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PEA Work Plan N G I a 1 R I N G Martin Metals Finishing Inc. (Former) Stockton, California rock sources (DTSC, 1998 and EDR, 2022; Appendix B). These soils are characterized as Class C, indicating moderately fine-grained soils with slow infiltration rates. Gravelly and sandy loam at the Site extend to approximately 4 feet below ground surface (ft bgs). Beneath 4 ft bgs, clay loam and cemented soils restrict the downward movement of water. Bedrock at the Site is deep due to the well-formed alluvial deposits, and bedrock in the general Site vicinity was deposited in the Quaternary period of the Cenozoic era (EDR, 2022; Appendix B). Storm drainage at the Site is directed to a concrete walled and bottom surface trench running south to north,and then into an underground drainpipe, eventually discharging to Mormon Slough via a 24-inch storm drain outfall at Mormon Slough. Mormon Slough is an intermittent east-to-west flowing creek that is dry most of the year, having had its hydraulic function mostly supplanted by the Stocking Diverting Canal in 1911. Local storm flow into the Mormon Slough will infiltrate, evaporate or eventually flow into the San Joaquin River. Groundwater is encountered nearby4 at-18 ft amsl, or approximately 50 ft bgs and appears to have an easterly flow gradient. 2.4Site History The Site has been occupied by industrial users since sometime prior to 1950. MMF occupied the Site from approximately 1981 to 1990 and performed metal plating and painting onsite. The operations included chemical dip tanks and rinse tanks, and wastewater pretreatment. Wastes generated by the operations included solvents and metal-laden filter cake from pretreated wastewater. In 1984, the City of Stockton (City)discovered that MMF had been illegally discharging wastewater to Mormon Slough via concrete storm drain trenches and storm drainpipes. The storm drainpipe from the trench to the Mormon Slough outfall was replaced sometime around 1979, reportedly before MMF operations commenced. The Site was cleaned by MMF after their operations ceased in 1990, which included the removal of plating-related materials, chemicals, and equipment and the cleaning of Site surfaces (e.g., concrete floor and walls). 2.5 Previous Investigations 2.5.1 Preliminary Assessment Under the authority of the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) and the Superfund Amendments and Reauthorization Act of 1986 (SARA), United States Environmental Protection Agency (USEPA) Region IX requested that DTSC prepare a Preliminary Assessment (PA) for the Site. The 1998 PA reviewed existing information on the Site and its surroundings to determine if the Site posed a threat to public health, welfare, or the environment and if further investigation under CERLCA or SARA was required (DTSC, 1998). The PA included a review of information available from federal, state, and local agencies and a Site reconnaissance visit, then an evaluation using the USEPA's Hazard Ranking System (HRS)to assess the relative threat of actual or potential releases at the Site and to determine the Site's eligibility for placement on the National Priorities List(NPL)for remedial response action. The PA found that the apparent problem at the Site was the illegal discharge of wastewater to the storm drain leading to Mormon Slough, which resulted in elevated levels of chromium, lead, and zinc in storm drain 4 Groundwater was monitored in 2016 an adjacent closed surface impoundment (Class I Landfill). https://documents.geotracker.waterboards.ca.gov/esi/uploads/geo report/6152533173/L10007726536.PDF. Groundwater elevations of-17.51, -18.38, and -18.48 ft amsl were observed in three wells. CDIM—Fairgrounds Industrial Park Page 3