EVALUATION OF PLANTS ON WATER BALANCE OF EARTHEN LANDFILL COVERS
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Abstract
Landfilling is one of the most common methods to dispose municipal solid waste (MSW) and coal combustion residues (CCRs) in the U.S. Sustainable landfilling will be the key for the future advancement of solid waste management. Sustainable landfilling will include zero discharge of leachate constitutes, converting the methane into energy and sequestering carbon dioxide to reach the ideal target of zero greenhouse gas (GHG) emissions. While we are far from zero leachate and zero GHG emissions goals, my research is the next step towards both of those goals. Alternative final covers (AFCs) have been increasingly permitted due to the financial benefits and environmental sustainability offered using native soil. However, often due to lack of validated plant data and transpiration models, practitioners ignore effect of plants when designing AFCs. This dissertation focused on evaluating the effect of plants on hydrological performance and water balance of earthen final covers by conducting field-scale and laboratory-scale experiments coupled with numerical modeling. The field-scale experiment was carried out in the southcentral U.S. It consisted of two 11 m long x 11 m wide x 0.9 thick earthen cover test sections, one planted with vetiver and the other was bare representing control. Each test section had a lysimeter to collect percolation. Both test sections were fully instrumented. About 14 sensors were installed in each test section to monitor matric suction, water contents, soil temperature, and lysimeter water levels. A weather station was installed at the site. The hydrological performance of each test section was monitored over a period of one year. Field data shows that the average ET rates for control and vetiver test sections were 0.31 cm/day and 0.30 cm/day, respectively. Twenty-two (22) water removal events of the field test sections were analyzed starting October 2017 to August 2019 when the lysimeters were allowed to flood by turning off the drainage valves. This analysis showed that the average ET rates during the 23-month period were 0.34 cm/day and 0.25 cm/day for control and vetiver test sections, respectively. More than 3-year field-scale data analysis indicates that the difference between soil water storage and evapotranspiration (ET) of the two test sections was less than 2%. The water balance model UNSAT-H was used to simulate the field water balance. The percolation predictions of UNSAT-H model for both test sections were relatively accurate. While the model predicted percolation accurately, it under-estimated ET for the vetiver test section by about 9% and overestimated ET for control by about 19%. A large-scale column experiment was set up in the southeastern U.S. Three identical laboratory soil columns (one without vegetation, second planted with vetiver grass and third planted with switchgrass) consisting of two layers of soil: topsoil (35 cm) underlain by compacted sandy silt (76 cm) were built and instrumented to mimic an ET landfill cover. The diameter of the soil columns was 25.5 cm. Data collected over a 14-month period showed that ET from switchgrass and vetiver columns was greater than ET from the bare column by 2% and 9%, respectively. By the end of the 14-month large-scale study, vetiver grass and switchgrass canopies were about 50% and 75% of their mature height, respectively. The columns experienced a 25-day drought starting mid-May 2019. Consequently, ET from vetiver grass and switchgrass columns increased significantly due to plant root water uptake. The four-month ET from switchgrass and vetiver columns was 23% and 37% higher than ET from control, respectively. The results indicate that in humid geo-climatic regions, the use of plants with canopies that restrict solar radiation to the surface of the cover may restrict evaporation and it does not enhance ET. However, if the plants go through drought and are under stress, the plant water increases while evaporation may be impeded due to relatively low unsaturated hydraulic conductivity of the uppermost dry exposed soil. More research on plants with smaller canopies and extensive root systems is recommended as it may enhance ET from ET covers.