COUPLED NITROGEN AND OXYGEN ISOTOPE STUDY OF NITRATE AT A RURAL UNLINED LANDFILL IN WESTERN VICTORIA, AUSTRALIA

Understanding of site-scale physical and chemical heterogeneities will inform remediation strategies for unlined rural landfills worldwide. The aim of this study was to characterize potential sources of nitrogen contamination to groundwater at an unlined landfill in rural western Victoria, Australia. Results revealed simultaneously high concentrations of both nitrate and ammonia within several wells, indicative of heterogeneous redox potentials within the unconfined aquifer. Combined isotopic analyses of δ¹⁵N and δ¹⁸O in nitrate identified a leachate-derived source and active denitrification across two sites hydraulically down-gradient from the landfill cell. Groundwater at an up-gradient site, as well as nearby surface water samples from a riparian creek, reflected primarily the isotopic signature of agriculturally-derived nitrate with overprinting from atmospheric exchanges of nitrogen and oxygen. Nitrate and ammonia in the creek were interpreted to result from a mixture of leachate and agricultural sources. Results illustrate how redox gradients across a landfill leachate plume impact biogeochemical nitrogen cycling.