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u 10.0 LEACHATE CONTROL In some areas of the United States, where precipitation is high or when cover on landfills is poor, rain may infiltrate through the landfill cover soil and percolate into the waste fill. Leachate is produced when the infiltration of water into the landfill exceeds the moisture -holding capacity of the wastes. When the absotptiT ve� pacity of the refuse is exceeded, leachate (water containing soluble substances) is generated. To evaluate leachate generation potential in landfills, the U.S. Environmental Protection Agency uses the water -balance method developed by C.W. Thorn- waite. This method is based on the relationship between precipitation, surface runoff, evapotranspiration, and soil -moisture storage. Using this method, the soil moisture remaining in the vegetated cover soil profile can be estimated monthly by accounting for gains from precipitation and losses from evapotranspiration. Thornwaite's water -balance method was used to evaluate the leachate genera- tion potential of the final cover at the NCSL (see Table 10-1). Based on the data, it was determined that infiltration would not occur and, therefore, that leachate generation should not be significant. The completed landfill will be covered with at least 4.5 feet of cover material con- sisting of the following: A minimum 24 -inch foundation layer of compacted soil placed over waste fill • A minimum 12 -inch layer of clayey soil compacted to achieve a hy- draulic conductivity of 1 x 10-6 cm/sec or less, placed over the foundation layer • A minimum 18 -inch layer of vegetative cover soil placed over the low -permeability layer The vegetative cover will be fertilized, as necessary, and seeded with moderately rooted, erosion -resistant vegetation native to the surrounding area. 43 Rev. 0 July 19, 1989