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Geosyntec consultants Within the interior of RRI, the groundwater has a general westward flow direction from Site 4 in the east, which has the highest groundwater elevations, towards the landfill sites in the west, which have the lowest groundwater elevations (Figure 20; Appendix D). The interior groundwater flow at RRI appears to follow the drainage channel system, which discharges to the Burns Cutoff on the west side of RRI(Figure 20). Local variations in the groundwater flow direction may occur within the interior of RRI in proximity to the drainage channels and/or areas with abundant plants. Groundwater generally discharges into the unlined drainage channels but following precipitation events, recharge to groundwater may occur along some portions of the unlined drainage channels (Tetra Tech, 2002; PRC, 1997b). The geochemistry of the surface water in the drainage channels and groundwater is similar during the dry season, providing a line of evidence that groundwater is discharging into the surface water drainage channels (Figures 22 and 23). During the summer months, evapotranspiration from abundant plants (trees, grasses, and brush) in some areas may extract groundwater and cause localized depressions of the potentiometric surface (Tetra Tech, 2002). During the most recent groundwater monitoring events, the groundwater flow direction and gradient in each area was as follows: • The groundwater flow direction at Site 3 was generally to the north at a gradient of approximately 0.004 feet per foot(ft/ft) in the third quarter 2021. • The groundwater flow direction at Site 4 was generally to the southwest at a gradient of approximately 0.006 ft/ft in the third quarter 2021. • The groundwater flow direction in the Central Area was generally to the west- southwest at a gradient of approximately 0.003 ft/ft in the third quarter 2021. • The groundwater flow direction in the southern portion of the Landfill Area was to the north at a gradient of approximately 0.002 ft/ft in the third quarter 2020. • The groundwater flow direction in the northern portion of the Landfill Area was generally to the south-southwest at a gradient of approximately 0.002 ft/ft in the third quarter 2020. While the magnitudes of the gradients may have some daily fluctuation with the tides, they remain low, ranging from approximately 0.002 ft/ft to 0.006 ft/ft. Slight fluctuations in the magnitude of the gradient up and down several times a day would have little impact on the average groundwater flow velocity and migration of COCs in groundwater over time. RRI Groundwater Monitoring Work Plan 14 June 2022