ARCHIVED REPORTS HUMAN HEALTH SCREENING EVALUATION

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GEOMATRIX majority of the risk is from ingestion of groundwater. The noncancer HI for an adult and child resident is 0.03, which is several orders of magnitude below the target HI of 1. A summary of the theoretical lifetime excess cancer risk and noncancer HI at the site is presented in Table 4. Appendix A presents the equations, models, and input parameters used to calculate the poten- tial lifetime cancer risk and noncancer HI; all input assumptions were based on conservative default parameters provided in the PEA. The process of estimating risk has inherent uncertainties associated with the calculations and assumptions used. Consistent with agency guidelines for health risk assessments, health pro- tective conservative assumptions and methods were used to address these uncertainties. As a consequence of using these health-protective assumptions and procedures, the calculated excess cancer risk and noncancer HI presented in this screening evaluation likely overestimate the actual health risks posed by residual petroleum at the site. Based on the screening health risk evaluation, the presence of residual petroleum constituents in soil and groundwater at the site does not pose an unacceptable human health risk to hypo- thetical future residents at the site, under the conditions evaluated. In addition, based on the conservative nature of the PEA evaluation, the presence of residual petroleum constituents in soil and groundwater should also not pose an unacceptable human health risk to future com- mercial and construction workers at the site. 4.0 ECOLOGICAL SCREENING EVALUATION Geomatrix reviewed the Natural Diversity Database(NDDB; June 2002) maintained by the California Department of Fish and Game for the Tracy and Union Island quadrangles. Ac- cording to the NDDB, two adult burrowing owls (i.e., athene cunicularia) were identified in September 1999 within a one-mile radius of the site. No other endangered, threatened or sen- sitive species identified by state and federal agencies were located within a one-mile radius of the site. The surrounding areas north of the site have since been developed into residential homes and are not likely to support ecological habitats. The residual petroleum hydrocarbons present at the site reflect historic spills from the OVP which was likely buried at depths of at least 2 to 4 feet bgs. During site investigations, the majority of the residual petroleum was found in soil at depths between 4.5 and 13.5 feet bgs. Most ecological receptors, including bur- rowing owls, are not likely to be directly exposed on a regular basis to affected soil located at these depths. Burrowing animals create dens in the upper four feet of soil and the area of af- fected soil is below the root zone of most plants. The results of the deeper soil samples indicate that the potential health risks to human receptors are at or below the acceptable levels. Given I:\Doc_SafeV,000s\6U7.007THRA�Su and HRA.ea 4