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Ms. Marilyn Petruescu September 30, 2024 In 1988, Simplot removed the 10,000-gallon diesel UST and the 10,000-gallon gasoline UST that were located adjacent to each other within the same excavation pit. During UST removal, the diesel UST was observed to be corroded and leaking along a seam. The gasoline UST was not observed to be leaking. Staining was observed beneath each UST. Two confirmation soil samples were collected from the bottom of the diesel and gasoline excavation pit at 13.5 feet and 15 feet below ground surface (bgs), respectively (Figure 3). As shown on Figure 3, petroleum hydrocarbons, benzene, toluene, ethylbenzene, and xylenes (BTEX, collectively)were detected in the soil samples (Weston 1989). 1989 Site Investigation In 1989, Roy F. Weston, Inc. (Weston) completed a site investigation which included the installation of three monitoring wells (MW-1 through MW-3), advancement of three soil borings (BH-1 through BH-3), and collection of soil and groundwater samples. Petroleum hydrocarbons and BTEX were detected in the soil and groundwater samples. Site features and the associated historical soil analytical data is shown on Figure 3. Additional site activities associated with the LUST case were not performed until 2009 when the CVRWQCB requested that Simplot locate the previously installed monitoring wells, advance confirmation soil borings, and collect groundwater samples in their letter dated March 27, 2009 (CVRWQCB 2009). Annual groundwater monitoring and sampling was completed between 2009 and 2023 in response to the March 27, 2009 letter. Confirmation soil borings were not completed. 2009 through 2023 Groundwater Monitoring Annual groundwater monitoring and sampling was completed between 2009 and 2023 at monitoring wells MW-2 and MW-3. Monitoring well MW-1 has not been located since sampling began in 2009 and is believed to be covered by asphalt(AMEC, 2009). During the 2009 sampling event, groundwater samples were analyzed for total petroleum hydrocarbons as gasoline and diesel (TPHg and TPHd, respectively), BTEX, 1,2-dibromo-3- chloropropane (DBCP), and 1,2-dibromoethane (EDB). Groundwater samples collected during the 2011 through 2023 monitoring events were analyzed for BTEX. TPHg and TPHd has not been analyzed since 2009. During the most recent groundwater monitoring event in October 2023, BTEX was not detected in the groundwater samples collected from MW-2 and MW-3. PROPOSED SCOPE OF WORK Based on review of the historical records of the LUST site, and in response to the CVRWQCB's June 11, 2024 letter request for additional investigation activities, Arcadis, on behalf of GSH, has proposed the following scope of work to assess petroleum hydrocarbons in soil and groundwater near the former USTs: • Install two groundwater monitoring wells to further evaluate the concentrations of petroleum hydrocarbons in groundwater and to determine the groundwater flow direction in the vicinity of the former UST area. www.arcadis.com 2/7 Lathrop UST Site Investigation and Well Installation WP 9.30.24_final