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April 23, 1998 96-1113008.80 State of California Department of the Youth Authority Facilities Planning Division Attention: Ms. Corinda Yep, Chief-Hazardous Materials and Waste Program Subject:LETTER-REPORT-Re-Sampling of Former Diesel Underground Storage Tank Locations Northern California Youth Correctional Center 7650 South Newcastle Road Stockton, California Page 6 only low concentrations of TPH-d were found in the stockpiles/backfills of the west UST in the Dewitt Nelson School(64 mg/kg), the west and east USTs in the Karl Holton School (44 mg/kg and 470 mg/kg, respectively), and the east UST in the 0. H. Close School (57 mg/kg). There is no evidence of TPH-d impacts in the native soil associated with the six USTs. The facility schools are built upon native soils of low permeability that, based upon geologic logs of two 50-foot deep borings drilled in the facility maintenance area, extend to approximately 50 feet bgs. Groundwater was not encountered in these borings. Based upon State Department of Water Resources information, the depth to groundwater is expected to be approximately 70 feet bgs. To evaluate potential impacts on groundwater quality, the TPH-d concentrations in soil were analyzed using the Leaking Underground Fuel Tank (LUFT) Field Manual, issued in October 1989 (updated 1991) by the State of California Leaking Underground Fuel Tank Task Force. A leaching potential analysis for TPH-d was conducted using Table 2-1 (page 29) from the LUFT manual. A copy of the table is included as Attachment D of this letter-report. Table 2-1 uses a scoring method to estimate TPH-d concentrations that can be left in-place without threatening groundwater. As shown in Attachment D, conditions at the DYA schools elicit a score of 48 points, the upper boundary of the middle range. This score provides a maximum allowable TPH-d concentration that can be left in-place of 1,000 mg/kg. If the score had been 49 points, 10,000 mg/kg TPH- d would be allowed. It should be noted that the LUFT leaching potential analysis does not include a score based upon soil types. Based upon the leaching potential analysis,the soil impacted by TPH-d could be silty sand or fat clay. Given that the sites being evaluated are underlaid by low permeability clayey silt, it appears that soil impacted by 10,000 mg/kg TPH-d diesel could be left in-place. RECOMMENDATIONS No further action is considered to be warranted for these USTs and it is recommended that the case be closed. These is based upon the following: ■ The potential sources of impact,the USTs, have been removed; ■ Only low TPH-d concentrations (44 mg/kg to 470 mg/kg) were detected in the