Grantee Research Project Results

Final Report: Field Evaluation of Evapo-Transpiration (ET) Caps

EPA Grant Number: R830845
Title: Field Evaluation of Evapo-Transpiration (ET) Caps
Investigators: Abichou, Tarek , Chan-Hilton, Amy , Tawfiq, Kamal , Abdelrazig, Yassir
Institution: Florida Agricultural and Mechanical University
EPA Project Officer: Hahn, Intaek
Project Period: April 1, 2003 through March 31, 2005 (Extended to June 30, 2006)
Project Amount: $395,548
RFA: Superfund Minority Institutions Program: Hazardous Substance Research (2002) RFA Text | Recipients Lists
Research Category: Land and Waste Management , Safer Chemicals , Hazardous Waste/Remediation

Objective:

The objective of this research project is to assess the field performance of evapo-transpiration (ET) caps through a field study. This project was developed to continue the effort led by the research team in designing 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 Hazardous Waste Management. Data from this study will be incorporated into Alternative Cover Assessment Program data to assist engineers and site owners in designing alternative covers, such as ET caps, and to provide researchers with data to calibrate existing percolation models. The objective of this research project is achieved by constructing, instrumenting, and monitoring three types of lysimeters to evaluate the performance of ET caps. One lysimeter was constructed in accordance with Resource Conservation and Recovery Act (RCRA) Subtitle D requirements and is refereed to as a prescribed cover. One lysimeter has cottonwood trees with grass for vegetation. A third lysimeter has eucalyptus trees and grass.

Summary/Accomplishments (Outputs/Outcomes):

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.

Conclusions:

Three test sections, which simulated conventional and ET covers, were constructed at the Leon County Solid Waste Management Facility in Tallahassee, Florida. The three test sections were instrumented for measuring key water balance components (precipitation, runoff, soil water storage, and deep percolation), soil temperature, and tree growth. Eucalyptus and cottonwood trees were used in the ET covers.

Surface runoff was very small for all two ET covers (1% of precipitation) but was much higher for the conventional cover (13% of precipitation). The average percolation rate during the first year of monitoring ranged from 47% of precipitation in the ET cover with eucalyptus to 56% of precipitation for cottonwood. The percolation rates after the tree establishment period was 17% and 24% of precipitation for eucalyptus and cottonwood, respectively, as compared to 33% in the conventional cover. The evapotranspiration to precipitation ratio was 81% for eucalyptus t, and 75% for cottonwood. The percolation performance ratios of 0.63 and 0.88 for the eucalyptus and cottonwood, respectively, indicate that ET covers performed better than the conventional cover. The results also show that eucalyptus performed better than cottonwood. Soil water storage was slightly higher in the lysimeters as compared to in the unlined test sections (408 mm). The higher soil water storage inside the lysimeters may be attributed to the presence of the geocomposite, which may have induced capillary effects and increases the water content above it. Soil temperature was slightly lower inside the lysimeters than in the unlined test sections.

Analysis of variance (ANOVA) comparison of tree growth, (tree height and DBH), inside and outside lysimeters, indicated that there was no significant difference in tree growth. This study on the effects of lysimeter’s lower no-flow boundary provides a basis for tentative conclusion regarding performance of covers. More studies still are required to assess the effect of the bottom no-flow boundary so that these results can be compared with others. The thermodynamic effects of this boundary need to be considered in future designs.

Project Practical Implications:

Field assessments have shown the potential for ET covers in Tallahassee, Florida, because they perform better than conventional covers. This information will aid landfill owners, designers, and regulators to consider ET covers as an alternative to conventional covers. ET covers will result in reducing the cost of landfill construction and operation as they work with nature and their performance improves with time. An additional feature of this study was the inclusion of unlined test sections along with the lysimeters to study the water balance in the alternative covers. This study established the effect of lysimeter’s bottom no flow boundary to performance of lysimeters in measuring percolation, soil temperature, soil water storage, and tree development.

