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Scott Mansholt - 2 - 29 June 2006 <br /> Mountain House #4 and #5 0 <br /> Soil and Groundwater Investigation <br /> • Residual PAHs were less than their respective PRGs and are not considered a risk to <br /> human health. The residual hydrocarbons in the underlying soils will naturally degrade <br /> and the human health risks are at acceptable levels. <br /> • A final determination for Site #4 cannot be made until a human health risk screening <br /> evaluation is completed and submitted for review. <br /> • Soil and groundwater impacted by TPHc is contained beneath Byron Road and/or <br /> within the current pipeline corridor. Removal of impacted soil may not be feasible given <br /> that the majority lies beneath Byron Road or the pipeline corridor. <br /> • The petroleum hydrocarbons identified at this site by fuel fingerprint analysis appear to <br /> be related to a refined products pipeline(s) and will be addressed separately. <br /> • Similarly, it appears that TPH in soil at Site #5 has been defined in extent to less than <br /> 150 feet in width and less than 32 feet deep, and occurs beneath Byron Road and/or <br /> within the pipeline right-of-way corridor. However, groundwater has not been <br /> adequately delineated downgradient, north of boring MH5-7 and additional borings and <br /> analysis are needed. In addition to the two borings proposed by SAIC, Regional Board <br /> staff recommends that a third boring be included approximately 75 feet northwest of <br /> proposed boring location MH5-8. <br /> • A well survey for domestic wells identified four private wells within a one-half mile radius <br /> of Sites #4 and #5. Considering the depths of the sanitary seals coupled with the <br /> distance from the project site, dissolved petroleum constituents have likely not affected <br /> these wells. <br /> We concur with the proposal to advance downgradient borings at Site #5 to include a third <br /> boring as discussed above. <br /> By 15 September 2006 please provide the results of the additional characterization for Site #5 <br /> and the results of the human health screening evaluation for Site #4. Additionally, a human <br /> health screening evaluation for Site #5 should be included provided the extent of impacted <br /> groundwater downgradient of MH5-7 is successfully delineated as discussed above. <br /> If you have any questions or comments, please contact Dean Hub and at (559 445-5179. <br /> SHELTON R. GRAY C. DEAN HUBBARD <br /> Senior Engineering Geologist Engineering Geologist <br /> P.G. No. 6357 <br /> cc: Michael J. Infurna, San Joaquin County EHD, Stockton <br /> Eric Ted-Bose, Trimark Communities, Tracy <br /> Luis Mercado, SAIC, Sacramento <br /> CDH/RlChevron 0VP1M1n.House 4&5 cover letter,June 06 <br />