Environmental stable isotopes (δ15N, 13C, 18O and 2H) are used to understand the biogeochemical pathways that affect carbon and nitrogen pools at the Trail Road Landfill (TRL) leachate plume, located at about 25 km West of Ottawa City. Samples of leachate and leachate-polluted groundwater were collected at TRL site from 2003 to 2005. A variety of analytical techniques (ion-selective combination electrode, simultaneous distillation and titration, ion chromatography, denitrifier method, diffusion sampler, gas extraction line, elemental analyser (EA), total inorganic/organic carbon (TIC/TOC) analyzer and isotope ratio mass spectrometry (IRMS)) were used to determine the concentrations and isotopic compositions of ammonium, nitrate and dissolved carbon in leachate and leachate-polluted groundwater. Landfill leachate has high NH4+ concentrations (average of 418 mg/L, 189 mg/L and 313 mg/L for leachate from leachate pumping station (LPS), leachate pipes (LP) and M32, respectively). The shallow groundwater underneath and near the landfill border, which has relative high NH4+ concentration up to 54 mg/L, was clearly impacted by landfill leachate. However in the deep aquifer the NH4+ concentration are low mainly reflecting background concentrations. Based on these data, the deep aquifer has not been impacted by leachate. However, the enriched 13CDIC values of groundwater (-5.0 ‰ VPDB and -1.9 ‰ VPDB for the shallow and deep aquifers respectively) in comparison with the 13CDIC value of upstream pristine groundwater (-15.0 ‰ VPDB) showed the deep aquifer is impacted by leachate plume. Therefore, the NH4+ is partly attenuated by nitrification and denitrification in the fringes of the plume. Low NO3- concentrations in leachate taken from LPS and M32 (lower than 2 mg/L) and groundwater (lower than 1 mg/L) show that natural attenuation is occurring through denitrification which take place in anoxic conditions. This is confirmed by the enriched δ15N values of nitrate (13.5‰ for the leachate and -7.6‰ to -0.6‰ for the groundwater, respectively), the measured excess N2 from diffusion sampler (0.2 mg/L to 4.7 mg/L) and δ15N of dissolved N2 (-1.8‰ to 3.1‰). In addition, the deviation of groundwater samples from the mixing lines on the cross plot of DIC vs. 13CDIC and DOC vs. 13CDOC confirm that DOC is consumed through a diverse array of biogeochemical reactions, including denitrification, within the aquifers. The significantly enriched 13C values of acetate from leachate (-14.5 ‰ VPDB) above the 13C of leachate bulk DOC (-25.7 ‰ VPDB), also indicate DOC consumption by redox reactions.

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