COMPLIANCE INFO_EW-5 INSTALL PERMIT PACKAGE 2/14/25

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Mr. Kippen November 4, 2022 www.arcadis.com Technical Memorandum 11-04-2022 2/4 creating a drawdown cone of depression in the vicinity of the newly installed piezometers PZ-1 or PZ-2, also screened in the shallow water bearing zone approximately 30 feet and 60 feet fro m CDCR-EW-1, respectively. The limited amount of groundwater drawdown and ultimately groundwater capture from the location of even the nearby piezometers is due to limited formation capacity in the shallow water bearing zone in this area of the CDCR property, caused by thin permeable sand zones and low hydraulic conductivity. Therefore, the results of hydraulic testing at CDCR-EW-1 indicated insufficient capture of the >25 µg/l shallow zone plume mass at AMW-22S or the piezometer PZ-1, which is located approximately 30 feet from the shallow water bearing zone extraction well. Consequently, Arcadis recommended additional site assessment activities before installing additional groundwater extraction wells on the CDCR property. The first phase of this additio nal site investigation was completed from June to July of this year. The results of the investigation determined that the center of the >25 µg/l plume mass was in the shallow water bearing zone beneath the CDCR property approximately 750 ft south-southwest of AMW-22S. Additionally, the subsurface lithology was assessed in greater detail, along with chemical analysis indicating plume migration pathways laterally and vertically through the shallow zone and with movement downgradient into the intermediate water bearing zone. Based on the results of the hydraulic testing and first phase of site assessment activities, we propose 3 additional extraction wells be installed and screened in the intermediate water bearing zone. The additional extraction wells will be positioned to hydraulically capture the >25 µg/l plume mass in the shallow water bearing zone beneath the CDCR property by inducing a downward vertical hydraulic gradient to the intermediate zone with placement of extraction wells hydraulically downgradient in area(s) where migration to the intermediate zone has already occurred. Following the hydraulic testing of the new extraction wells, additional extraction wells may be proposed if capture of the plume mass is not established. Additional groundwater monitoring wells are also proposed to further delineate the center of plume mass and monitor the performance of the groundwater plume remediation system. Water Board Question 3: Now that Forward has conducted aquifer testing, please identify how many wells Forward anticipates will be needed to create an inward gradient, measured from the wells at the edge of the plume. Will Forward revisit the capture z one analysis done for the point of compliance about 1000 feet down gradient, which has at least five wells? Does Forward propose to install a large diameter well(s) (like an agriculture well) to overcome the forces of the nearby agriculture wells in the area pumping from the impacted aquifer intervals? Response: The objective of the interim remedial action is to capture the plume mass in accordance with Section 3.c) of CAO R5-2017-0703, which requires a plan for “the installation of additional groundwater e xtraction wells to remediate areas north of the landfill where the discharger identified total VOC concentra tions in groundwater exceeding 25 µg/l”. Arcadis has completed desktop calculations to capture and remediate groundwater exceeding 25 µg/l and resto re beneficial use of all zones affected by the release for the entire plume. As noted above the initial hydr aulic testing at CDCR-EW-1 did not demonstrate that capturing the plume mass beneath the CDCR property was successful in the shallow zone and additional hydraulic testing is required to appropriately finalize the design for the remedial action. However, Arcadis does not anticipate the need for agricultural-like high-capacity (i.e., 1,000s of gpm) wells to achieve capture of the plume mass beneath the CDCR property. An aquifer hydraulic and groundwater capture assessment was conducted by calculation of water bearing zone thickness(es) and the relative permeability of the materials (i.e., proportion of sands and gravels), combined with the hydraulic gradient to estimate capture zones, which were also used to estimate the number of wells required