Evaluating Intrinsic Bioremediation At Four Sour Gas Processing Facilities In Alberta

Abstract

Mass attenuation through intrinsic bioremediation of the aromatic hydrocarbons-benzene, toluene, ethylbenzene, and xylene isomers (BTEX)-was studied at four natural gas processing facilities in Alberta. These compounds are commonly found in hydrocarbons associated with the natural gas processing industry, and their environmental behaviour and fate are relatively well understood. This study used four techniques to assess whether intrinsic bioremediation could attenuate BTEXcontaminated groundwater plumes at the four sites. Source mass data was not available at any site. At each site, historical data indicate that BTEX plumes were delineated and show no evidence of extensive migration. Geochemical evidence was reviewed to compare the distribution of dissolved BTEX concentrations to distributions of common electron acceptors (dissolved oxygen, nitrite+nitrate and sulphate) and metabolic byproducts of biodegradation (iron and manganese). Analysis of the data suggests that BTEX biodegradation is occurring at all four sites. Electron acceptors were depleted and metabolic byproducts were enriched within the BTEX plumes. Bacterial plate counts were generally higher in BTEXimpacted wells versus unimpacted wells at three of the sites. The opposite case was found at the other site. Microcosm experiments indicated aerobic biodegradation at three sites, while anaerobic biodegradation was interpreted at only two sites after incubation periods of approximately four to five months. Theoretical estimates of the biodegradation potential were calculated for each site, and indicate intrinsic biodegradation to have remediation potential at three locations. More field research is required to examine the relative role of biodegradation compared to other attenuation mechanisms (volatilization, sorption, and dispersion and chemical oxidation). Introduction Intrinsic bioremediation refers to the removal of contaminant mass through a series of microbial-catalyzed reactions occurring under natural conditions. Other naturally-occurring processes capable of attenuating contaminants (volatilization, dispersion, sorption and chemical oxidation) are not considered in this study. In general. these processes may also playa significant role in mitigating environmental impact by many contaminants. Biodegradation occurs because microorganisms are able to derive energy for reproduction and cell growth during electron transfer from the contaminant to electron acceptors. Biodegradation reactions are commonly classified as either aerobic or anaerobic, depending on the presence of oxygen. A third type of reaction, microaerophillic biodegradation(1), is considered an aerobic reaction, occurring under locally aerobic conditions in an environment generally classed as anaerobic. Conditions can be considered aerobic when dissolved oxygen (DO) concentrations are greater than 1 – 2 mg/L(2). Biodegradation of BTEX compounds under aerobic conditions has been quite clearly established(3, 4). During aerobic biodegradation, oxygen is consumed (reduced) by the bacteria to create water. The contaminant is correspondingly oxidized, freeing up carbon either to be released as carbon dioxide, or incorporated by the microorganism as cell mass. Ultimate end-products of aerobic biodegradation include water, carbon dioxide, and cell mass(4). Anaerobic biodegradation describes a series of reactions where microorganisms use any of several alternate electron acceptors because there is insufficient oxygen availability. The order in which electron acceptors are used reflects the decreasing relative energy derived by the microorganisms from the