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Geosyntec consultants msl)towards the lower elevation groundwater underlying RRI (1 to 12 feet below msl). The Stockton Deep Water Channel is up to 29 feet deeper than Burns Cutoff and has a larger influence on RRI groundwater than Burns Cutoff or the San Joaquin River because it intersects a much larger cross-section of the water-bearing zone (Tetra Tech, 2002). Surface water elevations in all three surface water bodies should be almost identical at any given time. Figure 10 shows a comparison of groundwater elevations in monitoring wells along the perimeter of RRI to surface water elevations measured at USGS stream gauge 11304810 in the San Joaquin River. The surface water elevations are always higher than the groundwater elevations, indicating that recharge from the surface water bodies and inward gradients towards the interior of RRI are continuously maintained within the top 70 to 80 feet bgs water-bearing zone and there is no tidal influence on the groundwater flow direction. 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 11; Appendix B).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 11). Figure 11 shows the general estimated potentiometric surface at RRI. The mean surface water elevation during the fourth quarter 2015 from stream gauge 11304810 was used as the average groundwater elevation at the surface water and groundwater interface around the perimeter of RRI. Groundwater elevations were calculated based on depth to water measurements taken during the fourth quarter 2015 and the surveyed top of casing elevations. If the well was surveyed in 2007 the top of casing elevations from the ERS 2007 survey were used to calculate the groundwater elevation. If the well was not surveyed in 2007,then the 1990s Navy surveyed top of casing was used.The groundwater elevations from the fourth quarter 2015 were used to generate the potentiometric surface contours. As described above, the depth to water varies by about 2 to 5 feet between the dry and wet season and the depth to water measurements were taken over a three-month period. Therefore, the potentiometric contours may not be representative of the groundwater potentiometric surface at a specific time. However, they provide a general representation of the groundwater flow direction at RRI on an island-wide scale. RRI Groundwater Monitoring Work Plan 13 1 June 2018