) sources (δ(15)NNH4(+) from -14‰ and +5‰) within the contaminated zone of both aquifers. Nitrate dual isotopes co-varied (δ(15)N: -3‰ - +60‰; δ(18)O: 0‰ - +50‰) within the range expected for coupled nitrification and denitrification of the identified sources. The fact that δ(15)NNO2(-) values were 50‰-20‰ less than δ(15)NNH4(+) values in the majority of wells confirmed that nitrification controlled N turnover across the site. However, the fact that δ(15)NNO2(-) was greater than δ(15)NNH4(+) in wells with the highest [NH4(+)] shows that an autotrophic NO2(-) reduction pathway (anaerobic NH4(+) oxidation or nitrifier-denitrification) drove N attenuation closest to the contaminant plume. This direct empirical evidence that both autotrophic and heterotrophic biogeochemical processes drive N attenuation in contaminated aquifers demonstrates the power of multiple N isotopes to untangle N cycling in highly complex systems.
authors.affiliation: Department of Catchment Hydrology, Helmholtz Centre for Environmental Research - UFZ, Theodor-Lieser Str. 4, 06112 Halle (Saale), Germany. Electronic address: naomi.wells@scu.edu.au.
authors.forename: Naomi S
authors.initials: NS
authors.lastname: Wells
authors.affiliation: Université de Liège, Département ArGEnCo, Hydrogéologie et Géologie de l'Environnement, Bât. B52/3 - Sart-Tilman, B-4000 Liege, Belgium.
authors.forename: Vivien
authors.initials: V
authors.lastname: Hakoun
authors.affiliation: Université de Liège, Département ArGEnCo, Hydrogéologie et Géologie de l'Environnement, Bât. B52/3 - Sart-Tilman, B-4000 Liege, Belgium.
authors.forename: Serge
authors.initials: S
authors.lastname: Brouyère
authors.affiliation: Department of Catchment Hydrology, Helmholtz Centre for Environmental Research - UFZ, Theodor-Lieser Str. 4, 06112 Halle (Saale), Germany.
authors.forename: Kay
authors.initials: K
authors.lastname: Knöller
mesh_terms: D064751:Ammonium Compounds; D004784:Environmental Monitoring; D060587:Groundwater / Q000737:chemistry; D009566:Nitrates; D009584:Nitrogen; D009587:Nitrogen Isotopes; D014874:Water Pollutants, Chemical / Q000737:chemistry
chemical_list: D064751:Ammonium Compounds; D009566:Nitrates; D009587:Nitrogen Isotopes; D014874:Water Pollutants, Chemical; D009584:Nitrogen
keywords: Ammonium attenuation; Groundwater; Industrial pollution; Nitrate reduction; Nitrite reduction; Stable isotopes