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Cindy Chain-Britton Cindy Chain-Britton Page 2 of 3 Page 3 of 3 April 2,2020 April 2,2020 BACKGROUND 3. Soil Gas Well Equilibration Time, The Workplan should be revised to indicate the length of time that the newly installed soil gas monitoring wells will equilibrate The Tracy Comers Shopping Center Site(Site),also known as Quality Cleaners Tracy, prior to sampling. According to the Advisory, the establishment of equilibrium in entered into a Voluntary Cleanup Agreement(VCA)with DTSC on May 6, 2015. low permeability soils is expected to take approximately two weeks. Contaminants of Concern(COCs)currently identified include tetrachloroethylene(PCE) and benzene in soil vapor and groundwater.The PCE soil vapor plume,which is only 4. Soil Gas Analysis. The proposed soil gas analytical method, EPA Method partially delineated, is located 45 feet north from a residential community. PCE has also 82608, should be revised. As previously noted by GSU, EPA Method TO-15 is been detected in soil gas along the sewer line that runs along the western boundary of needed to fully delineate a soil gas plume and support vapor intrusion risk the shopping center,where another neighborhood is adjacent to the west. PCE evaluations because it achieves sufficiently low method detection and reporting concentrations in soil gas in this area also remain undelineated. limits to allow risk to be determined from residual contamination (Comment#3, GSU Memorandum dated March 16,2019).The Workplan should also be revised The Workplan has been submitted in response to multiple requests from DTSC for the to include the list of laboratory reporting limits and method detection limits to completion of soil vapor and groundwater contaminant delineation. The Workplan ensure they are below risked based screening levels. proposes the installation and sampling of soil vapor wells,and states a separate workplan will be submitted to address groundwater sampling and delineation. 5. Replacement of Soil Gas Wells SG-10. SG-12 and SG-13. The Workplan should be revised to provide a rationale for the replacement of soil gas wells SG-10, GENERAL COMMENT SG-12 and SG-13,and the need for this data. These three wells are located in a 60-foot by 90-foot area located between the Quality Cleaners building and the The technical approach presented in the Workplan is unlikely to provide the data souhem site boundary. Currently, there are eight other soil gas wells in this area needed to complete soil vapor delineation and should not be approved in its current (SG-11, SG-26,SG-27, SG-28, SG-29, SG-30, SG-31 and SG-32),which were form. Revisions are needed to address the following areas of technical deficiency: last sampled at the end of May 2019. The extent of the soil gas plume to the west is defined by SG-33 and proposed soil gas well SG36. The eastern extent SPECIFIC COMMENTS of the plume is defined by SG-35. Previous sampling indicates that the soil vapor PCE plume extends beyond the southern site boundary, and off-site sampling is 1. Proposed Soil Gas Well Locations on Scarlett Place. The Workplan should be needed regardless of additional on-site results. revised to relocate proposed soil gas wells SG-40 and SG-41,so that they are directly south of the known soil vapor plume on-site. SG-40 should be relocated 6. Decision Looic Regarding Data Along Western Site Boundary. Partner has approximately 40 feet to the east of the proposed location, and SG-41 should be elected to pursue a phased approach to delineate PCE soil vapor along the relocated approximately 25 feet west of the proposed location. sewer line at the western site boundary.The phased approach includes resampling, or replacement and sampling, of existing soil gas well locations 2. Soil Gas well Construction. The step-out contingency plan described in Section along the sewer line that runs north-south along the western site boundary.The 4.4.6 for encountering clay at the proposed five-foot depth interval should be installation and sampling of two new soil gas wells(SG-38 and SG-39) located revised. Cal/EPA's Advisory—Active Soil Gas Investigations(Cal/EPA, 2015) east of the sewer line will also be included in this phase of delineation. states that soil gas wells should be collected a minimum of five feet below ground surface(bgs), in a permeable layer,and a greater depth is advisable. Not included in this current phase is off-site sampling in the neighborhood west of Groundwater is encountered at approximately ten feet bgs, and the following the sewer line.The workplan should be revised to include decision logic stating if approach is warranted: the second sampling event confirms soil vapor concentrations along the sewer a. Partner should plan to log soils at each location to groundwater prior to line are above residential screening levels, with application of a 0.03 attenuation well installation. The exact depth of the soil gas probe will depend on factor,then a workplan for soil gas well installation in the neighborhood west of location-specific lithology. The screen should be placed in a permeable or the site,to provide western plume delineation,will be provided for DTSC's semipermeable stradgraphic layer above the capillary fringe. review. b. The alternative installation method described in Appendix D of the Advisory(Cal/EPA, 2015),which provides methods for well installation in 7. Number of New Monitoring Wells. Section 4.1 should be revised to state that low permeability soils should be followed. there are six new soil gas wells: SG-36,SG-37,SG-38, SG39,SG-40, and c. The step-out method described in the Workplan should only be followed if SG-41. soil gas samples cannot be collected after attempting both measures described above.