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North County RC&SL Page 17 of 64 Public Works/Solid Waste Joint Technical Document County of San Joaquin – Revised July 14, 2025 mapped in an area containing potentially damaging earthquakes that have produced magnitude 5 or greater from 1800 through 1999 (T. Toppozada and others, 2000). Based on historic data, it does not appear that the Tracy-Stockton Fault represents a significant hazard to the landfill. Fault proximity was assessed in three reports [Preliminary Closure/Postclosure Maintenance Plan, February 10, 2010 (Appendix B and Plate 13), Geologic and Hydrogeologic Report, January 1988 (Appendix C) and draft Seismic Hazard report, December 8, 1994 (Appendix D)]. These reports discuss fault proximity and activity. One fault (the Tracy-Stockton Fault) is not discussed in either the 1988 or 1994 reports. Additional information on this fault is provided in this Section. 4.4 HYDROGEOLOGY 4.4.1 Surface Water NCRC&SL is not within the 100-year floodplain (FEMA digital Q3 flood maps for the landfill and areas surrounding the landfill, as provided on the FEMA and ESRI map server). Surface water from the landfill is directed to the seasonal wetlands on site. If these wetlands become full, surface water flows into a tributary of the South Paddy Creek, which flows into the San Joaquin River. 4.4.2 Regional Hydrogeology Regional hydrogeology is described in 1988 Geologic and Hydrogeologic Report (Appendix C). 4.4.3 Hydraulic Conductivity In situ hydraulic conductivity tests were performed at the landfill using slug tests in a groundwater well and falling head testing in two permeameters. Based on these tests, the hydraulic conductivity in the groundwater well is estimated to be between 8.3x10-3 and 9.7 x10-3 cm/sec, depending upon the assumed depth of penetration of the groundwater well into the aquifer. Permeameter tests yielded hydraulic conductivities ranging from 2x10-7 to 3x10-7 cm/sec. Laboratory hydraulic conductivity on remolded samples ranged from 1x10-4 cm/sec (silty sand) to 2x10-8 cm/sec (very clayey fine sand) (Appendix C).