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WASTE DISCHARGE REQUIREMENTS ORDER R5-2015-0058-01 -15- SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS FOOTHILL SANITARY LANDFILL, INC. FOOTHILL LANDFILL SAN JOAQUIN COUNTY GROUNDWATER CONDITIONS 62. First encountered groundwater at the site varies from about 200 to 300 feet bgs. Groundwater elevations generally range from about 36 feet MSL on the southern side of the site to about 61 feet MSL on the northern side of the site +/- 1 foot of seasonal variation. The average groundwater elevation at the site is about 45 feet MSL. The uppermost aquifer beneath the site occurs in Mehrten alluvium (e.g., sandy gravel) under confined or partially confined conditions. The overall permeability of these deposits is estimated to be about 9 x 10–3 cm/sec based on monitoring well slug testing data for similar alluvial deposits at the North County Landfill about eight miles northwest of the site. The groundwater gradient is typically about 0.0025 ft/ft toward the southeast, corresponding to a groundwater flow velocity of about 84 ft/yr. In the southwest corner of the site, the gradient appears to turn toward the southwest. Additional definition of the gradient direction in this area will be possible after collecting a sufficient amount of monitoring data (e.g., at least two semiannual periods) from recently-installed monitoring well MW-6 (see Finding 65). 63. Background groundwater quality in the uppermost aquifer at the site is relatively good including total dissolved solids (TDS) at about 210 mg/L, electrical conductivity at about 270 mg/L, chloride at about 6 mg/L, sulfate at about 8 mg/L, and bicarbonate alkalinity at about 140 mg/L. 64. The beneficial uses of underlying groundwater stated in the Basin Plan are municipal and domestic supply, agricultural supply, industrial service supply, and industrial process supply. 65. The groundwater monitoring system at the site currently consists of one background well (MW-4), two side gradient wells (MW-1A, and MW-3), and three down gradient wells (MW-2R, MW-5, and MW-6).8 MW-2R and MW-3 are Point of Compliance wells for LF-1 and LF-2 (Module 1), respectively, while MW-5 (installed in 2010) and MW-6 (installed in 2015) contiguously monitor both landfill units along the southern site boundary. MW-1A will also service as the background well for the proposed surface impoundment and proposed MW-8 will service as the down gradient well (detection well). See MRP Section A.1.a and Attachment C: Site Map. 66. General water quality monitoring and system requirements (Title 27, § 20415, subd. (e)(6)) require initial background sampling for each WMU. At a minimum, the Discharger must collect analytical data obtained during the quarterly sampling of all the Background Monitoring Points for a period of one year. For any new WMU, this data shall be collected before waste is discharged at the WMU and background soil-pore liquid data shall be collected from beneath the WMU before it is constructed. These WDRs require the Discharger to evaluate, propose, and implement a Detection Monitoring Program (DMP) for the proposed surface impoundment, as specified in Provision H.9.l. Subsequent changes to these networks will be reflected in a Revised MRP (operative MRP). 8. Monitoring well MW-1A was installed in 2013 as a replacement well for MW-1, which could no longer be sampled due to declining water levels. Another historical monitoring well, MW-2, was abandoned and replaced with nearby well MW-2R in 2003 to accommodate development of LF-2, Module 1.