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J.R. Simplot Company - 2 - 11 June 2024 San Joaquin County Site data must be compared to the criteria in the State Water Resources Control Board's Low-Threat Underground Storage Tank Case Closure Policy (LTCP) to determine case closure eligibility. The LTCP is composed of eight General Criteria and three media-specific criteria. The LTCP media-specific criteria is discussed below: 1. Groundwater: The historical Site well network is composed of three wells installed in 1989 (MW-1, MW-2, MW-3). At the time of installation, MW-1 and MW-2 groundwater samples contained TPH and BTEX and MW-3 groundwater samples contained only BTEX. Benzene was reported at 2,800 micrograms per liter (ug/L) at MW-1 . The Water Quality Objective (WQO) for benzene is 1 ug/L. At the time of installation in 1989, Weston states that the groundwater in the immediate vicinity of the Site flowed northeast due to onsite groundwater treatment pumping (fumigant remediation). Water Board staff has observed groundwater flow toward the north regionally at nearby cleanup cases. MW-1 has not been located by subsequent environmental consultants after the 1989 sampling event. The annual samples collected from MW-2 and MW-3 did not adequately characterize petroleum hydrocarbon constituents dissolved in groundwater at the Site. Site data does not explain how most of the benzene mass observed in 2017 (8,760 ug/L benzene in MW-2) migrated. 2. Petroleum Vapor Intrusion to Indoor Air: Depth to water in 2023 was around 11 feet below ground surface (bgs). Site data appear to meet Scenario 3 criteria of the LTCP. 3. Direct Contact and Outdoor Air Exposure: Soil samples BH-1 through BH-3 and soil samples collected from MW-1 through MW-3 installation appear to meet direct contact criteria; with the exception that naphthalene was not analyzed. LTCP General Criteria are not met. A conceptual site model that assesses the nature, extent, and mobility of the release has not been completed. Secondary source petroleum hydrocarbon impact to soil and groundwater has not been adequately investigated; therefore, secondary source may not have been removed to the extent practicable. Based on the increasing trend of petroleum hydrocarbons immediately following drought years, it is likely that contaminant mass is still adsorbed to soil near the source area. Please submit a work plan to assess petroleum hydrocarbon constituents present in soil near the former USTs and to assess the nature and extent of impact to groundwater. By 12 August 2024, upload to GeoTracker a work plan to assess petroleum hydrocarbon constituents present in soil and groundwater near the former USTs and to investigate the extent of petroleum hydrocarbon impact to groundwater. If you would like to discuss the components of the work plan or the path forward for the UST investigation, please reach out to schedule a planning meeting with me.