Reso PC17-07 0032 PC Reso Approving CEQA - 860 Charter St

Browse Search

LSA ASSOCIATES, INC. SEPTEMBER 2016 ROCKETSHIP REDWOOD CITY ELEMENTARY SCHOOL PROJECT INITIAL STUDY/ MITIGATED NEGATIVE DECLARATION P:\RWC1401B Rocketship School\PRODUCTS\IS-MND\Public\Rocketship Public Review 09-29-16.docx (09/29/16) PUBLIC REVIEW DRAFT 54 hydrocarbons (TPH) in the diesel range, TPH in the motor oil range, TPH in the gasoline range, and VOCs. Four shallow soil samples (one from the location of the sump, one from under the metal storage shed, one from behind the existing warehouse, and one from the proposed location of the school garden) were also analyzed for metals and polycyclic aromatic hydrocarbons (PAHs). Shallow soil samples collected from behind the existing warehouse near a pole-mounted transformer and the proposed location of the school garden were analyzed for PCBs and asbestos. Grab groundwater samples were collected from five of the borings, and the groundwater samples were analyzed for TPH in the diesel range, TPH in the motor oil range, TPH in the gasoline range, VOCs, and semi-volatile organic compounds (SVOCs). Four soil gas (vapor) samples were collected at 5 feet bgs and four sub-slab vapor samples were collected under the warehouse structure and analyzed for VOCs. The investigation found that the concrete slab section for the existing buildings consisted of approximately 6 to 8 inches of concrete over 4 to 6 inches of aggregate base rock. Subsurface soil observed in the exploratory borings primarily consisted of clay. Underlying the clay, at about 12 to 14 feet bgs, sand containing groundwater was encountered. The analytical results of the soil, groundwater, and vapor samples collected at the project site were compared to DTSC-determined health-risk screening values based on residential land use to determine whether the concentrations of chemicals reported in the soil, groundwater, or vapor samples might represent a health risk to future users of the project site. Concentrations below these screening values are not considered a significant health risk for a residential receptor. This is a conservative approach since screening values based on residential land use assume long-term and frequent exposures; students or school staff typically have less exposure, both in terms of duration and frequency, than a residential receptor.63 For the vapor samples, the screening assumed default vapor attenuation rates from the subsurface into indoor air and may under- or over-estimate the health risk.64 Based on the results of this investigation and the health-risk screening evaluation performed, the soil and groundwater do not contain chemicals for which they were tested at levels that represent a significant health risk. Also, no asbestos was detected by the analytical laboratory in the soil samples. However, benzene, 1,1-dichloroethene, and vinyl chloride were reported in one or more of the sub- slab vapor samples at levels that indicated subsurface VOCs may migrate into buildings at concentrations above residential indoor air screening values. The concentration of TCE and vinyl chloride in one of the soil gas sample collected at 5 feet bgs were also elevated. The TCE indoor air concentration estimated from the vapor samples collected also exceeded the U.S. Environmental Protection Agency’s (U.S. EPA) “prompt response action level.”65,66 63 Residential screening values assume an exposure scenario of 24 hours a day, 350 days per year, for 70 years. 64 Vapor concentration decrease (attenuate) as they move from the subsurface into indoor air. The attenuation occurs as a result of the processes that control vapor migration, which are diffusion and advection, coupled with the dilution that occurs when the vapors enter a building and mix with indoor air. The default attenuation rate is conservatively based on the 90th percentile of the U.S. EPA’s national empirical vapor intrusion database; the actual attenuation rate will depend on a number of factors such as chemical volatility, temperature, amount and magnitude of sub-slab cracks, and building interior pressure. 65 U.S. Environmental Protection Agency, 2014. EPA Region 9 Response Action Levels and Recommendations to Address Near-Term Inhalation Exposures to TCE in Air from Subsurface Vapor Intrusion, July, 9. ATTY/RESO.0032/PC RESO APPROVING CEQA - 860 CHARTER ST. EXHIBIT A REV: 06-15-17 PR Page 62 of 124