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FIELD EVALUATION OF EVAPO-TRANSPIRATION (ET) CAPS
Impact/Purpose:
Surface caps are frequently used to reduce the quantity of water that infiltrates into contaminated soils and/or into waste deposits to prevent further movement of contaminants or reduce leachate quantities. At many sites, the applicable rules and regulations (ARARs) require that the surface caps employ resistive principles, i.e., layers having low saturated hydraulic conductivity (compacted clay barriers, geosynthetic clay liners with or without a geomembrane). An alternative approach is to use Alternative Covers, such as Evapo-Transpiration (ET) caps, which exploit the water storage capacity of finer textured soils and the water removal capability of vegetation to reduce infiltration into the underlying waste. In a way, ET caps are designed to work with the forces of nature rather than attempting to control them. Before the acceptance of alternative landfill caps by regulatory agencies, and the full-scale implementation of alternative caps can be achieved, field studies are needed to verify the effectiveness of the designs.
The objective of this study is to assess the field performance of ET caps through a proposed filed study. This study was developed to continue the effort led by the research team in developing guidelines for using ET caps in the state of Florida and beyond. The first two steps of this process have been started by two mini-projects funded by the Florida Center for Solid and Hazardous Waste Management. Data from this study will be incorporated with ACAP data to assist regulators in formulating rational regulations toward the acceptance of the use of alternative covers such as ET caps and provide researchers with data to calibrate existing percolation models.
Description:
A field study was conducted to assess the ability of landfill covers to control percolation into the waste. Performance of one conventional cover was compared to that of two evapotranspiration (ET) tree covers, using large (7 x 14 m) lined lysimeters at the Leon County Solid Waste management facility in Tallahassee, Florida. Additional unlined test sections were also constructed and monitored in order to compare soil water storage, soil temperature, and tree growth inside lysimeters and in unlined test sections. The unlined test sections were in direct contact with landfill gas.
Surface runoff on the ET covers was a small proportion of the water balance (1% of precipitation) as compared to 13% in the conventional cover. Percolation in the ET covers averaged 17% and 24% of precipitation as compared to 33% in the conventional cover. On average, soil water storage was higher in the lined lysimeters (429 mm) compared to unlined test sections (408 mm). The average soil temperature in the lysimeters was lower than in the unlined test sections. The average tree height inside the lysimeters was not significantly lower (8.04 m for eucalyptus and 7.11 m for cottonwood) than outside (8.82 m for eucalyptus and 8.01 m for cottonwood). ET tree covers vegetated with cottonwood or eucalyptus are feasible for North Florida climate as an alternative to GCL covers.