SR0080452_SSNL - Laserfiche WebLink

Home Browse Search

Appendix A Site Closure Request July 2,2010 Speedy Foods-Stockton Page 11 of 13 16.3 Groundwater Vapor Emissions to Indoor Air—MTBE and Benzene Data from petroleum impacted monitoring wells indicate that benzene and MTBE concentrations are -below the Tier I ESLs (Table E-1). The chemicals of concern are not found in concentrations that would pose a risk to human health or the environment. • Tier I ESLs for benzene hi groundwater are 540 micrograms per liter (µg/L). The most recent analytical results indicate concentrations of 210 gg/L in EWL The results indicate that benzene is below laboratory detection limits in MW3 and MW4. • Tier I ESLs for MTBE in groundwater are 24,000 gg/L. The most recent analytical results from EWl, MW3, and MW4 indicate concentrations of 390 gg/L, 60 gg/L, and 1.6 gg/L respectively. 16.4 Conclusions Based on Risk Assessment Model ESLs assume a groundwater depth of 10 feet bgs and the groundwater beneath the Site ranges between approximately 56 and 76 feet bgs. The assessment of groundwater vapor emissions to indoor air is extra protective based on the low groundwater elevation at the Site. The risk assessment indicates residual contarnination poses no health risk due to direct exposure or vapor intrusion. A soil vapor survey is not warranted for the Site. 17.0 Discussion of Fate and Transport Model Ground Zero performed a BIOSCREEN groundwater model that simulates remediation through natural attenuation of dissolved hydrocarbons at petroleum fuel release sites. BIOSCREEN is based on the Domenico three-dimensional analytical solute transport model and has the ability to simulate advection, dispersion, adsorption, and aerobic decay as well as anaerobic reactions that have been shown to be the dominant biodegradation processes at many petroleum release sites. Details of the model are discussed in Ground Zero's Site Closure Request and Send-Annual Groundwater Monitoring Report— Winter/Spring 2010 dated March 24,2010. Two TPHg scenarios were run. The first assumed an infinite mass of TPHg remaining in the source zone and the second assumed 50 Kg of soluble mass in the source zone. The output data is for a period of 30 years. Heavily weighting the model on the side of caution using elevated core concentrations, an infinite mass in the source zone, a long TPHg half-life, and doubling the plume size still indicates the leading edge of the TPHg plume represented as 0.001 mglL would remain under the site and highway over the next 30 years. Two similar benzene scenarios were run. The model was weighted on the side of caution using elevated core concentrations, an infinite mass in the source zone, a long benzene half-life, and. doubling the plume size still indicates the leading edge of the benzene plume does not reach 240--- feet in 30 years. Both scenarios predict the benzene plume will stay well within the confines of the site and highway.