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Port of Stockton - 2 - 20 September 2018 Rough and Ready Island Draft UST Data Gap Work Plan ,no further action' eligibility under the LTCP that the Draft Work Plan may not fulfill. This includes: • Petroleum vapor intrusion to indoor air: It is necessary to comply with the LTCP media- specific criteria for petroleum vapor intrusion to indoor air. As specified in the LTCP, the media-specific criteria for petroleum vapor intrusion to indoor air can be accomplished by either 1) collecting TPH data, in addition to the benzene data, to demonstrate that TPH concentrations in the upper five feet of soil have attenuated to less than 100 mg/kg; or 2) collecting soil vapor data to demonstrate compliance with the threshold values provided in Appendix 4a or 4b of the LTCP; or conduct a site-specific risk assessment for the vapor intrusion pathway; or 4) control exposures through the use of mitigation measures. o Groundwater benzene data: 2008 groundwater samples at UST Site 817B yielded benzene concentrations of 2,500 pg/L at monitoring wells 917-3 and 917- 4. The benzene concentration at monitoring well 917-10 was also elevated at 600 pg/L. The LTCP media specific criteria for vapor intrusion to indoor air requires benzene concentrations in groundwater to be less than 1,000 pg/L and have a sufficient bioattenuation zone in soil. o TPH in soil: The 2008 soil samples also had exceedances of 100 mg/kg at 4 feet depth below ground surface (bgs) at samples 817-D2, 817-D3, and 817-D4. However, soil samples adjacent to these locations are not proposed in the Draft Work Plan. A sufficient bioattenuation zone according to the LTCP requires the upper five feet of soil to be less than 100 mg/kg to meet the media specific criteria for vapor intrusion to indoor air. • 2008 Soil Sample 817-D4 benzene: The Draft Work Plan does not propose sampling adjacent to the 817-D4 location from 2008. However, the benzene concentration from this sample (8.7 mg/kg) also exceeded the media specific criteria for direct contact and outdoor air exposure for commercial/industrial settings (8.2 mg/kg) in the LTCP. Recommended Actions: 1. Central Valley Water Board staff recommend the Port also collect soil samples for TPH and benzene analyses adjacent to the 2008 soil samples at 817-D2, 817-D3, and 817-D4. 2. Central Valley Water Board staff recommend the Port also collect groundwater samples from monitoring wells 917-3, 917-4, and 917-10 simultaneously with soil sampling to evaluate concentrations of TPH and benzene in groundwater. Groundwater samples should be collected following protocols appropriate for volatile organic carbon analysis. Comment 2: UST Site 9168 Issues: In 2008, soil sample C-5 at 5-8' bgs had a TPHd detection of 790,000 mg/kg. A concentration this elevated may indicate the presence of free product in groundwater. The groundwater-specific criteria of the LTCP requires there to be no free product. Recommended Actions: Central Valley Water Board staff recommend the Port also collect a grab groundwater sample at the 2008 C-5 sample location to determine if concentrations have attenuated and evaluate the existence of free product.