ARCHIVED REPORTS FINAL SITE INVESTIGATION

Home Browse Search

FMS #24 Site Investigation Report California Army National Guard 3.5.5 Health risks for the carcinogenic chemicals (benzene and naphthalene) are estimated in terns of the incremental probability of developing cancer over a lifetime (that is, above the background rate of cancer) as a direct result of the estimated exposure at the site (EPA, 1989). For example, an incremental cancer risk of one -in -one -million (variously displayed as 1 in 1,000,000, 1 x 10-6, 10"6, or 1E-06) means that an individual's chance of developing cancer has increased by 0.000001. The cumulative cancer risk is obtained by summing the cancer risk estimates of the individual carcino- genic chemicals. A 10"6 cumulative cancer risk represents the point -of -departure for regulatory agencies, with risks below this considered de minimus (inconsequential) and risks above this value requiring continued investigation. A 10-6 increase is very small, considering that the background rate of cancer from any and all causes in the United States is approaching 50% (0.5) (American Cancer Society, 2006). 3.5.6 A noncancer hazard estimate is the ratio of an exposure estimate to a "safe" (no -adverse - effect) reference concentration (RfC) for inhalation exposure, and is expressed as a hazard quotient (HQ). The HQ is a mathematically derived index that expresses the relationship between predicted exposure concentrations and an RfC. If an HQ is greater than 1 (exposure is greater than the RfC), then this is an indication that the exposure pathway should continue to be evaluated. If the HQ is less than 1, then exposure is less than the RfC, and adverse effects are not expected. A hazard index (III) is a summation of HQs from all the noncarcinogenic chemicals, and is an indicator of the potential for cumulative noncancer effects. 3.5.7 Risk -assessment results for the Stockton FMS #24 Site (Tables 3-7 and 3-8) indicate that the cumulative cancer risks for existing commercial use (5x 10-) and hypothetical residential use (1 x 10"6) do not exceed the 1 x 10-6 point -of departure. Thus, there are no significant cancer risks expected for existing commercial or future residential uses of the site based on VOC concentrations measured in soil vapor. Cumulative noncancer hazards are also below the 1-1I=1 point -of -departure 40 (HI=0.004 for existing commercial use, and HI=0.024 for hypothetical residential use) and, consequently, there are no significant noncancer health hazards expected for commercial or future residential uses of the site based on the VOC concentrations measured in soil vapor. 3.6 Contaminant Migration Potential 3.6.1 Quantitative information has not been obtained regarding the physical properties of surface and vadose zone soils in the former UST area; therefore, the rate of surface water infiltration and the rate of migration of possible contaminants cannot be evaluated. Surface water runoff at the site flows toward the west. The potential for surface water infiltration exists because approximately 90 percent of the site is paved. Therefore, any contaminants present in the subsurface soils could continue to migrate to groundwater. 3.6.2 Potential migration pathways at FMS #24 include indoor/outdoor air, surface water, soil, soil gas, and groundwater. The primary release mechanism is the former UST location; thus, leaching of contaminants downward within the subsurface to groundwater is the potential pathway. 3.6.3 Based on the limited information available and site conditions at FMS #24, the potential for the exposure of receptors (e.g., workers at the FMS) to any contaminants from the former UST location through inhalation, ingestion, or dermal contact is possible; therefore, there is a potential health risk at the site. Potential contaminant releases at the former UST location occurred primarily below grade. Most of the site is paved; therefore, the transport of near -surface contamination through indoor and outdoor air or surface water runoff is minimal. The transport of contaminants through outdoor air, surface water runoff, and infiltration to the subsurface is possible beyond the former December 2007