DETERMINATION OF BIODEGRADABLE DISSOLVED ORGANIC CARBON (BDOC) IN A LANDFILL LEACHATE- CONTAMINATED AQUIFER (NORMAN, OK)

The potential for microbial degradation of dissolved organic carbon (DOC) was examined at an anoxic aquifer contaminated with landfill leachate in Norman, OK. Despite closure of the landfill in 1985, high amounts of DOC remain in the contaminant plume with concentrations of 225 mg/L at the landfill and >75 mg/L in the center of the plume. The persistence of DOC down-gradient suggests that much of the material is not biodegradable. Previous carbon fractionation work has confirmed that 65-70% of the DOC is comprised of the hydrophobic fraction. The availability of biodegradable carbon will exert a large control on microbial processes in the contaminant plume.

The goal of the current work was to develop a bioassay technique to examine the percentage of biodegradable DOC (BDOC) along the contaminant plume. A sulfate-reducing bacterial consortium (SRC) was enriched from the landfill and then inoculated into a subset of anoxic, filtered groundwater samples. In order to evaluate the potential for DOC consumption at the landfill, preliminary experiments were performed to evaluate the effect of the addition of (1) the SRC, (2) sulfate as an electron acceptor, (3) and nitrogen and phosphorus (N+P) as nutrients. The maximum amount of DOC consumption (9-10% in a 10-day period) occurred with the simultaneous addition of the SRC, sulfate, and N+P; 4% of the total DOC was consumed with the sole addition of the SRC. In contrast, indigenous bacteria only degraded DOC with additions of sulfate and N+P, consuming 4-5% of the total DOC.

In subsequent experiments, groundwater samples from 9 sites along the axis of the plume were amended with SRC, sulfate, and N+P. Relatively high amounts of BDOC (10-20%) were observed through the plume, including at sites furthest from the landfill. These results represent an estimate of biodegradation potential under non-limiting conditions of microbial biomass and electron acceptor availability. The in situ biodegradation may be overestimated using this approach, especially at sites where these factors may control carbon consumption. The use of bioassays to evaluate the potential for DOC consumption in contaminated aquifers will provide a useful tool for understanding the controls on microbial processes, many of which are tied to the natural attenuation of organic contaminants.