Multi-species measurements of nitrogen isotopic composition reveal the spatial constraints and biological drivers of ammonium attenuation across a highly contaminated groundwater system

作者：Wells, Naomi S.^*; Hakoun, Vivien; Brouyere, Serge; Knoeller, Kay

来源：Water Research, 2016, 98: 363-375.

DOI：10.1016/j.watres.2016.04.025

• Ammonium attenuation
• Groundwater
• Industrial pollution
• Nitrate reduction
• Nitrite reduction
• Stable isotopes

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 similar to 1 mg l(-1) DIN ([DIN] = [NH4+-N] + [NO3--N] [NO2--N]). Mean delta N-15-DIN increased from similar to 2 parts per thousand to +20 parts per thousand 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 (delta N-15-NH4+ from-14 parts per thousand and +5 parts per thousand) within the contaminated zone of both aquifers. Nitrate dual isotopes co-varied (delta N-15: -3 parts per thousand - +60 parts per thousand; delta O-18: 0 parts per thousand- +50 parts per thousand) within the range expected for coupled nitrification and denitrification of the identified sources. The fact that delta N-15-NO2- values were 50 parts per thousand-20 parts per thousand less than delta N-15-NH4+ values in the majority of wells confirmed that nitrification controlled N turnover across the site. However, the fact that delta N-15-NO2- was greater than delta N-15-NH4+ 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.

• 出版日期2016-7-1