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