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EnviroTech Consultants, Inc. Page 1 Site Investigation: Sampling Report Louise Avenue Water Treatment Facility EXECUTIVE SUMMARY EnviroTech Consultants, Inc. (EnviroTech) conducted a soil assessment at the request of the City of Lathrop at the Louise Avenue Water Treatment Facility (LAWTF), located at 2112 E. Louise Ave, Lathrop, California 95330 (Site). The potential for elevated concentrations of arsenic and vanadium were first noted in 2016 during a site visit by the San Joaquin County Environmental Health Department. Historic information indicates that filter cake and sludge were once stored temporarily on the ground surface before 2016, but has since been removed. On April 16, 2018, EnviroTech collected soil samples in three areas of the Site using a hand-auger. Area 1 is the location of the release of the filter press sludge that drained into the storm drain inlet, Area 2 is the location that the filter cake was placed on the surface, and Area 3 is the portion of the retention pond that contained residual sludge from the filter press spill (Figure 2). At each sample location, samples were collected from the surface to 6 inches below ground surface (bgs) and at 1 to 2 feet bgs. The selected samples were analyzed by a certified laboratory for the presence of arsenic and vanadium. A soil sample was taken from a location significantly separated from the selected sample areas to determine the background levels of arsenic and vanadium within the area. The San Francisco Bay Regional Water Quality Control Board (SFBRWQCB) Environmental Screening Level (ESL) for Direct Exposure - Dermal Contact for a Construction Worker is 10 mg/kg for arsenic (Soil Direct Exposure S-1, February 2016)1. The Environmental Protection Agency (EPA) Carcinogenic Target Risk Dermal Screening Levels for arsenic is 17 mg/kg. These ESL values are used in this study as the minimum indicators for arsenic concentrations of concern. The San Francisco Bay RWQCB does not currently have an ESL indicator for direct exposure for vanadium. Following analytical results of the initial sampling on April 16, 2018, EnviroTech recommended resampling at location COL2-1 from surface to 6 inches, collecting an additional background sample, and collecting additional samples in Area 3 to determine the full vertical and lateral extents of the arsenic impacted soil within the retention pond. On September 12, 2018, at the original sample locations in Area 3, COL3-4 was advanced to 5 feet using a hand auger and samples were collected from 2 to 3 feet bgs, 3 to 4 feet bgs, and at 5 feet bgs. Due to the high density of cobbles and boulders, sample locations COL 3-1, 3-2, 3-3 and 3-5 were not extended to 5 feet bgs and secondary samples for these locations were not collected. Eight additional sample locations in the retention pond were selected south of COL3-5 and east of COL3-4 in an area approximately 20 feet by 40 feet. In each sampling location (where possible)the hand auger was advanced to 5 feet bgs. On December 27-28, 2018, EWMI excavated approximately 260 yards of soil and stored the soil in 12 roll- off bins on Site. EnviroTech collected composite samples from each roll-off bin. Each composite sample was analyzed for CAM-17 metals by EPA Method 6010-B. Analytical results for each of the roll-off bin composite samples were compared to the Potrero Hills Landfill Special Waste Acceptance Guidelines — 1 The San Francisco Bay Regional Water Quality Control Board (SFBRWQCB) Environmental Screening Levels (ESLs)Workbook was updated on January,24,2019.The ESL for Direct Exposure—Dermal Contact for a Construction Worker for arsenic is the same as the February 2016 ESL. 5400 Rosedale Highway Bakersfield, CA 93308 Tel 661.377.0073 Fax 661.377.0074