Sediment-bound sulfate and ferric iron are proposed electron acceptors in the bacterial degradation of organic contaminants (DOC to 200 mg/L) in a ground water leachate plume from a closed municipal landfill, Norman, Oklahoma. We characterized sulfate and ferric iron in Canadian River alluvium variably affected by the landfill leachate and estimated the impact these electron acceptors have on organic degradation.
Five cores and adjacent wells located up to 400 m from the landfill were sampled. Mud samples contain more total sulfur (avg. 0.14 wt.%) than the sand (avg. 0.02 wt.%) but no systematic differences were detected between background and contaminated sediments. Iron sulfides are the dominant sulfur form. Acid-soluble sulfate was detected only in mud samples (avg. 0.02 wt.%). The groundwater concentration of sulfate was markedly lower in the leachate (<0.05 mM) relative to background ground water (up to 5 mM). The sulfur isotopic composition of dissolved sulfate is consistent with bacterial reduction except for samples from sand >3 feet thick that has been exposed to leachate for >5 years. The isotopic composition of these samples was similar to background sulfate (10 per mil) and is attributed to the dissolution of detrital barite. The distribution of sulfate-reducing bacteria was estimated using sulfur-35 imaging. Results indicate no activity in mud, spotty activity in sands, and peak activity near sand/mud contacts. The higher activity near sand/mud contacts may be related to sulfate diffusion from mud.
Rapidly reducible ferric iron (RRFe) (0.5 N HCl soluble) up to 0.25 micromoles/g was detected only in background and recently contaminated sediments. More slowly reducible iron (SRFe) (hematite) was estimated using a Ti3+/EDTA extraction. Sand contains about 5 micromoles/g and mud approximately 35 micromoles/g of SRFe, with no significant differences between background and contaminated sites. Apparently, SRFE abundance has not been modified by the leachate.
Sulfate and ferric iron are present in the alluvium but their bioavailability is limited by low abundance, form, and accessibility. Organic degradation supported by solid phase electron acceptors in the alluvium is likely to be slow relative to flow rates within sand intervals.
Jason Masoner