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Toyota Town Inc., Stockton - 2 - 19 May 2022 Central Valley Water Board staff concurs with ATC's proposals presented in the Work Plan, as long as the following comments are incorporated into the scope of work: 1. ATC proposes to collect grab groundwater samples at two depths between 60- 110 feet below ground surface (bgs), with the depths being dictated by observed lithology. While Central Valley Water Board staff concurs with this approach, please ensure that the shallow grab groundwater sample is collected from approximately the same depth as the screen interval of well MW -12 (60-65 feet bgs). Additionally, in order to ensure proper vertical delineation, please do not collect the deep grab groundwater sample less than 20 feet deeper than the shallow sample. 2. ATC proposes to advance the soil gas well boreholes to 5 feet bgs. However, the July 2015 joint -agency guidance document Advisory — Active Soil Gas Investigations (Advisory) recommends that soil gas samples not be collected from shallower than 5 feet bgs to minimize the possibility of breakthrough of ambient surface air. The Advisory also recommends that soil gas probes be centered vertically within the filter sand pack, with at least six inches of sand above and below the probe. As such, Central Valley Water Board staff recommends that you advance the boreholes deeper than 5 feet bgs and construct the soil gas well with the top of the sand pack no shallower than 5 feet bgs. 3. ATC proposes to collect samples in accordance with the Advisory but does not include specifics. For clarification purposes, Central Valley Water Board staff recommends that you utilize quality assurance measures recommended in the Advisory, including shut-in testing and quantitative leak testing (Section 4.2.1 and Appendix C of the Advisory, respectively). Quantitative leak testing involves placing a shroud over the sampling train and placing a leak detection compound (LDC) under the shroud, with the under -shroud LDC concentration being quantified either by direct measurement equipment or collection of an under. shroud air sample. The under -shroud concentration can then be compared to the LDC concentration in the corresponding sample. A sample LDC concentration greater than 5% of that under the shroud would indicate significant leaks and sample invalidity. 4. The complete scope of work, including ATC proposals and Central Valley Water Board staff comments is summarized in Table 2 below.