Abstract
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.
Highlights
Effective remediation of groundwater contamination by leachate from unlined landfill sites requires site-scale hydrogeological characterization (Tiller, 1992; McLay et al, 2001; Allen, 2001)
Investigations were conducted at the Beaufort Transfer Station (BTS) in 20092010; groundwater samples were collected from three usable boreholes (MB01, MB02 and MB04), which penetrated the unconfined aquifer to shallow depths of 3, 4.5 and 4.5 meters, respectively (Fig. 2)
Groundwater field parameters (Fig. 4) showed that pH remained consistent over sampling timescales, while redox potential (Eh) exhibited variability across the landfill with the most oxidized conditions (~200mV) found up-gradient hydraulically of the landfill cell in MB04
Summary
Effective remediation of groundwater contamination by leachate from unlined (and possibly decommissioned) landfill sites requires site-scale hydrogeological characterization (Tiller, 1992; McLay et al, 2001; Allen, 2001). Variations in hydraulic conductivity in a sedimentary aquifer, e.g., due to changes in lithology (Genereux and Guardiario, 2010), can induce redox gradients that control the distribution of microbial consortia and the potential for contaminant biodegradation (Bjerg and Christensen, 1992; Jankowski and Beck, 2000). Understanding of site-scale physical and chemical heterogeneities is important for effectively harnessing microbial activity for bioremediation (Cozzarelli et al, 2011), at unlined rural landfills, a common historical type of landfill worldwide (e.g., USGS, 2003). Various approaches exist for understanding the distribution of potential electron donors for denitrification in leachate plumes (Cozzarelli et al, 2000)
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