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determine if the initial indication of elevated methane at the property boundary was actually related to methane migration or to installation of probes PGP-1 and PGP-2 into refuse- containing areas that are in communication with the rest of the landfill. 2.3 GEOLOGIC CONDITIONS 2.3.1 REGIONAL GEOLOGY The Cove Contractors Landfill is located near the center of the Great Valley province of California. The Great Valley is a large, northwest trending structural trough, bounded on the east by the Sierra Nevada province and the Cascade Mountains province, on the west by the Coast Ranges province, and to the south by the Stockton Arch. The Great Valley drains west to the Pacific Ocean through the Stockton delta and into the San Francisco Bay (Hackel, 1966). The site is underlain by a thick sequence of flat lying marine and non-marine sediments (Harden, 1998). Unconsolidated Quaternary sediments derived from the 120-foot thick Victor Formation and underlying Laguna Formation overlie the marine sediments. The Victor Formation is indicative of a fluvial depositional environment and is generally coarser grained than the underlying Laguna Formation but is somewhat clayey in the Stockton vicinity. The interfan and basin sediments in this area generally have low infiltration rates based on their texture. The Laguna Formation is composed of semi-consolidated silt, sand, and gravel. 2.3.2 REGIONAL FAULTING According to a review of the "San Joaquin County Council of Governments Seismic Safety Element, May 1973," there do not appear to be mapped Holocene Faults in the site vicinity. The nearest potentially active fault, according to this publication, appears to be the buried Tracy- Stockton Fault, which passes approximately one mile to the north of the site and trends west- southwest to east-northeast. The position of this fault was determined from oil well-log data; no surface expression of this fault has been mapped. Subsurface data indicate that no appreciable movement has occurred on the Tracy-Stockton Fault since Mid-Pliocene time, perhaps more than five million years ago. Numerous active faults have been mapped in both the Coast Ranges (10 to 30 miles west) and the Sierra Foothills (40 to 50 miles east). 2.3.3 SITE GEOLOGY Based on previous site investigations, the surface geology of the site appears to be interfan or flood plain sediments deposited by influence from Duck Creek and/or the San Joaquin River. The sediment beneath the site consists of relatively horizontal bedded clay, silt, and sand. The available regional geological data suggests that on a broader scale, outside the study area, these beds may interfinger. 3 Mice NNOC E idna P:\707 Cove Contractors\Feasibility Study and Corrective Action Plan\Corrective Action Plan_Final.doc