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2-8 Forward Composting Facility SWT Engineering Report of Composting Site Information - August 2019January 2022 z:\projects\allied waste\forward\resource recovery facility\5 yr permit rvw 2019-2020\rcsi 2019\text\sec 2.doc As described above in this section, the composting pad will be constructed of native earth. The permeability of soils below the surface of the composting pad is lo w. Operations will be suspended, if necessary, during periods of wet weather to prevent damage to the composting pad or the surface will be paved with gravel or asphalt if the economics justify the higher cost. Due to the low-permeability of native soils and depth to groundwater, monitoring of soil moisture is not warranted. In accordance with WDR Order No. R5 -2014- 0006 and WDR Order No. 2020 -0012-DWQ , the soil in the upper one foot of the composting pad and compost storage pad areas are tested and if necessary, conditioned and compacted to achieve 1 X 10 -5 cm/sec vertical hydraulic conductivity. The site and pad will have a minimum of a one percent slope and windrows will be constructed normal (or perpendicular) to the slope, so there will be no standing water on the pad. Detailed design calculations are presented in Appendix B. 2.1.6 Drainage Control Run-on to the composting site is prevented by an existing berms and ditches located on the eastern and a portion of the south edgesperimeter of t he siteRRF /Compost site area. S lopes generally decrease from the northeast to the southwest. An existing ditch prevents run-on from the north. The grading and drainage plan (Figure 6) shows the proposed grading of the RRF/Compost Facility. The grading plan is the same for all of the potential composting alternatives. A one percent minimum grade is provided at all points in the composting site. The composting windrows are arranged normal to the slope to allow free drainage and minimize ponding. The site is sloped to drain to two ditches designed to carry the water from the 25 -year, 10- minute duration storm (0.368 inches) assuming all the water that falls on the composting site runs off. The ditches will carry the water to a sedimentathe lined compost retention basin designed to hold the net average annual rainfall during months where rainfall exceeds evaporation (5.67 inches) assuming all the water that falls on the site runs off . This exceedscontain the 25-year, 24-hour rainfall event (2.84 inches), with adequate freeboard. As described above in Section 2.1.5, the composting pad will be constructed of native earth. The permeability of soils below the surface of the composting pad is low. Operations will be suspended, if necessary, during periods of wet weather to prevent damage to the composting pad or the surface will be paved with gravel or asphalt if the economics justify t he higher cost. Due to the low -permeability of native soils and depth to groundwater, monitoring of soil moisture is not warranted. In accordance with WDR Order No. R5 -2014- 0006, the soil in the upper one foot of the composting pad and compost storage pad areas are tested and if necessary, conditioned and compacted to achieve 1 X 10 -5 cm/sec vertical hydraulic conductivity. 2.1.7 Leachate Control (14 CCR, Section 17867(a)(12)) Since additional water will only be applied to the compost when necessary for maintaining adequate internal moisture, significant leachate generation is not antici pated. As stated in Section 3.5.3, the site is in a region of net evaporation. Additionally, compost windrows typically form a light crust on the outside, which repels rainfall. The site's drainage control system, including the lined compost retention ba sin, was designed to handle all runoff , from a 25-year, 24-hour storm, including runoff from the compost piles. This system is illustrated in Figure 6 and runoff calculations are attached in Appendix C.