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WASTE DISCHARGE REQUIREMENTS ORDER R5-2015-0058-01 -12- SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS FOOTHILL SANITARY LANDFILL, INC. FOOTHILL LANDFILL SAN JOAQUIN COUNTY Facility Specification E.15, SPRR. 5 See also Finding 92, MRP Section A.4, and Attachment D: Drainage Controls & Monitoring. 52. WMUs must be constructed to accommodate stormwater runoff from 24-hour precipitation events with a return period of 1,000 years for Class II WMUs. (See Title 27, § 20320). According to National Oceanic and Atmospheric Administration’s (NOAA) Precipitation Frequency Atlas 14, Volume 6 (rev. 2014), the Facility’s 1,000-year, 24- hour rainfall event is estimated to result in 5.45 inches of precipitation, respectively. Source: NOAA Precipitation Frequency Data Server (https://hdsc.nws.noaa.gov/hdsc/pfds). 53. All landfill drainage facilities, including overside drains, perimeter ditches, culverts, and sedimentation basins were designed to handle a 24-hour, 100-year storm event. UNSATURATED ZONE CONDITIONS 54. The maximum estimated capillary rise in the unsaturated zone is estimated to be about 3.3 feet based on soil type. The minimum separation from waste to groundwater, taking into account the estimated capillary rise, is about 170 feet. See Findings 27 and 62.6 Soil Gas 55. In 2003, the Discharger installed 11 single-completion soil gas monitoring probes (SG-1 through SG-11) within 400 feet of the landfill unit to monitor for landfill gas migration. Ten of the probes, SG-2 through SG-11, are shallow probes completed to depths ranging from 25 to 68 feet bgs. The other probe, SG-1, is a deep probe completed to a depth of 255 feet bgs. SG-1 is also the closest probe to the landfill, installed 100 feet from the landfill unit. Probe SG-8 was abandoned in 2012 in preparation for construction of Module 2. The existing and former probe locations are shown in Attachment E: Gas Controls & Monitoring. 56. Landfill gas has generally been detected at low concentrations in all of the above soil gas probes, except for SG-11 on the west side of LF-1, where methane and carbon dioxide have been detected up to 17% and 16% by volume, respectively. All of the probes have indicated the presence of volatile organic compounds (VOCs), however. The most VOC-impacted probes included SG-6, SG-11 and deep probe, SG-1, which had the highest concentration of vinyl chloride (418 ppbv). The results of these three soil gas probes are summarized below. 5. State Water Resources Control Board Quality Order No. 97-03-DWQ National Pollutant Discharge Elimination System (NPDES) General Permit No. Cas000001 Waste Discharge Requirements for Discharges of Storm Water Associated with Industrial Activities Excluding Construction Activities. 6. Calculation based on lowest elevation of waste (216 feet MSL) and estimated highest groundwater elevation (46 feet MSL, including capillary rise) at M-1’s LCRS sump.