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NNE ) Calculated vertical hydraulic gradients for the March 2023 monitoring event are presented in Table 5,as well as a summary of vertical hydraulic gradients for the same well pairs observed during four previous monitoring events (March 2021, March 2019,September 2017,and September 2016) for comparison.The vertical hydraulic gradients between co-located wells in successive horizons during March 2023 were as follows: Gradient Direction/Vertical Hydraulic Gradient(ft) Monitoring Well Pair March 2023 MW-9 (Upper) and MW-10 (Middle) down/0.014 MW-11 (Upper) and MW-12 (Middle) up/0.006 MW-13 (Upper) and MW-14(Middle) down/0.224 MW-14 (Middle) and MW-15 (Lower) down/0.032 Current and historical results summarized in Table 5 show that the strongest and consistently downward vertical gradients occur in the upper and middle well pair MW-9 and MW-10, respectively,closest to the Site(Figure 3).This well pair is closest to the cluster of private supply wells located approximately 700 to 1,200 feet to the west-northwest(Figure 3). In general, and with some exceptions,the magnitude of the vertical gradients decrease by an order of magnitude with increasing distance toward the northeast and shift from downward to upward as measured in successively distant monitoring well clusters MW-11/MW-12 and MW-13/MW-14/MW-15 (Table 5). Downward vertical gradients near the site, likely influenced by industrial and private supply well pumping, has likely contributed to downward migration of COCs in the middle and lower hydrostratigraphic zones near the Site Static or slightly upward vertical gradients toward the northeast may help retard downward vertical migration of COCs in the downgradient portions of the plume. Laboratory Analytical Results Results of groundwater analyses for samples collected in March 2023 are summarized in Table 6. Duplicate analytical results are summarized in Table 7 and further discussed under"Field Quality Assurance/Quality Control." Historical VOC analytical data are presented in Appendix D (Table D-1 and Figures D-1 and D-2) and historical inorganic analytical data are presented in Appendix E (Table E-1 and Figures E-1 to E-4), respectively.Analytical results for VOCs detected in groundwater are discussed below. Volatile Organic Compounds Carbon Tetrachloride Carbon tetrachloride was detected above the California maximum contaminant level (MCL) of 0.5 micrograms per liter(Ng/L) in the sample collected from Middle Horizon well MW-12 at a concentration of 1.6 pg/L in March 2023.Carbon tetrachloride was detected below the MCL in Middle Horizon well MW-10 (at 0.11 Jug/L), Upper Horizon well MW-9 (at 0.064 J ug/L) and the primary and duplicate samples from Lower Horizon well MW-15 (at 0.42 ug/L).The compound was also detected at trace-level concentrations https://woodplc.sharepoint.com/teams/olintracy/shared documents/general/03.0 docctrl/2021 biennial report/01_text/tcy 2021 biennial_text_rev.docx Page 4