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ekienvironment <br /> & water <br /> The 2023 Spring (i.e. First Semiannual) regional groundwater elevation contours are shown on <br /> Figure 3. The regional contours shown on Figure 3 are calculated using the kriging interpolation <br /> method applied to the data from all wells. Horizontal hydraulic head gradients were calculated <br /> using the triangulation method applied to the data from three wells in the vicinity of each <br /> specified well, LAA, or pond. The calculations were performed using the EPA's "3PE: A Tool for <br /> Estimating Groundwater Flow Vectors" spreadsheet.' Table 4 compares Fall 2022 and Spring <br /> 2023 gradients and azimuths. The estimated azimuths may be different than the direction of <br /> groundwater flow inferred from the regional contours because the gradients and azimuths are <br /> calculated using data from just the three nearest wells whereas the regional contours are <br /> calculated using data from all wells. The inferred flow direction is delineated perpendicular to <br /> the regional contours. Water levels and hydraulic head gradients are expected to change <br /> temporally because of seasonal conditions, construction activities, disposal operations, and land <br /> use changes. The gradients were calculated in the vicinity of the following selected sites: <br /> • Near compliance well MWM-12 and Mossdale LAA A23; <br /> • Near compliance wells at Central Lathrop Specific Plan Area A36; <br /> • Near well MBRMW-1, P13-1, and former LAS-2 site located at the CTF; <br /> • Near compliance wells at River Islands LAAs A28 and A31. <br /> Gradients near Pond S6 could not be calculated as there are no longer three active water level <br /> monitoring wells around the pond. Gradients could not be calculated near compliance wells at <br /> River Islands A30 as MWR-12 was dry during the 2023 Spring Monitoring Event. <br /> 8 3PE:A Tool for Estimating Groundwater Flow Vectors prepared by United States Environmental Protection Agency, <br /> EPA 600/R-14/273, September 2014.The 3PE spreadsheet is designed to estimate the direction and magnitude of <br /> the hydraulic gradient vector from the coordinates of three wells and the respective groundwater elevations. <br /> City of Lathrop 8 13 July 2023 <br /> EKI B90088.03 <br />