BIODEGRADABLE CARBON AS A POTENTIAL CONTROL ON MICROBIAL COMMUNITY STRUCTURE AND FUNCTION IN AN AQUIFER CONTAMINATED WITH LANDFILL LEACHATE

Carbon availability is a dominant controlling factor for microbial activity in subsurface environments, including contaminated aquifers. We recently have initiated a study examining the quantity and quality of dissolved organic carbon (DOC) in an anoxic aquifer contaminated with landfill leachate in Norman, OK. The goals of the current study are to (1) develop a rapid method to quantify the “microbially available” or biodegradable fraction of the DOC (BDOC) and (2) determine how BDOC might influence rates of the dominant terminal electron-accepting processes (TEAPs) and the physiological profile of the microbial community.

To determine the amount of BDOC in the contaminated aquifer, a bioassay technique was developed where DOC consumption was measured in anoxic groundwater inoculated with an enriched sulfate-reducing microbial consortia (SRC). Preliminary experiments indicated that the maximum amount of DOC consumption (9-10% in a 10-day period) occurred with the simultaneous addition of the SRC, sulfate, nitrogen, and phosphorus. The amount of BDOC varied along the plume flowpath, with no BDOC found at some sites and the maximum amount being 10-20% of the total DOC. The absence of detectable organic acids in the plume also suggests that the DOC is largely refractory, and the high DOC at sites farthest from the landfill indicates this DOC persists long-term. C-limitation was further supported by the stimulation of sulfate reduction, occurring at very slow rates in situ, with the addition of 10 mM lactate. Thus, the low amounts of biodegradable carbon may limit microbial function. Differences in C availability also may affect the physiological profile of the microbial community, as investigated with BIOLOG. For example, the highest functional diversity of microorganisms occurred at the landfill, the site with the highest amount of BDOC.

Obtaining information about the amount of BDOC in a subsurface system will help us to understand the controls on microbial community function and structure and allow for a better prediction of ecosystem rates of TEAPs and biodegradation. Perhaps more importantly, this rapid method for estimating the degradation potential of DOC can be applied to other contaminated systems where such information is critical for making long-term decisions about bioremediation and other treatment options.