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Wait Plan and Time Schedule for Analyses of Background Groundwater Quality City of Stockton Regional Wastewater Control Facility Page 2 result in flow reversal. Flow reversal may also be caused in low water times by pumping at Tracy for Delta water export. The river stage elevations average approximately four (4) feet above sea level and range from one (1) to ten(10) feet above average stage during short rainy periods. The river is channeled behind levees which halt flood deposition of new material on surrounding farmland, and natural consolidation of the organic-rich former fluvial deposits has resulted in land subsidence. 4.0 HYDROGEOLOGY Much of the information presented in this section is summarized from the Report of Groundwater Conditions in the Vicinity of the City of Stockton Regional Wastewater Control Facility Oxidation Ponds prepared by Condor dated September 2006, and from the Groundwater Monitor Well Installation Report prepared by Geotechnical Consultants Inc. (GTC)in October 2004, contained in an Appendix therein. The RWCF facility is located near the San Joaquin Delta-river boundary. To the west are flat fine-grained fluvial delta deposits rich in organic matter and clay-rich flood plain deposits. To the east, fine-grained fluvial deposits grade to, and inter-finger with, distal alluvial fan deposits shed from the Sierra foothills. To the south,abandoned river or slough channels are evident on aerial photographs taken in the late 1940s (Appendix 6). Shallow groundwater occurs in the clay-rich flood plain deposits, which contain organic debris. Channel sands and natural levee sands isolated within fluvial clay and silt overbank deposits are indicated in the well logs prepared for monitoring well installation on-site. The gravelly clay encountered in the boring for MW-12, for example, likely is a buried stream channel. The sand and silt channel deposits are laterally discontinuous; however, the channel sand stringers may be continuous in the northerly direction of stream flow for a considerable distance. The flood plain aquifer has low transmissivity and storage. Water quality of the shallow background groundwater is poor, with naturally high salinity. The relatively high salinity may be attributed to any or all of the following: rising connate water from deep marine sediments, leaching soil salts and agricultural soil amendments by way of irrigation and precipitation,and encroaching saline tidal water. West of the SJR, the oxidation ponds and circulation ditches at the RWCF have surface water levels maintained higher than surrounding land surface. The average SJR stage is also higher than adjacent land. To the west the depth-to-water in the monitoring wells ranges from about three (3) feet to more than ten (10) feet. Thus, in general, the water elevations of the river and ponds create groundwater gradients toward the water table underlying surrounding lands. Monitoring well MW-15, located 1,700 feet south of the oxidation ponds (Figure 2), has been identified as an upgradient well, though groundwater at this location flows to agricultural drains south of the oxidation ponds. Inside the south and west facility boundaries groundwater gradients are controlled (flattened) by a system of groundwater interceptor ditches and subsurface drains that are pumped back to the treatment ponds. Groundwater gradients outside the interceptor system are controlled by a network of agricultural drainage canals and pumps operated by the West Bridge Irrigation District that discharge water to the SJR at the Bums Cutoff Pumping Station west of downgradient well MW-16 shown on Figure 5. There currently are no data on the water elevations in the constructed wetlands north of the oxidation ponds; however, recent well hydrographs of MW-8, MW-9, and MW-10 indicate gradients from the river to the wetlands occur in these reaches. Groundwater flowing westward from under the wetlands is either pumped back from the interceptor system or flows westward to the agricultural drains at the Butes Cutoff Pumping Station. Between Pond 1 and the river, water levels in monitoring wells MW-1 and MW-2 located on a flat area east of Pond 1 indicate a groundwater gradient toward the river from the ponds. Depths to groundwater in monitoring wells installed east of the river range from about ten(10)feet to more than 14 feet. Fast of the Sources of High-Chloride Water to Wells, Eastern San Joaquin Ground-Water Subbasin, California, USGS Open File Report 2006-1309. A �., CONDO