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r (3) and (4) The site drainage plan has been designed to control runoff the net average r precipitation in December, January, and February (5.67 inches), which is greater than the 25-year, 24-hour storm, which involves 2.28 inches of precipi- tation. Assuming 5.67 inches of precipitation over 33.23 acres of land, which r a runoff coefficient of 1.0 results in a total volume of 5,100,000 gallons. See the calculations in Appendix 4 for Runoff Retention Basin Sizing. The runoff would contain floatables and sediment from the compost areas and other areas of the site. The retention pond will allow sediments to settle out and will incorporate a removal of floatables. r Physical Site Conditions Continental Sediment Sequence. The Great Valley in the area around Stockton was uplifted at the end of the Eocene and was eventually cut off from the Pacific Ocean on the west by subduction and/or accretion events that uplifted or emplaced the present day Coast Ranges. Sediments derived from the eroding Sierra Nevada in the east and the Coast r Ranges in the west were deposited in the Great Valley by rivers and streams from about 35 million years ago to the present. In the vicinity of Forward, the marine sediments are generally overlain by approximately 2,500 feet of unconsolidated Tertiary and Quaternary r sediments of (from oldest to youngest) the Valley springs, Mehrten, Laguna, and Victor Formations. Sediments of these formations were deposited by the activity of the major streams in this area that meandered across the valley floor. rThe most recent deposit, the Victor Formation, is typically heterogeneous and laterally and vertically discontinuous, which is indicative of a fluvial depositional environment. The Victor Formation is generally coarse grained but slightly clayey in the interfan sediment areas in the vicinity of Forward. In this area, the Victor Formation is approximately 100 feet thick. rSite Geology. Site geology has been compiled from subsurface information provided on lithologic logs of soil borings and monitoring wells drilled during previous investigations. rMost of the boring information is clustered in the northern area of the site along the access road and in the southern area around Monitoring Well 5 (MW-5). Fence diagrams and two geologic cross sections were drawn for the site and are illustrated on Drawing 2 (Klein- felder 1991) and Drawing 3 (Beta Associates 1987), respectively. Overall, the northern and southern areas of the site are geologically comparable. For clarity, Drawing 2 does not include all of the borings available on the Fence Diagrams. The location and orienta- tion of the cross sections taken are indicated in plan on Drawing 1. In general, geology at the site is characterized by finely bedded silty, clayey, or sandy materials. Most of the beds are highly variable and discontinuous over distance. This variability is probably the result of deposition in a fluvial environment where the river is r variable in its course. The sediments observed appear to correspond with the description of Pleistocene-aged sediments of the Victor Formation. rwa10029B64.wp5/38 9ns/9a 38 1