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WORK PLAN FOR ADDITIONAL SUBSURFACE INVESTIGATION <br /> Former Helena Chemical Facility <br /> 2245 West Charter Way <br /> Stockton,California <br /> July 13,2020 <br /> 1.0 INTRODUCTION <br /> This work plan was prepared by Condor Earth (Condor) to present proposed monitoring well installation <br /> activities at the location of the former Helena Chemical facility at 2245 West Charter Way (Site) in <br /> Stockton,California(Figure 1,Appendix A). Condor prepared this work plan at the request of Ms.Beverly <br /> Brandon of Raymond Investment Corporation(property owner), and as directed by Ms.Aimee Phiri of the <br /> Central Valley Regional Water Quality Control Board(Central Valley Water Board)in a letter dated March <br /> 19,2020. <br /> As stated in Condor's Site Investigation Report, dated September 20, 2019, the purpose of the work is to <br /> further delineate concentrations of constituents of concern (COCs) associated with the historical <br /> agricultural chemical use at the Site. In a letter dated March 19, 2020, the Central Valley Water Board <br /> concurred with the report's recommendation for additional investigation and requested "a work plan to <br /> conduct an investigation that addresses the data gaps identified". The data gaps referenced by the Central <br /> Valley Water Board were: <br /> 1. Based on the step-out investigation results, additional investigations are warranted to the north and <br /> northeast of the Site to fully delineate the vertical and lateral extent of nitrates and <br /> [1,2,3-trichloropropane]1,2,3-TCP, define the Conceptual Site Model for the Site, assesspotential risks <br /> to human health and the environment, and to evaluate remedial action alternatives. Therefore, during <br /> the next phase of investigation, step-out borings should be advanced until the lateral and vertical <br /> boundaries with non-detect levels of nitrate and 1,2,3-TCP emanating from the Site are identified. In <br /> addition,a receptor survey should be conducted to identify potential receptors within a 1,000 foot radius <br /> of the Site. <br /> 2. Investigations conducted at 1904 Charter Way located to the south of the Site identified three <br /> groundwater bearing zones: a shallow zone referred to as the A Zone from approximately 20 to 40 feet <br /> below ground surface (bgs); an intermediate B Zone from approximately 70 to 90 feet bgs; and a deep <br /> zone C Zone from approximately 115 to 135 feet bgs. <br /> a. Although it is possible that groundwater at the Site also occurs in three groundwater bearing zones <br /> due to its proximity to the 1904 Charter Way property, onsite data needs to be collected to define <br /> the onsite stratigraphy and sample at least the next lower water bearing zone for nitrates and <br /> 1,2,3-TCP. <br /> b. The next phase of the investigation should identify all groundwater bearing zones present at the Site, <br /> delineate the vertical and lateral extent of the contaminant plumes in each of the groundwater zones, <br /> and install groundwater monitoring wells at appropriate locations and depths to monitor the deeper <br /> groundwater zones, if warranted. <br /> 3. During the May 2016 investigation, 1,2,3-TCP was detected in a stormwater sample collected from the <br /> stormwater retention basin located in the southeastern portion of the Site at a concentration of 0.00026 <br /> [micrograms per liter] ugIL. Central Valley Water Board staff recommend that at least one boring be <br /> �r�r CONDOR <br />