Lyle G. Bruce, Ph.D., CPG1, Jim Cuthbertson
2, J. Scott Ziegler
1, Arati Kolhatkar, M.S., C.E.
1 and Brent Graves, LPG
3, (1)BP Remediation Management, (2)Delta Consultants, (3)Hull & Associates Inc.
At this Central Indiana site, dissolved benzene levels were relatively high fifteen years after the service station closed. Dissolved concentrations were above those allowed for approved MNA, despite the facts that the UST source area had been over excavated at the time the facility closed and a pump and treat system had operated for a period of time. In 2004, dissolved benzene levels near the source area hovered at 1000 ppb. Inorganic analyses showed that dissolved sulfate was present naturally in the aquifer outside the hydrocarbon plume with a mean concentration of 78 mg/l. In the dissolved phase BTEX plume area, however, sulfate concentrations were substantially reduced to concentrations less than 10 mg/l. Additionally, a shadow of reduced sulfate concentration extended downgradient of the plume area. This was a very strong indicator that the hydrocarbon plume had gone anaerobic and was undergoing sulfate reducing conditions. The hydrocarbon plume was apparently being degraded by sulfate reducing bacteria, but at a slow rate. In 2004, high concentration sulfate solutions were added to the source area through an infiltration trench three times over a five month period. Within 841 days from the last application, dissolved benzene concentrations had decreased an order of magnitude and the site met criteria for MNA. Monitoring wells in the plume area progressed from stable benzene concentrations to attenuation rates between -.0022 to -.0064 per day (half lives between 315 and 108 days). This site demonstrates that if a plume is already undergoing natural sulfate reduction, the addition of concentrated sulfate solution to the source area will increase the attenuation rate. Ethylbenzene did not decrease in concentration in some wells that had substantial benzene declines. Toluene and xylenes did decrease. This is evidence of selective degradation by the microorganisms and that dilution was not a factor in the attenuation rates.
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Manuscript Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Remediation® Conference
