SECTION A

Processes that Control the Natural Attenuation of Hydrocarbons and Fuel Oxygenates at Gasoline Release Sites

Gasoline releases are perhaps the most frequently cited cause of ground-water contamination. The most soluble constituents of conventional gasoline are the aromatic hydrocarbons - benzene, toluene, ethyl benzene, and xylenes (BTEX). Processes that control the transport and natural attenuation of BTEX in ground water have been the focus of longstanding activity in the Toxic Substances Hydrology Program through research projects conducted at Bemidji Minnesota; Galloway Township, New Jersey; and Laurel Bay, South Carolina. Results of this research are often cited by the petroleum industry as the concept of natural attenuation has gained acceptance over the past ten years. Regulators have realized that, in many cases, the use of engineered remediation to re-establish pristine conditions at a spill site is more ideological than practical. This awareness has culminated in the recent release of the U.S. Environmental Protection Agency's "Monitored Natural Attenuation Policy Directive 9200.4-17" from the EPA's Office of Solid Waste and Emergency Response.

Extensive monitoring of the geochemical signature of contaminated ground water at these sites has lead to fundamental understanding of BTEX degradation by indigenous microbes through aerobic and anaerobic reactions involving various terminal electron acceptors. In particular, this monitoring led to the development of using hydrogen (a ubiquitous intermediate of hydrocarbon degradation) concentrations to facilitate the evaluation of the efficiency of natural attenuation processes. Mathematical models of reactive transport of BTEX in the plumes have been applied to quantify reaction rates and study the dynamics of plume migration subject to natural attenuation processes.

BTEX mass loss from ground water to the unsaturated zone due to volatilization and aerobic degradation in the capillary zone also is a factor contributing to natural attenuation. Unsaturated-zone gas composition data collected at the Galloway and Laurel Bay sites and transport modeling allows for quantification of contaminant mass loss. For BTEX these two processes are coupled, that is aerobic degradation enhances the rate of volatilization. The overall process is significant near the source, where spilled product is at the water table.

Increased usage of oxygenated gasoline in recent years challenges the natural attenuation paradigm because methyl tert-butyl ether (MTBE), the most widely used fuel oxygenate, is much more soluble, less readily adsorbed, and believed to be much less reactive in many aquifer systems than BTEX. MTBE has been added to gasoline in small quantities as an octane enhancer since the late 1970s. However, MTBE use is now (1999) substantially greater; since the winter of 1992, the compound has been blended in gasoline in urban areas throughout the country to meet the gasoline oxygen-content requirements of amendments to the Federal Clean Air Act. The gasoline that contaminates shallow ground water at Laurel Bay, S.C. contains MTBE. The contaminated ground water discharges to an adjacent surface-water body. Consequently, this site has become the focus of gasoline release research in the Toxic Substances Hydrology Program.

Collaborative studies on MTBE at Laurel Bay include the following hydrologic compartments:

Combined, these collaborative studies suggest that pathways for natural attenuation of MTBE do exist; however, the magnitude of each pathway needs to be examined and compared to BTEX to understand the environmental liability associated with releases of MTBE-oxygenated gasoline.

For additional information contact:

James E. Landmeyer,
USGS, Columbia, South Carolina
(email: ), or

Arthur L. Baehr,
USGS, W. Trenton, New Jersey
(email: )