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GSI Job No.: 140 I G [�a1T Issued: 15 November 2017 ENVIRONMENTAL The Site is located within the Eastern San Joaquin Groundwater Basin, a sub-basin of the San Joaquin Valley Groundwater Basin, approximately 17 feet above mean sea level (msl; Geosyntec Consultants, 2012). The Eastern San Joaquin sub-basin is bounded by the Mokelumn River to the north, Stanislaus River to the south, San Joaquin River to the west, and bedrock to the east (SJCDPW, 2004). Regional groundwater flow patterns have been significantly altered since pre- development conditions. Under pre-development, steady-state conditions, groundwater flowed from the natural recharge areas along the perimeter of the valley to the low-lying areas along the San Joaquin River. Under post-development conditions, groundwater flows in an east-northeast direction towards a central groundwater depression located outside the city limits of Stockton (SJCDPW, 2004; Geosyntec Consultants, 2012). A 12 September 2011 Semi-Annual Groundwater Monitoring and Remediation Progress Report prepared for the adjacent former ARCO Service Station No. 0548 indicated that the depth to groundwater was approximately 24 to 26 feet bgs based on monitoring well water level measurements (Arcadis, 2011). Based on groundwater elevation measurements at this adjacent property,groundwater generally flows in the same general direction as the regional east-northeast groundwater flow pattern (SJCDPW, 2004; Geosyntec Consultants, 2015). 3.3 Results of Recent Domestic Well Sampling As described in the Order, from July 2007 to December 2013, eight groundwater samples were collected from Colonial Heights Maintenance District municipal water supply Well 01, located on Pershing Avenue between Paloma Avenue and Valencia Avenue, approximately 1 mile northeast of the Site. Concentrations of PCE were detected in all eight of the groundwater samples. Six of the eight samples reported concentrations of PCE that exceeded the United States Environmental Protection Agency (US EPA) Maximum Contaminant Level (MCL) for drinking water of 5 micrograms per liter (µg/L). On 2 August 2017, Water Board staff collected groundwater samples from seven domestic wells (two along Thornton Road and five along EI Camino Avenue). The samples were collected as part of an investigation into a potential petroleum hydrocarbon release from the former service station located at 1120 West Hammer Lane. PCE was detected at or above the MCL in five of the seven groundwater samples. Between 16 and 29 August 2017, Water Board staff collected additional groundwater samples from three domestic wells along EI Camino Avenue, approximately 600 to 1,100 feet northeast of the Site. Concentrations of PCE detected in these samples ranged from 8.9 to 88 pg/L. In total, between 2 August 2017 and 29 August 2017,Water Board staff collected groundwater samples from 32 domestic wells in the vicinity of the Site. PCE was reported in 12 of the 32 wells above its MCL. 4.0 APPROACH AND RATIONALE FOR SAMPLING LOCATIONS Based on the initial project meeting with the Water Board, historical Site use information identified to date, potential Site tenant space dry cleaning operations layouts, and preliminary identification of the location of on-Site sewer laterals, GSI has selected 13 sampling locations for sampling soil and soil gas to a depth of 15 feet bgs to assess the presence of PCE and other volatile organic compounds (VOCs). Proposed sampling locations are shown on Figure 1: • Four locations in each of the two tenant spaces where dry cleaning was conducted (B1 through B4 in 1762 and B7 through B10 in 1744); • One exterior location adjacent to where a former tenant reported waste PCE drums were temporarily stored (1311); and Site Investigation Work Plan Park Woods Cleaners