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7.0 HUMAN HEALTH RISK ASSESSMENT A human health risk assessment was conducted using the RISCO risk assessment software. The RISCO is published by Groundwater Software.com,a software development company specializing in geosciences applications. The program has been (unofficially) approved by some regional boards of the State of California Water Resources Control Board to evaluate health risks at Leaking Underground Storage Tank (LUST)sites. The RISCO program consists of a series of linked workbooks using equations generally provided in screening guidelines promulgated by the SFRWQCB. These spreadsheets allow for the calculation of baseline human health risks based on user-provided inputs.The calculation incorporates site-specific data to identify source media, transport mechanisms, exposure pathways and potential receptors and to calculate baseline carcinogenic risk and noncancer toxic hazards relative to regulatory thresholds(i.e., 1 x 10-6 for excess cancer deaths and a Hazard Index of 1.o for noncancer toxic effects). Constituents of concern(COCs) at the Site were identified as TPH-d, MTBE, Benzene and 1,2-DCA.The exposure scenarios are limited to a worker(commercial or construction worker) potentially exposed to contaminated soil,groundwater and indoor air vapors(e.g.,the latter exposure would potentially occur in a building situated above areas where soil and groundwater impacts have been reported). Direct exposure to shallow non-potable groundwater is assumed to be an incomplete exposure pathway,and has therefore not been evaluated quantifiably. Based on the current depth to groundwater beneath the Site,the absence of any remaining vadose zone soil impacts beneath the Site following extensive soil removal actions, and the generally low volatility of constituents currently present in Site groundwater(primarily petroleum hydrocarbons in the diesel range)a soil gas survey was not deemed necessary at this time. Potential indoor air concentrations of(volatile) COCs were calculated using the Johnson &Ettinger(1991) fate and transport model. Available soil data were used to quantify risks; maximum (historic) reported concentrations were used as data inputs to calculate exposure point concentrations for soil and for potential indoor air vapors. Exposure point concentrations for potential indoor air vapors were determined based on fate and transport modeling using maximum reported soil and grab groundwater concentrations from borings collected post- excavation and the maximum of the last five quarterly monitoring results. Since the reported levels of volatile COCs in Site soils were approximately equal to or below laboratory detection limits, the fate and transport modeling associated with propagation of vapors from groundwater sources was used to determine potential indoor air concentrations of volatilized COCs. A summary of the potential human health risks calculated for commercial/industrial occupants exposed to COCs in indoor air is provided in Table 7 below, along with maximum groundwater concentrations over the last five quarters, and calculated potential indoor air levels for each COC. A summary of risks calculated for surface soil exposure is shown in Table 8. Copies of RISCO Model runs, including input and output data and assumptions,are included in Appendix F. The risks are calculated for a worker with a reasonable maximum exposure("RME") to Site contaminants. The calculated potential excess cancer risk associated with potential inhalation exposure to CDCs in indoor air was 3.28 x 10-9 —or approximately three in one billion-which, as noted above, is well below the typical threshold for acceptable excess cancer risk(i.e., 1 x 10-6, or one in a million). 10