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Southwest Pond Assessment Work Plan Lathrop Facility,San Joaquin County,California 1.0 Introduction J.R. Simplot Company (Simplot) owns and operates a fertilizer manufacturing facility that uses three double-lined process water ponds (the West, North, and South Ponds) located at 16777 Howland Road in Lathrop, California (Figure 1). Discharge to the ponds is regulated by the Central Valley Regional Water Quality Control Board (RWQCB) under Waste Discharge Requirements Order No. R5-2019-0010 (RWQCB, 2019a). In addition, an inactive asphalt-lined pond, known as the Southwest Pond, is located south of the West Pond (Figure 2). On April 22, 2019, the RWQCB issued a letter to Simplot requesting that Simplot submit a closure plan for the inactive Southwest Pond (RWQCB, 2019b). On October 25, 2019, Simplot submitted the Closure Plan— Southwest Pond(Closure Plan), which described the general steps that will be performed to close the pond, including a soil assessment (Wood, 2019a). On November 20, 2019, the RWQCB reviewed the Closure Plan and requested that Simplot submit a work plan that describes the details of the soil assessment mentioned in the Closure Plan (RWQCB, 2019c). On November 22, 2019, Simplot informed the RWQCB that preparation of a work plan with the soil assessment details and implementation of that soil assessment was planned for 2020 (Wood, 2019b).This work plan proposes site characterization activities to address the RWQCB's request. 2.0 Background Simplot indicated that records describing the date of construction and design details of the Southwest pond are not available (Wood, 2019a). Simplot purchased the Lathrop facility from Occidental Chemical Company in 1983, and it is not known if the Southwest Pond was operating at that time.The Southwest Pond operated as part of a system of ponds, including the West Pond. Historical records are available for the West Pond, which was retrofitted with a double synthetic liner in 1994 and is currently operated in conjunction with two additional double-lined ponds (the North and South Ponds). Prior to retrofitting with a synthetic liner, the West Pond was operated with an asphalt liner that, based on visual appearance, was similar to the one installed in the Southwest Pond. Observations of the subsurface of the West Pond were made during the liner retrofitting. Records of these observations were recorded in the April 25, 1996, Closure and Post-Closure Maintenance Plan (Geomatrix, 1996) for the West, North, and South Ponds and are summarized below. The West Pond is approximately 3.5 acres in area and was constructed in 1975 with an asphalt liner. The Closure and Post-Closure Maintenance Plan indicates the West Pond accepted phosphoric acid wash water and filtrate process water from about 1975 until 1982. In 1994, the West Pond was retrofitted to meet California Code of Regulations (CCR), Title 27 performance standards, including a double synthetic liner.The liner system included a trench for the leachate collection and recovery system (LCRS) pipe that was cut through the existing asphalt liner into the underlying soils. During construction of the LCRS, a layer of highly cemented material approximately 6 to 12 inches thick was encountered below the asphalt. The cemented material is believed to be a fluorosilicate produced as a precipitate of phosphoric acid wash water infiltrating into the soil underlying the asphalt liner of the West Pond. The base of the Southwest Pond has a linear shape, which indicates it may have been used to settle particulate material associated with the phosphoric acid wash water. The Southwest Pond was asphalt- lined but was apparently not in use in 1994 when the West Pond was retrofitted. As such, no data describing subsurface conditions related to retrofitting are available for the Southwest Pond. Given its • wood.