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WASTE DISCHARGE REQUIREMENTS ORDER NO. R5-2010-0016 -5- SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS NORTH COUNTY LANDFILL SAN JOAQUIN COUNTY <br /> <br /> <br />GEOLOGY 21. The regional geology in the site area represents a transition area between Cretaceous to Quaternary Period alluvial deposits of the Great Valley flood plain and Jurassic Period metamorphic rocks of the Sierra Nevada foothills. The valley deposits thin out within a few miles east of the site where the surface geology is primarily foothill terrain dominated by dissected alluvial uplands and exposed, uplifted bedrock. 22. There are no known Holocene faults within 1000 feet of the facility. The closest active fault is the Bear Mountains fault zone within the Foothills Fault system about 18 miles (29 km) east of the site in the Sierra foothills. Recorded magnitudes of seismic events along this fault zone range up to 5.8 on the Richter scale (1975 Oroville event). The Foothills Fault system has been characterized as producing a maximum credible earthquake of 6.5. 23. The Central Valley Coast Range Fault, approximately 54 km from the site, has a maximum probable earthquake (MPE) of 6.4, and the San Andreas Fault, approximately 126 km distant has an MPE of 8.0; peak horizontal ground accelerations associated with each MPE event are .10g and .09g, respectively. 24. Surface soils at the site consist of interbedded silts and clays to about 3 feet bgs, underlain by a thin (i.e., ½ foot) layer of hardpan. Beneath the surface soil layers, soils consist of laterally discontinuous Riverbank (northern half of site) and Turlock Lake (southern half of site) formation alluvium, including silts, clays, sand and gravel layers. These deposits show coarsening-up patterns typical of Pleistocene Age alluvial stream deposits flanking the eastern Sierra foothills. <br />UNSATURATED ZONE 25. The lowest elevation of solid waste in the landfill is about 60 feet MSL, corresponding to the base of M3. The lowest elevation of leachate in the landfill is about 56 feet MSL, corresponding to the base of the LCRS sump for M3. 26. The estimated capillary rise in the unsaturated zone is estimated to be less than five feet based on soil type. The minimum separation from waste to groundwater, taking into account the estimated capillary rise, is about 83 feet. See Findings 25 and 34. 27. Hydraulic conductivities ranging from 1 x 10-6 cm/sec to 1 x 10-8 cm/sec (based on testing of remolded laboratory samples) have been measured in the upper 10 feet of the unsaturated zone, which contains a greater percentage of clay and silt than in underlying layers (e.g., 30 to 60 feet bgs), where higher hydraulic conductivities (ranging from 1 x 10-4 to 2 x 10-7 cm/sec based on permeameter and laboratory testing) have been measured. See Finding 24.