Abstract

Effective utilization of remedial strategy may be the most constructive way for the hydrogeologist to promote rational science in the environmental world today. Among the skills utilized by the seasoned environmental professional, quantitative calculation of risk or performing risk assessment may be one of the most productive. Sites with known subsurface groundwater contamination are commonly assigned cleanup standards based upon stringent maximum contaminant levels (MCLs). MCLs are commonly known as federal drinking water standards and are well accepted by environmental regulators. Due to their availability and regional acceptance, they are often applied as cleanup standards across the board. Risk assessment cleanup levels are now being incorporated as groundwater standards in several states including Utah.

The universal application of MCL cleanup standards to contaminated, shallow, poor quality groundwater may cause ineffective use of resources. Accordingly, in many cases, natural volatilization and biodegradation (passive remediation) are viable options. Installation and operation of active remediation systems at sites of lower contaminant concentrations often have insignificant cleanup effect. A preferable use of resources would be to limit active remediation to sites of higher contaminant concentrations. The key is knowledge of where to divide between lower and higher contaminant concentrations. This knowledge is most effectively defined by risk assessment.

A case study of risk assessment was carried out in Woods Cross, Utah. The hydrocarbon-contaminated site is adjacent to a refinery, oil pipeline, and railroad transportation facility. On-site hydrocarbon impacted soil and groundwater generated during past and ongoing hydrocarbon spills and releases, effectively condemned the property from further development.

Historical and recent soil and groundwater analysis data collected on- and off-site were compiled and categorized by MCLs, Utah recommended cleanup levels (soil, level 1), and mean concentrations and ranges of elements for the Western United States. Benzene was the only constituent of concern, based upon its widespread persistence and toxicological sensitivity. The risk assessment evaluated potential risks to human health and the environment from exposure to benzene at the highest level detected on-site. Exposure point concentrations for ambient air were developed using conservative models that, when combined with standard intake assumptions, were used to estimate potential exposures and potential carcinogenic health risks. The upper bound excess lifetime cancer risk to a future adult resident living on our case study site from birth to age 30 from inhalation exposures were calculated to be within the acceptable risk range of 10 ⁴ to 10⁻⁶ of the United States Environmental Protection Agency (EPA). Based upon submittal of the calculated risk and the EPA acceptable range of risk, rational decisions concerning abatement, investigations, remedial programs, and closure are more easily negotiated.