Abstract

Aggressive and costly remediation measures are often taken at petroleum fueling facilities, with results that commonly achieve only minor reduction of risk to human health and the environment relative to the cost of the remediation.

In order to curb excessive remedial measures, the Utah Department of Environmental Quality’s Leaking Underground Storage Tank Section is implementing and encouraging innovative and less costly methods of assuring protection of human and environmental health. One of these methods is called natural attenuation, also known as intrinsic remediation.

Natural attenuation is a cost-effective passive cleanup measure in which the reduction of contaminant concentrations is achieved due to natural transport and fate processes such as adsorption/desorption, advection/dispersion, diffusion, and microbial degradation. Natural attenuation also reduces the risk of exposure by significantly reducing contaminant phase partitioning and media transfer often caused by some conventional, but costly methods of remediation such as pump and treat, vapor extraction, or dig/haul/aerate. These conventional methods which usually involve and rely upon media and phase transfer of contamination from the subsurface to the sub-aerial breathing zone. Alternatively, natural attenuation ensures the complete destruction of contaminants in situ.

The facility selected as the topic of this paper is typical of over 2,000 sites managed by the LUST section. The hydrogeologic conditions and fate and transport processes beneath the facility are typical of the shallow unconfined aquifer that characterize many of Utah’s intermontane basins. Natural attenuation of gasoline-related benzene was demonstrated at a fueling facility located north of Salt Lake City, Utah, wherein the observed soil and ground-water concentration decreased over time due to natural processes, including microbial degradation. This phenomenon was confirmed by long-term ground-water monitoring and supported by contaminant fate and transport modeling.

The results of this study show that adsorbed and dissolved contaminant plumes attenuate rapidly, even when adsorbed phases continually leach to ground water. It also demonstrated that limited site characterization data are often sufficient for modeling. If applying the basic principles of natural attenuation results in a low risk of exposure to potential receptors, numerous LUST case files can be closed out.