6.2 SR 10-03-2022 - Laserfiche WebLink

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3.6 Leachate System Management The leachate handling systems collect leachate and transfer it to one of the leachate storage tanks. Leachate is either recirculated through the waste in accordance with the recirculation plan or hauled to a WWTF for treatment. The leachate collection system pipes will continue to be cleaned at a minimum of once per year. Leachate will be sampled and analyzed in accordance with the SAP (Barr Engineering Feb. 2021, or as modified). The HELP model shows that after final closure, leachate generation approaches zero. This is due to the extremely efficient performance of the FML/geonet combination. However, the tremendous number of variables utilized in the HELP model usually allows only a gross approximation of leachate quantity. Therefore, an alternative method for estimating post -closure leachate generation will be used. The other method used to estimate leachate quantity is by direct comparison to existing landfills with leachate collection. Using this latter method the MPCA suggests that appropriate estimations can be obtained by expecting five percent of total precipitation which contacts a landfill final cover. Utilizing this method, the maximum average annual leachate production for the facility during post -closure is estimated to be 4,655,325 gallons. Based on experience, the volume of leachate produced will decrease over the postclosure period. Assuming the maximum leachate generated during the first year of post -closure is 4,655,325 gallons, the quantity generated decreases by 50% by year five (2,327,662 gallons), and leachate generation is negligible by the end of the post -closure period (year 20); the average annual volume of leachate produced during post -closure is 1,745,747 gallons. Landfill gas condensate and liquid from the gas extraction wells is also managed by the leachate collection system and hauled off -site for treatment at a WWTF. The quantity of condensate managed during post -closure for the purposes of deriving treatment costs was estimated at 4% of the leachate produced. The average annual condensate production is calculated as 69,830 gallons. Therefore, the average generation of leachate and condensate will be approximately 1,815,577 gallons/year (1,745,747 gallons leachate + 69,830 gallons condensate). Leachate may be recirculated back into the waste to maximize landfill gas production during landfill operation, however, for purposes of financial assurance cost estimation, it is assumed that leachate is discharged directly to Elk River WWTF for treatment. It should be noted that the Landfill continually reviews their options with regard to leachate management, both on- and off -site, and will continue to do so into the post -closure period. 3.7 Gas Monitoring During post -closure, landfill gas monitoring will be performed at minimum on a quarterly basis. Monitoring of certain aspects of the gas system will likely be conducted more frequently in accordance with other permits and regulations (i.e. air permit). A portable methane gas meter will be used to measure the gas concentrations at designated monitoring points. A portable meter will also be used for conducting surface scans. The in -place meters measuring flow and other parameters at the flares and gas plant will also likely continue to be used into the post -closure period. At the perimeter gas probes, the detection of methane above 25 percent of the LEL at any of the monitoring points will trigger an investigation as to the cause, and likely result in an adjustment of flow rate in the landfill gas wells associated with the area in question. MProject Number: 227705133 3.3