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WASTE DISCHARGE REQUIREMENTS ORDER NO.R5-2009-0049 SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS HARNEY LANE LANDFILL SAN JOAQUIN COUNTY 19. The site receives an average of 16.5 inches per year of precipitation as determined from Rainfall Depth Duration Frequency data provided by the State Department of Water Resources for the Linn Ranch Station about three miles northeast of the site. The 100-year, 24-hour precipitation event for this station is 3.3 inches. The estimated mean Class A pan evaporation rate is about 65 inches per year. GEOLOGY 20. 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 19 miles 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 to 7.0 on the Richter scale and a peak bedrock acceleration of 3.5g to 4.5g (1977 Butte County General Plan). 21. The regional geology in the site area represents a transition between Cretaceous to recent age 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. Monitoring well boring logs indicate that the site is underlain by the Victor formation to about 100 feet bgs, and then by the Laguna formation. The former consists of alluvial deposits, generally as follows, from top to bottom: - Silty sand and/or sandy silt (10 to 40 feet thick); - Fine to course grained sand (20 to 45 feet thick); - Clay or clayey sand (20 to 30 feet thick); - Gravelly sand and/or sandy gravel (to interface with Laguna). Similar alluvial deposits are found in the underlying Laguna formation, except that it also contains volcanic and/or metamorphic rock fragments. UNSATURATED ZONE 23. The minimum separation from waste to groundwater at the site is about 83 feet (see Finding 28). 24. In 1991,. the Discharger installed an LFG monitoring system along the site perimeter in accordance with Chapter 15 (now Title 27) regulations. The system consisted of 12 LFG monitoring wells with nested probes screened in the upper, middle, and lower portions of the unsaturated zone, as shown in Attachment C. 25. Subsequent perimeter monitoring of LFG in the unsaturated zone showed high concentrations of methane (up-to 60 percent by volume) and the presence of several volatile organic compounds (VOCs), including tetrachloroethene (PCE), which has also been detected in groundwater at the site (see Finding 30). Since initiation of LFG extraction in 1996, methane concentrations detected along the site perimeter have been greatly reduced (see Findings 43 and 44).