The impact on groundwater imparted by the infiltration of high dissolved organic carbon (DOC) leachate from capped, unlined landfills can be attenuated by biogeochemical reactions beyond the waste source, although such reactive loss in the aquifer is difficult to distinguish from conservative advective dispersion. Compoundspecific measurement of δ13C in carbon species, including CH4, dissolved inorganic carbon (DIC), and the major DOC compounds (acetate, humic acid, and fulvic acid) provides a constraint in this assessment that can assist in exercises of modeling and prediction of leachate transport. The Trail Road municipal landfill near Ottawa, Ontario, Canada, hosts AQ1 an unlined sector which produces a highly enriched leachate (DOC >4500 mg/L) that provides a good site to examine reactive attenuation within the receptor aquifer. Acetate, a sentinel component of leachate DOC (∼1000 mg C/L), is absent in impacted groundwater. Mass balance calculations together with reaction modeling suggest continued acetate fermentation with calcite control on DIC and δ13CDIC evolution. In groundwater within 50 m of the landfill, methane concentrations are elevated (∼10 mg/L), consistent with acetate fermentation, whereas δ13CCH4 measurements in deeper groundwater range down to −51‰ compared with −60‰ in the landfill demonstrating oxidative loss. DOC in the deep aquifer is remarkably depleted to values less than −40‰ suggesting methanotrophic bacteria selectively consume isotopically light CH4 to fix carbon. Continued reaction of leachate DOC in groundwater is demonstrated by evolution away from conservative mixing lines on diagrams of δ13C vs. concentrations of DIC and DOC.

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