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Memo to Mr. Chris Flower - 3 - 29 July 2021 <br />residential screening levels were used to demonstrate compliance with the most <br />conservative cleanup goals. <br />EXTENT DEFINED <br />Using a PID and red dye diesel additive as a visual indicator, the extent of impact was <br />determined to be approximately 44 feet long by 11 feet wide. Sidewall samples were <br />collected approximately every 30 linear feet along the perimeter of the excavation and <br />floor samples were collected approximately every 350 square feet. Confirmation sample <br />analytical results indicate that the extent of contamination was captured and removed <br />by the excavation. <br />THREAT TO GROUNDWATER <br />The remaining constituents do not pose a threat to groundwater due to their low <br />solubility in water and low remaining concentrations in soil. <br />THREAT TO HUMAN HEALTH <br />The threat to human health is low considering the confirmation samples were two to <br />three orders of magnitude below the residential shallow soil ESLs. Also, soil containing <br />residual concentrations has been covered with approximately three feet of clean fill and <br />railroad tracks. <br />SUMMARY <br />Based on the above evaluations Central Valley Water Board staff concurs with UPRR <br />that cleanup of the 22 July 2020 diesel release was completed to the extent practicable, <br />and residual concentrations that could be related to this incident do not appear to <br />present either a threat to water quality or human health.