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

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 Cl⁻gradient 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.