Distinguishing Sources of Ammonium in Groundwater at Former Gasworks sites using Nitrogen Isotopes.

Ammonium is a common contaminant found in the soils and groundwater at former gasworks, associated with historical gas production and the storage and disposal of by-product. However, it can also be present in groundwater at gasworks sites from a variety of natural and other anthropogenic sources. This study evaluates the use of nitrogen isotope analysis at eight former gasworks sites in the UK, as a forensic tool to differentiate between ammonium from gasworks and non-gasworks sources. It also provides an understanding of how the parent coal, gas making technology and by-product processing can influence the presence of ammonium on a former gasworks and the importance of understanding the site layout when sampling.Results of this study indicate that, gasworks sources would be typically indicated by a δ15N of between -3.2‰ and +10.7‰, which correlate to published isotopic ranges specifically for coal and coal by-products. This broad range includes published values for the isotopic signature of parent coal (δ15N -3.2‰ to +6.3‰,), Coal pyrolysis residue/tar (δ15N +4.2‰ to +10.7‰,), gas purifier waste (δ15N +2‰ to +5‰) and coking works derived ammonium sulfate (δ15N -0.5‰). This suggests that gas purification waste may have a distinct isotopic range as compared to coal tar, a finding supported by results from Site A. Gasworks-sourced nitrogen typically had a lower δ15N value than non-gasworks sources and predominantly in the δ15N(NH4) form. This study demonstrates nitrogen isotope analysis, coupled with traditional hydrochemistry and a detailed site investigation and is shown to have potential for use as part of the environmental forensic toolkit.