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Taj M. Bahadori -2- 14 December 2018 Foothill Landfill Stockton The Liner EAD Report proposes replacing the LCRS gravel blanket layer over the base liner with a geocomposite drainage layer due to the high cost and lack of availability of suitable local gravel. No modification of the LCRS sump and sideslope EAD designs under the WDRs is being proposed. As such, the proposed design modification does not involve elimination of any liner components per Item 1 above. The relevant engineering properties of the blanket LCRS layer (Item 2 above) include the following: • Hydraulic Head — must be less than 30 cm (11.8 inches) per Subtitle D (40 CFR 258.4(a)(2)) • Flow Capacity— LCRS must be able to handle at least twice the anticipated maximum daily leachate flows per Title 27, section 20430(b); and • Material Strength - must be able to withstand compressive loads of overlying wastes, cover and landfill equipment per Title 27, section 20430(e). The proposed modified EAD design was evaluated for the above properties using field data from adjacent Module 2A for comparison. Hydraulic modeling of the proposed design under estimated critical conditions indicated a maximum head on the base liner of 0.06 inches, much less than the typical geocomposite thickness of 0.3 inches.' The peak leachate drainage rate for the LCRS geocomposite was calculated to be 0.34 inches/day (9,240 gal/ac/day), about 2.6 times higher than the actual peak leachate drainage rate of 0.13 inches/day (3,460 gal/ac/day) at Module 2A based on 2017 leachate monitoring data.These results support a conclusion that the proposed LCRS design modification meets Title 27 and Subtitle D minimum standards for.design head and flow capacity.2 After correction for various safety factors (clogging, material creep, and design), a design transmissivity for the geocomposite was then calculated as a function of the peak drainage rate from the HELP analyses, maximum distance to LCRS pipe, pipe hydraulic conductivity, and slope of the drainage layer. Design transmissivities for module construction quality assurance (CQA) testing were then calculated for various landfill development stages, including early development (15 vertical feet), interim development (145 feet of waste), and the maximum future build-out for LF-2 anticipated by the Discharger (300 feet). The results indicated corresponding minimum required 100-hour design transmissivities of 7.1x10-4 m2/s, 2.9 x10-4 m2/s, and 3.5x10-4 m2/s, respectively. Corresponding minimum required compressive strengths (including a safety factor of 1.5)were 1,500 psf, 16,500 psf and 38,250 psf. The report indicates that geocomposite LCRS products are available with minimum transmissivities and compressive strengths capable of withstanding these corresponding loads, including safety factor. Given that the Discharger is not proposing any changes to the liner itself, nor to the LCRS sumps, the proposed LCRS design should be at least as protective of water quality as that prescribed in the WDRs (Factor 3 above). Slope stability and other module-specific engineering issues associated with the proposed liner design modification, including any necessary modifications to the above analysis or water quality issues associated with the design, will, however, need to be addressed in individual design reports for modules (or module phases) as they are proposed. 1, Hydraulic modelling for leachate head and flow rate was conducted using the Hydraulic Evaluation of Landfill Performance(HELP)program under assumed critical conditions(e.g.,15-foot waste column,wettest 10-year period on record). Only the LCRS blanket layer was included in this analysis, not the LCRS piping. 2. The results are believed to be conservative, given that the actual leachate drainage rate for Module 2A reflected both the gravel blanket layer and LCRS piping,while the HELP run included only the geocomposite blanket layer.