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of straw -bale dikes will be constructed approximately every 150 feet along the downgradient silt fences to limit downgradient flow of sediment -laden water until vegetative cover is established. Final cover and vegetation will be placed as soon as possible after the module is brought to final grade. Vegetative growth will reduce sheet flow runoff, limiting erosion and movement of sediment. 5.6 LEACHATE MANAGEMENT 5.6.1 Leachate Generation Potential 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 absorptive ca- 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 5-3). 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 the waste fill PJ9 9390218A.00W 44 Rev. 1 September 8, 1989