OBJECTIVES :

      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 designs for 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 engineers and site owners in designing alternative covers such as ET caps and provide researchers with data to calibrate existing percolation models. 

Experimental Approach:

      The objective of this proposal will be achieved by constructing, instrumenting, and monitoring three lysimeters to evaluate the performance of ET caps.  One lysimeter will be constructed in accordance with RCRA Subtitle D requirements.   One lysimeter will have poplar trees with grass under story for vegetation.  The other will have local grasses and shrubs. The proposed research program consists of four major tasks.   Task 1 involves the pre-construction testing and design of an ET cap and a RCRA subtitle D cap.  Task 2 consists of the construction of lysimeters using the same cap designs obtained in Task 1.  Task 3 consists of monitoring the lysimeter constructed in Task 2 to assess the field performance of the cap design developed in Task 1.  Task 4 will consist of analyzing the data collected in Task 3 and calibrating existing water balance models to be used as predictive tools. 

Result:

     The primary anticipated result of this study will be a set of design steps intended for the use by site owners and design engineers. The documents will provide information on cost-effective ET caps. It is also anticipated that the permit process for cost-effective ET caps designs will benefit from this effort. Local and state regulators will be invited to review and observe the entire process, from site evaluation, to the design of the ET cap to be tested, to the evaluation of the field test results. The other valuable result of this study will be determining the exact percolation rate through a prescriptive cap.  This percolation rate is referred as the equivalent percolation rate and will be compared to those obtained by ACAP.