ARCHIVED REPORTS_XR0010358 - Laserfiche WebLink

Home Browse Search

WNILCPS 9ESCifR$CGIWATAMT3 ' byproducts Intrinsic bioremediation occurs when indigenous microorganisms work to bring about a reduction in the total mass of contamination in the subsurface without the addition of nutrients. 1 Microorganisms obtain energy for reproduction and cell maintenance by expediting the transfer ' of electrons from electron donors to electron acceptors. This results in the oxidation of the electron donor and the reduction of the electron acceptor Electron donors at the site are natural organic carbon and fuel hydrocarbons. Fuel hydrocarbons are completely degraded if they are ' utilized as the primary electron donor for microbial metabolism Electron acceptors are typically elements or compounds that occur in relatively higher oxidation states, typically electron acceptors include oxygen, nitrate, ferrous iron, sulfate and carbon dioxide. Microorganisms preferentially utilize electron acceptors while metabolizing fuel hydrocarbons. Dissolved oxygen is utilized first as the primary electron acceptor. After the dissolved oxygen is consumed, anaerobic microorganisms use electron acceptors in the following order of preference nitrate, ferric iron, sulfate and finally dissolved carbon dioxide. Depending on the type of electron acceptors present, pH conditions and redox potential, ' anaerobic biodegradation can occur by denitrification, ferric iron reduction, sulfate reduction, or methanogenesis Other, less common anaerobic degradation mechanisms such as manganese reduction may dominate if the physical and chemical conditions in the subsurface favor use of these electron acceptors. Evaluation Evidence supporting the effectiveness of intrinsic remediation of fuel hydrocarbons at the site is supported by trends in dissolved BTEX concentrations in groundwater at the site (see Figure 10) The highest BTEX concentrations have been found in MW-2 and MW-2A 1 In monitor well MW-2A, from 1995 though September 1997, the concentrations of the BTEX contaminants and TPH have decreased to approximately 5 to 23 percent of the July 1995 levels There has been limited sampling of MW-2 because of problems with the well, however, in the two samples taken at this location (1988 and 1997) there has been approximately a five-fold decrease in concentration levels of the BTEX contaminants. iThere is additional information supporting intrinsic remediation of fuel hydrocarbons Well MW-7 is directly downgradient of MW-2 and MW-2A. When concentrations in monitor well MW-7 are compared with those in MW-2 or MW-2A the concentrations of the BTEX ' contaminants decrease by more than 99 percent Since the distance between monitor well MW-2 and MW-7 is only 120 feet and the groundwater flow velocity is approximately 6 7 feet per ' year, dissolved contamination should have reached monitor well MW-7 in 6 to 12 years The BIOSCREEN model was used to simulate the movement and degradation of BTEX in the ' groundwater under the influence of advection, dispersion, sorption and biodegradation The purpose was to 1) predict the future extent and concentration of a dissolved contaminant plume, 2) to assess the possible risk to potential downgradient receptors 3) and to provide technical ' support for intrinsic remediation This model supports the fact that biodegradation is occurring • and that the contaminant plume will be limited in extent (see Appendix A) L1PR0lEC YSANJOQ%iSA\AM0NPL 33