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NNE ) collection logs (including field testing parameter measurements) are included in Appendix A. Groundwater samples were labeled and stored on ice in an insulated cooler pending delivery to the laboratory for analysis.Samples were collected, retained,and transported under chain-of-custody to a California state- certified analytical laboratory.Chain-of-custody forms and laboratory reports are included in Appendix B.Groundwater samples were analyzed for volatile organic compounds (VOCs) by U.S. Environmental Protection Agency(EPA) Method 8260E by Eurofins Test America in Tacoma,Washington.The practical quantification limit(PQL)for these samples was 0.2 micrograms per liter(pg/L).As noted in our Response to Comment letter for the 2019 Biennial Monitoring and Sampling Report(Wood,2019),this PQL is below the maximum contaminant limits for carbon tetrachloride(0.5 pg/L) and chloroform (80 ug/L) as well as most other volatile organic compounds reported for Method 8360B and thus suitable to meet monitoring needs. Groundwater Elevations Figures 4 through 6 illustrate the piezometric surface of the Upper, Middle, and Lower Horizons, respectively,as interpreted from the water-level elevations measured in March 2023.The inferred groundwater flow directions in the Upper, Middle, and Lower Horizons are to the northeast, which is consistent with regional trends. Table 4 summarizes the groundwater flow directions and horizontal hydraulic gradients estimated since 2008.The following gradients were calculated from March 2023 depth-to-water measurements: - Upper Horizon-0.0005 foot/foot,estimated between monitoring wells MW-51 and MW-13; - Middle Horizon-0.0018 foot/foot,estimated between monitoring wells MW-10 and MW-14;and The groundwater gradient for the lower horizon has historically been calculated between MW-15 and MW-16; however, because MW-16 has been eliminated from the MRP due to historically low or non-detect concentrations,the gradient for the lower horizon is not calculated. Groundwater elevation trends from February 2005 to March 2023 are shown on Figure 7.With the exception of MW-13,the groundwater elevations measured in March 2023 indicate a continued decline that began in 2012. The decline may be associated with the recent drought,or the extraction from the nearby Banta production wells. Due to the historically wet 2022/23 winter season,San Joaquin County is no longer classified as having drought conditions (www.drought.gov).This is reflected in the cessation of declining groundwater elevations and,as observed at MW-13,a reversal in the declining groundwater elevation trend. Historical groundwater data and trends are included in Appendix C. Historically,gradients for the Upper Horizon have been calculated between MW-1 and MW- 13, however, because MW-1 has been eliminated from the MRP due to historically dry conditions, MW-5 is used in its place. https://woodplc.sharepoint.com/teams/olintracy/shared documents/general/03.0 docctrl/2021 biennial report/01_text/tcy 2021 biennial_text_rev.docx Page 3