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Human Health Screening Evaluation <br /> Surland Homes—Bridle Creek February 12,2009 <br /> Previous subsurface investigations have shown that the site is predominantly underlain by silt <br /> and clay, with isolated sandy lenses, to approximately 16 feet bgs. Groundwater is typically <br /> encountered at approximately 5 to 7 feet bgs. Monitoring at the site has shown a north- <br /> northwesterly groundwater flow. <br /> 1.3 PREVIOUS ENVIRONMENTAL INVESTIGATIONS <br /> Geomatrix and SAIC performed environmental investigations at the site starting in 1998. The <br /> results of those studies are summarized in sections 1.3.1 and 1.3.2. <br /> 1.3.1 Initial Soil and Groundwater Investigations <br /> A soil and groundwater investigation was performed by Geomatrix in 1998. Eight soil borings <br /> were advanced. Nine soil samples and three groundwater samples were collected for analysis. <br /> Following this investigation, two groundwater monitoring wells (TWA and TW-2) were <br /> installed at the request of San Joaquin County. A third well was installed at the request of the <br /> RWQCB in 1999. Quarterly groundwater monitoring activities were performed through April <br /> 2000. Well TW-2 was destroyed prior to April 2000 due to development in the area of the well <br /> (Geomatrix, 1999). <br /> The RWQCB issued a letter in February 2003 requesting an additional quarter of groundwater <br /> monitoring and sampling to confirm that total petroleum hydrocarbons quantified as diesel <br /> ' (TPHd) concentrations were stable (or less than the water quality objective for the TPHd <br /> (without silica gel] taste and odor threshold), and/or attenuating in the two remaining monitoring <br /> wells (RWQCB, 2003). CEMC responded by forwarding a work plan, prepared by SAIC (SAIC, <br /> 2003a), to the RWQCB on April 30, 2003. The work plan outlined the methods to conduct one <br /> round of quarterly monitoring for the two on-site groundwater monitoring wells. The RWQCB <br /> issued a letter accepting the work plan on May 5, 2003, and a report detailing the sampling <br /> results was submitted in October 2003 (SAIC, 2003b). <br /> ' 1.3.2 Health Risk Screening Evaluation <br /> Geomatrix prepared and submitted a Health Risk Screening Evaluation (HRSE) for the site in <br /> August 2002. The HRSE concluded that the theoretical lifetime excess cancer risk for an adult <br /> and child resident at the site is 1 x 10-6, equal to the acceptable target risk of I x 10-6. The report <br /> noted that the majority of the risk was associated with ingestion of groundwater. The DTSC <br /> issued comments on the HRSE in a letter dated December 19, 2003, stating that there was <br /> insufficient data to reach a conclusion regarding the overall human health risk, and requesting <br /> additional analytical data from the site. The 2003 DTSC letter also noted that a conceptual site <br /> model (CSM) had not been developed for the site. <br /> 1.3.3 Soil Sampling Work Plan and Response to Comments <br /> SAIC prepared a work plan in August 2006 that described a sampling program to collect <br /> additional data. The work plan addressed concerns expressed by DTSC regarding the number of <br /> soil samples collected for the Geomatrix 2002 HRSE, the depths of the soil samples analyzed, <br /> ' the detection limits for some of the constituents of potential concern (COPCs), and several other <br /> issues discussed in the 2003 DTSC letter (SAIC, 2006). <br /> 1 3 <br /> jri1 irm <br /> From Science to Solutions <br /> I <br />