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Natural and Enhanced Remediation of Fuel Oxygenates and Gasoline at UST Spill Sites on Federal Facilities

Project Number: 2519-B9004
Project Chief: Jim Landmeyer
Cooperator: US Navy, SOUTHDIV
Period of Project: October 1, 1996 to October 1, 2008

SUMMARY

Map showing the location of the fuel leak and the resultant plumes of benzene and MTBE.

A priority issue to environmental managers is the isolation of waste and remediation of contaminated environments. Natural attenuation (passive bioremediation) is one strategy that can be used to remediate contaminated environments, although there is much to be learned about microbial and geochemical processes. Fuel released from underground storage tanks has caused contamination of groundwater by petroleum compounds and fuel oxygenates in Beaufort, SC.

PROBLEM

Oxygen addition to the two locations produced dramatically different results with respect to DO generation and contaminant decrease. Oxygen-release compound injected into the first location, a former underground storage tank (UST) source area, did not significantly change measured concentrations of DO, benzene, toluene, or MTBE. Conversely, oxygen-release compound injected 200 meters (m) downgradient of the former UST source area rapidly increased DO levels, and benzene, toluene, and MTBE concentrations decreased substantially. These different results appear to be related to differences in pre-existing hydrologic and microbiologic conditions that characterized the two locations prior to oxygen addition. For example, the aquifer downgradient of the former UST source area was anoxic, but received oxygen-saturated recharge through overlying permeable sediments during rainfall events. As such, the aquifer was characterized by high numbers of aerobic heterotrophic bacteria (as most probable number per milliliter [MPN/mL]), as well as bacteria capable of growth on dilute media containing benzene, toluene, ethylbenzene, and xylenes. In contrast, such oxygen delivery did not occur to the anoxic aquifer beneath the paved, former UST source area, and consequently the aquifer contained much lower numbers of aerobic heterotrophs and fuel-degrading bacteria prior to the addition of oxygen-release compound. Moreover, the absence of such oxygen-saturated recharge beneath the former UST source area resulted in the accumulation of reduced mineral- and soluble-inorganic species and attendant significant sediment/chemical oxygen demand. The results of this investigation indicate the important role that pre-existing hydrologic and microbiologic conditions, in addition to sediment/chemical oxygen demands, have on the outcome of oxygen-based remediation strategies, and suggest that these properties be evaluated prior to the implementation of oxygen-based remedial strategies.

OBJECTIVES AND SCOPE

Investigate the fate of BTEX and MTBE in groundwater at a release typical of those found on the MCAS Beaufort. Natural and enhanced bioremediation approaches to site restoration are being investigated. Oxygen addition to anoxic, fuel-contaminated groundwater systems is an accepted approach for promoting aerobic biodegradation of gasoline hydrocarbons and the fuel oxygenate methyl tert-butyl ether (MTBE). Some sites, however, do not exhibit increases in dissolved oxygen (DO) concentrations after oxygen addition, and at other sites initial decreases in contaminant concentration are followed by a "rebound" to original, pre-oxygen addition concentrations.

APPROACH

Results before and after ORC addition; MTBE decreased and oxygen increased.

Oxygen addition to anoxic, fuel-contaminated groundwater systems is an accepted approach for promoting aerobic biodegradation of gasoline hydrocarbons and the fuel oxygenate methyl tert-butyl ether (MTBE). Some sites, however, do not exhibit increases in dissolved oxygen (DO) concentrations after oxygen addition, and at other sites initial decreases in contaminant concentration are followed by a "rebound" to original, pre-oxygen addition concentrations. In order to more fully examine the environmental factors that control this common scenario, oxygen addition using an oxygen-release compound (a proprietary form of magnesium peroxide [MgO2]) was performed in two locations of a shallow, gasoline-contaminated aquifer in South Carolina characterized by benzene, toluene, and MTBE at concentrations greater than 1 milligram per liter (mg/L), respectively.

One reason why ORC injection worked in an anoxic contaminated aquifer is because natural recharge of oxygenated water keeps aerobic metabolism viable, as is shown in this plot of natural DO addition and then consumption during 3 rainfall events.

REFERENCES

Landmeyer, J.E., Chapelle, F.H., Herlong, H.H., and Bradley, P.M. 2001. Methyl tert-butyl ether biodegradation by indigenous aquifer microorganisms under natural and artificial oxic conditions: Environmental Science & Technology (35): 1118-1126.