Installation of nested monitoring well network where groundwater discharge to surface water occurs.

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Assessment of the Fate of Fuel Oxygenates and Fuel Additives in Contaminated Ground-Water Systems: TOXIC Substances Hydrology Field Investigation

Project Number: 2519-B8Z04
Project Chief: Jim Landmeyer
Cooperator: USGS Toxics Substances Hydrology Program
Period of Project: October 1, 1996 to October 1, 2008

SUMMARY

The use of the fuel oxygenate MTBE is widespread and of high volume in some gasolines. Hence, its release to the aqueous environment is ubiquitous and can approach concentrations in groundwater above acceptable levels. This research is designed to quantify the fate of MTBE, and potential replacement fuel oxygenates (such as TAME and DIPE), as well as the fuel additive EDB in groundwater and surface-water systems.

Map showing location of the Toxics investigation into MTBE-contaminated groundwater

PROBLEM

Use of multilevel piezometers to assess this discharge. The blue marks on the well casing indicate the level of groundwater in each well. The lowest groundwater level is in the shallowest well at far left, with higher groundwater levels to the left as the depth of the well increases. This is a classic groundwater discharge zone.

MTBE is biodegradable under aerobic and anaerobic conditions, and natural remediation processes can be enhances by the delivery of oxygen to anoxic systems. MTBE degrades to CO2 under oxic and anoxic conditions, and under methanogenic conditions to TBA. TBA biodegrades to CO2 under oxic and anoxic conditions.Oxygen addition to anoxic, petroleum-contaminated groundwater systems to promote aerobic contaminant biodegradation is a common approach used for site remediation. However, the addition of oxygen to anoxic aquifers does not necessarily result in increased aerobic contaminant bioremediation. Oxygen-based remediation at the Toxic Substances Hydrology Program Intensive Field Site in South Carolina produced dramatically different results of contaminant remediation. In one area, oxygen-release compound injected into the subsurface did not increase the low level of dissolved oxygen (DO), and concentrations of benzene, toluene, and methyl tert-butyl ether (MTBE) remained relatively unchanged. Conversely, the same oxygen-release compound injected less than 200 meters downgradient rapidly increased DO levels from 0 mg/L to greater than 12 mg/L, and benzene, toluene, and MTBE concentrations decreased by up to 87%. These different results were found to relate to differences in hydrologic conditions between the two areas prior to the oxygen addition, which gave rise to differences in ambient microbial community structure that ultimately affected remediation results.

OBJECTIVES AND SCOPE

Determine the fate of fuel oxygenates, both current and proposed, and fuel additives in surface- and groundwater systems, using natural and enhanced remediation processes.

Conceptual model of significant MTBE biodegradation in the rather narrow hyporheic zone compared to extensive MTBE transport in the aquifer.

The microbial community beneath the stream in the hyporheic zone rapidly degrades MTBE into carbon dioxide, as indicated in the results from laboratory experiments.

APPROACH

Use a combination of field and laboratory approaches to determine the fate of MTBE and alterative fuel oxygenates and additives in the surface-water and groundwater environments.

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.