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 indicated typically by a delta N-15 of between -3.2 and +10.7 parts per thousand, 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 (delta N-15 of -3.2 to +6.3 parts per thousand), coal pyrolysis residue/tar (delta N-15 of 4.2-10.7 parts per thousand), gas-purifier waste (delta N-15 of 2-5 parts per thousand) and coking-works-derived ammonium sulfate (delta N-15 of -0.5 parts per thousand). This suggests that gas-purification waste may have a distinct isotopic range compared to coal tar, a finding supported by results from Site A. Gasworks-sourced nitrogen typically had a lower delta N-15 value than non-gasworks sources, and predominantly in the delta N-15((NH4)) form. This study demonstrates nitrogen isotope analysis coupled with traditional hydrochemistry and a detailed site investigation is shown to have potential for use as part of the environmental forensic toolkit.