Comparison of Two Approaches for Assessing Public Health Effects from Vinyl Chloride in Shallow Groundwater Discovered During Reconstruction of an Earthquake-Damaged Freeway. K. M. Johnson and J. Loh, Tetra Tech, Inc., 3746 Mt. Diablo Blvd., Suite 300, Lafayette, CA 94549
Vinyl chloride was discovered in shallow groundwater and soil gas in a portion of the realigned transportation corridor for Interstate Highway 880 (I-880) near San Francisco, CA. The original freeway structure, known as the Cypress Freeway, was destroyed during the 1989 Loma Prieta Earthquake and reconstruction has progressed slowly because of public pressure to construct the freeway along a different route. Vinyl chloride, at concentrations up to 44 mg/l, and several other chlorinated compounds were detected in shallow groundwater sampled near a former chemical company. As a result of the discovery of hazardous materials in the subsurface environment, the public transportation agency was required to estimate the potential public health effects associated with subsurface installation of the freeway footings. Two approaches were required: 1) assessment of risks following U.S. EPA guidance and 2) assessment of risks according to the State of California’s Safe Drinking Water and Toxic Enforcement Act (also known as Proposition 65). Both sets of analyses were based on the prediction of airborne vapor releases during four components of the freeway footing construction process, including excavation of soil for the footings, pile driving and removal activities, coring (removal) of soil from the open-ended, 45-foot deep pilings used to support the freeway footings, and seepage of groundwater into the footing excavations. Following U.S. EPA guidance, carcinogenic risks were estimated for vinyl chloride exposures of nearby residents, children playing near the construction zone, and freeway construction workers. In contrast, following the Proposition 65 procedures, public health risks from vinyl chloride releases were determined by comparison of predicted daily exposures with an "acceptable daily intake level," also known as a "no significant release level" (NSRL). Although construction of the freeway footings was a one-time event, under the Proposition 65 process, exposures were defined legally as chronic daily intakes rather than the lifetime average daily intakes used in evaluating carcinogenic risks. Because of the differences in defining exposures, the USEPA and Proposition 65 results also differed, although the latter results were highly conservative. Redesign of three of the footings was agreed to in order to allow freeway construction to remain on schedule and provide the public relief from traffic congestion caused by the earthquake-related freeway collapse.