EPA Grant Number: R83-0633-010
Title: Identifying the Signature of Natural Attenuation in the Microbial Ecology of Hydrocarbon Contaminated Groundwater Using Molecular Methods and "Bug Traps"
Investigators: K.L. Sublette, L. Ford, D. White, A. Peacock
Institution: University of Tulsa
EPA Project Officer: Bala Krishnan
Project Period: 3-1-03 to 2-28-04
Project Amount: $147,303
Research Category: Intrinsic bioremediation
The most important mechanism of natural attenuation leading to destruction of the contaminants in groundwater is intrinsic bioremediation. It has now been clearly established that BTEX hydrocarbons are amenable to microbial degradation even in the absence of oxygen under the right conditions with respect to geochemistry, environmental parameters, and microbial ecology. More recently it has been shown that aliphatic hydrocarbons are also susceptible to anaerobic biodegradation by similar mechanisms. When clear proof of natural attenuation exists and environmental receptors will not be threatened during the life of a hydrocarbon plume, a risk-based management approach can be both cost- effective and protective of human health and ecological receptors.
However, gathering the necessary evidence for natural attenuation can itself be a costly undertaking. Conventional indicators of intrinsic bioremediation include the distribution of hydrocarbons, metabolites, and the correlation of temporal trends with the concentrations and distributions of geochemical parameters (electron acceptors, products of reduction of electron acceptors, DO, redox potential, hydrogen, etc.). These data are typically collected over the entire plume and in suitable control areas over a period of time at significant cost. The actual extend of site characterization required to support a risk-based management strategy varies from state to state. However, the goals remain the same: to deduce the prevalent bioprocesses in the subsurface and to determine whether natural attenuation will prevent exposure of environmental receptors to the hydrocarbon plume. With respect to the prevalent bioprocesses the key word here is deduced - that is, these data amount to circumstantial evidence of intrinsic bioremediation. What is needed is a definitive signature. We propose that this signature lies in the in situ microbial ecology and that this signature can be obtained a reduced cost compared to more conventional site investigations.
We propose to search for the signature of intrinsic bioremediation of petroleum hydrocarbons in the microbial ecology of the contaminated groundwater. The principal field site to be used in this project is the Brewer's pooling unit of a Ft. Lupton, CO gas production site currently operated by Kerr-McGee Corp. This site was formerly operated by Amoco Production Co. and was for seven years the subject of an intensive site investigation of the mechanisms of intrinsic bioremediation of gas condensate hydrocarbons which contaminate groundwater at the site. The site contains an elaborate network of groundwater monitoring wells in both the plume and an upgradient control or uncontaminated area. Over seven years (1993-1999) of quarterly groundwater monitoring data are available, as well as the results of an intensive investigation of the vadose zone (1998-1999) and the results of numerous microcosm studies using sediments from the site. All of these data support the intrinsic bioremediation of gas condensate hydrocarbons at the site by aerobic and anaerobic mechanisms (principally sulfate reduction and methanogenesis) and a stable or shrinking plume. These data were used to support the acceptance of a risk- based management strategy of the site by the Colorado Dept. of Environmental Quality. One of us (Sublette) was a principal investigator on both the groundwater and vadose zone studies and is very familiar with the site and the data. For comparison we will also use two other field sites near Hobbs, NM contaminated with gasoline by leaking USTs which have been characterized to a lesser extent.
It is hypothesized that the in situ microbiota contain a signature of past and present hydrocarbon exposure and utilization. A comprehensive measure of the in situ microbial ecology of a site will yield the identity of this signature. We maintain that a definition of hydrocarbon natural attenuation in terms of a site's microbial ecology is direct and definitive, whereas a definition in terms of contaminant chemistry and/or geochemistry or hydrology is indirect and inferential. It is well established that indigenous microbes generate various compounds within their cellular structure (biomarkers) that reflect in situ conditions. Therefore, the microbial community contains a record of the sum microbial response to the environment that is written from changes in the biochemistry of individual organisms. We propose that these responses, in association with knowledge of available electron acceptors and donors of a site, will define the signature of a successful hydrocarbon natural attenuation process at a low cost.