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Geosyntec O consultants and potential other residual COCs in soil and groundwater; and monitored natural attenuation (MNA) for groundwater at the off-site area. Presently, on-site phytoremediation and off-site MNA are the two remedial activities being performed. The phytoremediation system is presently composed of approximately 1,200 red gum eucalyptus trees (Eucalyptus camuldulensis) planted on a 5- to 10-ft grid pattern that covers about 2.3 acres of the site. Initially 1,500 trees were planted in 2003, with a further planting of approximately 250 additional trees in 2013. Morbidity and plantation thinning have reduced the total number of trees that presently comprise the phytoremediation system. 3. PURPOSE AND SCOPE OF SOIL SAMPLE COLLECTION 3.4 Purpose of Soil Sample Collection The purpose for collection of shallow soil samples at the site is to satisfy the regulatory requirements imposed on the site by the RWQCB. Specific guidance is indicated in the RWQCB's document titled Monitoring Well Destructions and Soil Sampling Schedule (RWQCB, 21 Dec 2012), as follows: "Soil Sampling Schedule The phytoremediation treatment system was implemented in 2003 to remove nitrate from groundwater, uptake nitrogen from soil, and degrade pesticide and herbicide compounds. Soil sampling was initiated to monitor the effects of phytoremediation on soil constituents. Pursuant to a letter dated 22 January 2008 by Central Valley Water Board staff, OxyChem discontinued sampling for pesticides and herbicides, because the concentration of these compounds were below cleanup goals. OxyChem may be required to resample soil for organic constituents prior to considering the site for no further action. The fall 2011 sampling data show that nitrogen still exceeds cleanup goals in many locations. Therefore, OxyChem needs to continue sampling for nitrogen compounds. However, the frequency may be reduced to biennially in even numbered years." 3.2 Written Approval from the Lead Oversight Agency In 2018 Geosyntec confirmed with Ms. Aimee Phiri, P.E. (the RWQCB case worker at that time for the site) the requirement for soil sample collection and analysis for nitrate, ammonia and pH. Correspondence from Ms. Phiri which confirms this requirement (including the RWQCB letter Monitoring Well Destructions and Soil Sampling Schedule [RWQCB, 21 Dec 2012] cited in her email communication) is presented in Appendix A. 2024 TR0712 Stockton Soil Sampling Work Plan 2 2024.08.2