Abstract
People generate a large and ever-increasing volume of waste, which originates in rural and urban areas, industrial operations and other activities. In spite of waste recycling, which is the solution to the huge amount of solid waste, a large part of it is deposited into landfills, dumps, etc.Landfills are, at present, the most widely used waste disposal facilities. Final cover systems are used at landfills and other types of waste disposal sites to control moisture and percolation, promote surface water runoff, and minimize erosion. In addition, covers prevent direct exposure to the waste, control gas emissions and odors, prevent occurrence of disease vectors and other nuisances, and meet aesthetic and other end-use purposes. The Resource Conservation and Recovery Act (RCRA) Subtitle D, requires the use of landfill covers to meet these needs.Prescribed covers, permitted by current regulations, are based on a barrier concept that requires them to employ resistive principles, i.e., a layer having low saturated hydraulic conductivity. The most important focus of this study, is the EvapoTranspiration cover (ET) concept, which utilizes one or more vegetated soil layers to retain water until it is either transpired through vegetation, or evaporated from the soil surface. These cover systems rely on the water storage capacity of the soil layer, rather than low hydraulic conductivity materials, to minimize percolation. ET cover designs are based on using the hydrological processes and water balance components at a specific site, which includes the water storage capacity of the soil, precipitation, surface runoff, evapotranspiration and infiltration. The objective of this study is to determine the feasibility of developing and using ET Covers in tropical locations with medium to high precipitation. Puerto Rico can be assimilated as a part and sample of the typical average “Tropical Zones,” so the study was concentrated there. The obtained conclusions can be applied later, along with the same principles, to Tropical Zones in general, which will have similar characteristics of rainfall, vegetation, soil texture and classification, and temperature. The study consisted of collecting climate, soil, and vegetation data and characteristics from six regions or “Ecozones” determined on the Island of Puerto Rico, as a representative of the “Tropical Sub-climate.” The collected data was then used as input of a software called PR-ET to calculate the PET or ETo of the location and then the basis data of a flow model (HYDRUS 1-D) to simulate infiltration into the waste during peak weather events. PET and ETo are similar terms and are defined as the “Potential Evapotranspiration.” Simulations were performed to evaluate the effect of the weather period, the effect of soil thickness, the effect of vegetation, the LAI (Leaf Area Index) and finally to determine what regions in Puerto Rico show potential for implementing ET covers.Results from this study showed some sub-regions (or locations) belonging to the six Ecozones (or regions) of Puerto Rico were able to meet the preliminary requirements for hydrological performance as required by the RCRA. However, field evaluation of these designs should be performed before full implementation of ET covers in these regions. Of the 21 locations studied in the preliminary design, 15 were adequate for study by modeling them to confirm the feasibility of using the Evapotranspiration covers in them. After this modeling or simulation was conducted, the results were as follows: eight locations can effectively use ET covers using as covering vegetation pastures. Five locations can use ET covers, but the vegetation needs to be changed using then shrubs and grass. The other locations are being rejected for this study. The selected locations are supposed to have thickness less than 2.0 m.
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