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Site Conceptual Model October 3, 2002 ARCO Facility No 6100 MW-2 is situated along the trace of cross-section A-A' (Figure 3), and also appears constructed with the screen situated across the Sand l/silt contact This construction suggests that small quantities of groundwater are flowing along the lithologic contact, within Sand 1, and when the water encounters the well, enters and slowly fills the casing Given the apparent lateral continuity of Sand 1 (at least based on data colleted during the most recent assessment activities), and the similar construction of all other wells with rechargeable water (VW-1, VW-5, VW-6, and VW-14), it appears likely that similar lithologic conditions exist at these other wells Those wells screened within Sand 1 that do not have rechargeable groundwater likely do not intersect the sand/silt lithologic contact, or are completed in an underlying sand stratum, such as well VW-15 The source of the groundwater encountered in Sand 1 appears to be due to meteoric water, likely due to percolation of rain water on the undeveloped lands surrounding the ARCO facility Groundwater elevations with respect to time for wells VW-1 through VW-6 and VW-14 are presented in Figures 5 and 6 It should be noted that, in general, the peaks, valleys, and inflection points on the hydrograph of each well appear in phase with the other wells In addition, the water levels appear lowest during the late summer and autumn monitoring events, and highest during the winter and spring monitoring events This cycle corresponds with the rainy season cycle These observations suggest a seasonal fluctuation in groundwater elevations measured in these wells Other potential sources that might contribute to groundwater in Sand 1 include the septic system northwest of the station building (Figure 2), or leaks from water supply piping or irrigation piping that transect the site Input from these sources appears to be minimal, as human habitation of this site is constant year-round If the water in Sand 1 was due primarily to the septic leach field or leaking water pipes, the hydrographs would not indicate seasonal fluctuations, as these systems would be in use all year The accumulation of water in well MW-1 appears to be due to a similar mechanism Well MW-1 is constructed to intersect the contact between Sand 2 and the underlying silty unit (Figure 4) Water flowing along this contact would accumulate in well MW-1 in a manner similar to that described above for wells VW-1 through V-6, and VW-14 However, as shown on Figure 3, wells MW-3, MW-5, and MW-7 (and well MW-9 on Figure 4) are constructed �rvr similarly to well MW-1, but do not contain groundwater This lack of water in any of the deep- screened wells except MW-1 suggests a localized vertical flow of groundwater through the silt between Sand 1 and Sand 2 3(�< ?-W) ,1-3 Ipi( C0.)z , s -�, 93'ew. V) ap, wss-„�,w. f G35 33 Pi) a�f c4M '6 6A.a It s,i t'Uta.- S,,j 2 Standing water levels in the water supply well have previously been measured at approximately 45 feet bgs Construction details for this well are included in Appendix E The supply well was constructed with a gravel pack from approximately 35 feet to 540 feet bgs, and is screened between 396 and 536 feet bgs This well appears to be screened in the regional water supply aquifer Based on the screen interval and the water levels measured within the well, the regional water aquifer appears to be confined This is supported by the possible presence of an artesian well approximately 2,000 feet southeast of the site (U S Geological Survey topographic Map, Midway, CA, photorevised 1980) The location of this artesian well is shown on Figure 1 P 1Bp-Arco for URS161001Reports16100 SCM doe Page 7 Stratus