Task by Task Summary

Task 1–Preconstruction Testing and Cap Profile Design (100 percent completed)

ET Soil Thickness Design. This subtask consisted of collecting climate data, soil samples, and vegetation characteristics for Tallahassee, Florida. Hydraulic properties of the soil samples, along with vegetation root density functions, were evaluated in the laboratory. The accumulated data were used as input into an unsaturated flow model (UNSAT-H) to simulate the water balance during peak weather events. Simulations were performed to estimate the soil thickness needed to construct an ET cap using different types of vegetation. Simulations of ET caps with trees and grasses having a thickness between 1.0 and 1.5 m had a balance of infiltration and ET resulting in neutralized percolation. The vegetation parameters, however, proved to have a significant effect on percolation.

ET Lysimeter Design. A modeling study using Windows program HYDRUS-2D was performed to investigate how lysimeter geometry and boundary conditions affect lateral diversion and percolation rates, which are measured using lysimeters. The break in texture that occurs at the interface between the base of the cover profile and the percolation collection layer at the base of the lysimeter was determined. Results of the modeling activities showed that cost-effective lysimeters can be 5 m wide, have sidewalls that are 0.35 m high, and be constructed on slopes not steeper than 5:1. Lysimeters constructed in accordance with these recommendations have a performance ratio greater than 0.80. That is, the percolation rate measured from such lysimeters underestimates the percolation through an actual cover by as much as 20 percent. The recommended design also results in less overcompaction typically associated with lysimeter construction.

Task 2–Construction of ET Cap Lysimeters (100 percent completed)

Construction of lysimeters is complete. Preparation of a firm foundation for the ET cap lysimeter involved the stripping of topsoil and proof rolling. Three lined test pads of 20 feet by 42 feet were prepared along the dominant slope in an east-west direction. Each of the constructed lysimeters consists of a system to collect percolation from the base, a system to collect surface runoff, and sensors to monitor the hydrologic variables. Each lysimeter consists of a “bath tub” lined with geomembrane and geocomposite. At the down slope, a drainage sump pump was installed and connected to a drainage monitoring system. A geocomposite drainage layer, followed by a thin gravel layer, was placed on top of the geomembrane to collect the percolating water from the cap soil and to rapidly transmit it to the sump area. This layer also acts as a cushion to protect the bottom geomembrane during soil placement. A root barrier was placed on top of the gravel layer. The root barrier was intended to prevent the roots from reaching the percolation collection system. The ET cap was subsequently placed in three layers of soils. A thin layer of mulch was added at each lift of soil to improve fertility to aid the growth of trees and grasses and to improve the water storage capacity of the soil. Fertilizer and lime also were added. The thickness of soil cover above the root barrier is approximately 4.5 feet.

Tasks 3 and 4–Data Collection and Analysis (100 percent completed)

Data collection has been performed since July 2004. A CR 23X Campbell scientific datalogger is used to collect water balance components data from the lysimeters. The downloaded data are used to evaluate and compare the water balance components through each lysimeter. Continuous data collected from test covers include: runoff, soil water storage, and percolation. A weather station was installed onsite to record precipitation, soil temperature, air temperature, solar radiation, wind speed and direction, and relative humidity. Percolations through the ET caps are compared to percolation through the prescribed cap. Field performance of the ET caps is assessed and compared to that of the prescriptive cap, which establishes the equivalency.

Journal Articles:

No journal articles submitted with this report: View all 2 publications for this project

Supplemental Keywords:

landfill covers, landfill caps, evapo-transpiration, ET, lysimeters, percolation rates, alternative covers, barrier layers, water balance, vegetative cover, contaminated sediments, environmental chemistry, hazardous waste, waste treatment, bioremediation, chemical contaminants, chemical kinetics, contaminant transport models, contaminated marine sediment, leaching, remediation, risk assessment, sediment caps, sediment treatment.
, RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Waste, Water, TREATMENT/CONTROL, Ecosystem Protection/Environmental Exposure & Risk, Waste Treatment, Contaminated Sediments, Environmental Chemistry, Fate & Transport, Hazardous Waste, Ecology and Ecosystems, Environmental Engineering, Hazardous, fate and transport, environmental technology, sediment treatment, hazardous waste management, hazardous waste treatment, risk assessment, landfill caps, evapo-transpiration caps, contaminated marine sediment, soil and groundwater remediation, sediment caps, contaminated sediment, chemical contaminants, contaminated soil, chemical kinetics, remediation, control technologies, leaching, contaminant transport models, bioremediation

Relevant Websites:

http://www.eng.fsu.edu/~abichou/projects/FEETC/

Progress and Final Reports:

Original Abstract

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.