2012-09-06_REVISION - M2008070 (25)

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Design and Operations Plan Western Gravel E & P Waste Disposal Facility 3.4.6 Leachate Collection and Removal System (LCRS) October 24, 2011 Page 25 of 51 The basis of constructing and operating a LCRS at each cell is to facilitate detection and removal of leachate so that the pressure head on the cell floor liner will not exceed one foot, in accordance with CDPHE Regulations and to prevent leachate leakage from the cell. Because of arid climatic conditions and HELP Model results, significant leachate will not be generated at the facility. Any leachate generation will mainly occur during waste placement when the cell is open to precipitation. Five key elements will be used to control leachate at the facility including the following. • Minimize precipitation run -on • Use of daily and interim cover • Construction of landfill cap (final closure) • Construction of cell floor and sidewall liners • Construction of LDS 3.4.6.1 LCRS Construction A permanent LCRS will be constructed in each landfill cell. Leachate collection sumps and leachate removal appurtenances will be similar for each cell. LCRS components consist of a drainage layer placed over cell floors and sidewalls to collect and convey leachate to a sump located at the lowest point of the cell. The LCRS of each cell is based upon facilitating detection and removal of leachate in order to prey ent cell floor liner head pressure from exceeding one foot and to prevent leachate leakage from the cell. A one -sided geonet will be placed fabric side up over cell sidewalls that will be constructed to a 3 (H) to 1 (V) slope configuration. The geonet will be placed directly over the 60 -mil HDPE primary liner and installed in the outside anchor trench, similar to underlying HDPE liner material. A minimum protective soil cover of at least 12- inches will be placed over the composite geonet within 30 days o f geonet installation. The cell floor will be sloped a minimum of 2 % to the low point of the cell and leachate collection sump. A minimum 12 -inch thick layer of leachate collection gravels will be placed across the cell floor and over protective materials (i.e., 6 -oz. non -woven geotextile) that are placed over the primary liner. The leachate collection gravels will consist of free draining gravels having a permeability of 1 x 10 -2 cm/sec or greater. The geonet placed along side slopes will overlap gravels a minimum of 2 feet at the toe of the sidewalls. A layer of 6 -oz. non -woven geotextile will be placed over the gravels in order to inhibit fine soil materials from migrating into the gravels. A minimum protective soil cover of at least 12 inches thic k will be placed over the fabric within 30 days of installation. Estimated travel time of leachate migration from the furthest distance (370 feet) to the collection sump is 25 days, assuming a hydraulic conductivity of 5x10 -2 cm/sec and a porosity of 0.32 for leachate gravels placed on the cell floor and geonet hydraulic conductivity of 1x10 -2 cm/sec and porosity of 0.25 that is placed on the 3:1 side slopes. Travel time calculations are included in Appendix G. The leachate sump has been sized to collect run -off from the 2 year, 24 hour storm event falling on a newly completed landfill cell encompassing an area of approximately four acres. However, since final cover is designed to shed precipitation, leachate generation following cell closure is not anti cipated. The following inputs were the basis used for sump size calculations. • The sump is filled with clean leachate gravels. • The porosity of the gravel is 0.32. NWCC, Inc.