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Ms Vicky McCartney Page 2 of 3 • requesting further gtoundwater sampling to confirm the trend of declining concentrations and asked that the potential for vapor intrusion into existing onsite buildings be evaluated APPROACH As an initial screening evaluation, existing concentrations of contaminants in groundwater were compared to the default(Tier 1) Environmental Screening Levels (ESLs) published by RWQCB in the document entitled Screening foi Environmental Concerns at Sates 1vith Contaminated Soil and Gi oundwater—Inter im Final(CA Regional Water Quality Control Board, San Francisco Bay Region, February 2005) The exposure and physico-chemical parameters used in the document and the methodology of quantifying risk are consistent ..,th current U S EPA Guidelines, and essentially equivalent to procedures outlined in the Prelnninai3, Endangeinient Assessment Manual (DTSC, 1994) and the California Hinman Health Screening Levels published in December 2004 by the CA Office of Health Hazard Assessment(OEHHA) The ESLs for human exposure are maximum contaminant concentrations in groundwater that would not be expected to create a significant risk to individuals occupying the contaminated property The ESLs were calculated using conservative default hydrogeological and exposure parameters and are presented in the document both for unrestricted Iand use (residential) and for commercial/industrtal property use The published ESLs for vapor intrusion were calculated by the Johnson and Ettinger equations in use by the US EPA and other regulatory agencies and are intentionally very conservative ('maximizing predicted risk) i� If the contaminant concentrations at a site do not exceed the Trei 1 ESLs no further evaluation is warranted to quantify risk CONTAMINANTS OF CONCERN, REPRESENTATIVE CONCENTRATIONS AND COMPARISON WITH TIER I ESLs Residual Contaminants in Groundwater Based upon the lack of detectable concentrations of toluene, ethylbenzene and xylenes during the last four sampling events, only benzene and TPHg were evaluated as contaminants of concern The concentrations used in the evaluation were the inean of the last four sampling events in samples collected from monitoirng wells MW1 and MW3 In calculating the mean, non-detectable results were treated as positive results at the detection limit concentration The contaminant concentrations in groundwater are compared with the Tier 1 ESLs below TIER I Residential TIER I Commercial COC Representative Cone Screenin Level' Exceeded) Screening Level' Exceeded� Benzene 44 parts per billion(ppb) 1,900 ppb NO 6,400 ppb NO I TPHg 61 ppb — NO* -- NO* Notes j Table E-I a,Gi oitndii ater Screening Lcilels for Evaluation of Potential Vapor Inas uvion Concerns � * ESLs for TPHg not included in Table E-1 a,use of soil gas reconiniended,see discussion below i 0 1GROUNDZE W L11ti51,W1RSLGWTxi doe