Recharge processes drive sulfate reduction in an alluvial aquifer contaminated with landfill leachate

Journal of Contaminant Hydrology
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Natural attenuation of contaminants in groundwater depends on an adequate supply of electron acceptors to stimulate biodegradation. In an alluvial aquifer contaminated with leachate from an unlined municipal landfill, the mechanism of recharge infiltration was investigated as a source of electron acceptors. Water samples were collected monthly at closely spaced intervals in the top 2 m of the saturated zone from a leachate-contaminated well and an uncontaminated well, and analyzed for δ18O, δ2H, non-volatile dissolved organic carbon (NVDOC), SO42−, NO3 and Cl. Monthly recharge amounts were quantified using the offset of the δ18O or δ2H from the local meteoric water line as a parameter to distinguish water types, as evaporation and methanogenesis caused isotopic enrichment in waters from different sources. Presence of dissolved SO42− in the top 1 to 2 m of the saturated zone was associated with recharge; SO42− averaged 2.2 mM, with maximum concentrations of 15 mM. Nitrate was observed near the water table at the contaminated site at concentrations up to 4.6 mM. Temporal monitoring of δ2H and SO42− showed that vertical transport of recharge carried SO42− to depths up to 1.75 m below the water table, supplying an additional electron acceptor to the predominantly methanogenic leachate plume. Measurements of δ34S in SO42−indicated both SO42− reduction and sulfide oxidation were occurring in the aquifer. Depth-integrated net SO42− reduction rates, calculated using the natural Clgradient as a conservative tracer, ranged from 7.5 × 10− 3 to 0.61 mM·d− 1 (over various depth intervals from 0.45 to 1.75 m). Sulfate reduction occurred at both the contaminated and uncontaminated sites; however, median SO42− reduction rates were higher at the contaminated site. Although estimated SO42− reduction rates are relatively high, significant decreases in NVDOC were not observed at the contaminated site. Organic compounds more labile than the leachate NVDOC may be present in the root zone, and SO42− reduction may be coupled to methane oxidation. The results show that sulfur (and possibly nitrogen) redox processes within the top 2 m of the aquifer are directly related to recharge timing and seasonal water level changes in the aquifer. The results suggest that SO42−reduction associated with the infiltration of recharge may be a significant factor affecting natural attenuation of contaminants in alluvial aquifers.

Publication type Article
Publication Subtype Journal Article
Title Recharge processes drive sulfate reduction in an alluvial aquifer contaminated with landfill leachate
Series title Journal of Contaminant Hydrology
DOI 10.1016/j.jconhyd.2006.03.005
Volume 86
Issue 3-4
Year Published 2006
Language English
Publisher Elsevier
Contributing office(s) Toxic Substances Hydrology Program
Description 23 p.
First page 239
Last page 261
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