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E C ® R Additional Site Assessment and Site Conceptual Model . 7-11 Store#14117 September 30, 2005 Page 4 flood basin deposits These deposits were interpreted to have been deposited by fluvial (water borne) processes as overbank flood plain deposits outside the main channel of the San Joaquin River The percentage of coarser- and finer-grained sedimentary sequences is principally controlled by distance from the main river channel, water depth, and velocity at the time of deposition Higher water levels and higher flow velocities tend to transport more coarse- grained materials that tend to be primarily sandy Lower velocity flow regimes are able to transport and deposit only fine-grained sediments consisting principally of silts and clays The morphology of these sedimentary sequences tend to be ribbon-shaped bodies of coarser-grained sediments enclosed in finer-grained silts and clays Since the majority of the coarser-grained deposits were deposited in relatively low energy environments, they typically contain a significant amount of fine-grained material in their matrix The coarse- grained deposits also are discontinuous because they tend to be truncated as flow direction changed Based on the description of the soil samples collected during the current assessment activities, and the boring logs generated from the CPT borings, the lithology at the 7-11 site appears to be consistent with the lithology described at the STTC site 2 2 Hydrogeology The Site is located in the Sacramento Valley Groundwater Basin Water use In 1990 was 58-percent agricultural, 32-percent environmental, 6-percent urban, and 4-percent other Up to approximately 6 million acre-feet per year of water is exported from the basin, principally to areas in Southern California, by local, State, and Federal conveyance facilities The Sacramento Valley is a major ground-water basin and can be considered a single-aquifer system The storage capacity of the Sacramento Valley ground-water basin is approximately 114 million acre-feet at depths of 20 to 600 feet below ground surface (bgs) (USGS, 1996) 2.2.1 Groundwater Flow Rates and Directions Groundwater at this site has been monitored since the third quarter 1999 Depth to water (DTW) has ranged from 6 69 feet to 10 15 feet in MW-1, and groundwater flows primarily to the southeast and northwest with an average gradient of 0 017 (Table 1 and Figure 5) Groundwater flow direction in the site vicinity has historically fluctuated The Shaw Group indicated that during 13 recorded quarterly events, groundwater flow directions have been to the south/southeast, east/northeast, and west (The Shaw Group, Workplan for Additional Offsite Assessment, Fate and Transport Evaluation Addendum, Sensitive Receptor Survey, and Fourth Quarter 2002 Monitoring and Sampling, 2003) Fate and transport modeling • conducted by SECOR for a site located approximately 1,000 feet east of the 7-Eleven site indicated the same time-dependent variability in groundwater flow direction at the 7-Eleven 117-ElevenW-11 stores1141171Reports114117 SCM and Additional Assessment doc Page 4