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Mr. Jeff Tierney, E.I.T. Geosyntecr-' July 1, 2022 consultants Page 2 concentrations from the maximum concentrations reported in off-site monitoring wells. The estimated off-site soil vapor concentrations were then compared to residential soil vapor screening levels to assess the potential for vapor intrusion. Results and conclusions from this assessment were described in the Vapor Intrusion Assessment Report3 (Assessment Report; Geosyntec, 2022). Based on the analytical results for the groundwater and soil vapor samples and the correlation factors developed using those results, the Assessment Report concluded that it did not appear that vapor intrusion was a concern for off-site residents, and that revisions to the groundwater threshold concentrations were not warranted. The Assessment Report also recommended the collection of additional paired groundwater and soil vapor samples in the dry season (i.e., Q3-2022) to help validate the correlation factor between groundwater and soil vapor. In its letter dated February 14, 20224 (CRWQCB, 2022), the CRWQCB described its review of the Assessment Report's findings, and requested that a soil gas sample be collected near MW-27 concurrent with the planned Q3-2022 routine groundwater monitoring event, to evaluate the concentration of 1,2-dichloropropane(1,2-DCP)in soil gas at that off-site location. The CRWQCB also gave its concurrence to the recommendation that additional paired groundwater and soil vapor samples be collected in the dry season (i.e., Q3-2022) to help validate the correlation factor between groundwater and soil vapor. To satisfy these requests, this Addendum has been prepared to describe (i) the installation of two new semi-permanent off-site SVPs which are planned to be located near off-site monitoring well MW-27, (ii) the planned analysis of soil gas samples collected from each of the on- and -off-site SVPs, and (iii) the planned analysis of groundwater samples collected from selected monitoring wells located near to the SVPs. FIELD ACTIVITIES Pre-Field Activities Prior to the beginning of field activities, the following will be performed: • Drilling permits will be obtained from the San Joaquin County Environmental Health Department; • An Encroachment Permit will be obtained from the City of Stockton for the installation of the two SVPs in the public right of way (i.e., either the sidewalk or roadway) adjacent to MW-27; s Geosyntec Consultants,Inc.2022. Vapor Intrusion Assessment Report,January 7,2022. a Central Valley Regional Water Quality Control Board.2022.REVIEW OF 7JANUARY 2022 VAPOR INTRUSION ASSESSMENT REPORT,FORMER OXYCHEM STOCKTON, 1904 WEST CHARTER WAY, STOCKTON, SAN JOAQUIN COUNTY,February 14,2022. engineers I scientists I innovators