Soil covers are used to reduce the quantity of water that infiltrates waste deposits at landfills. Reducing the volume of infiltrating water decreases the amount of leachate that is generated and the risk of groundwater contamination.
Generally, soil covers are designed with low-saturated hydraulic conductivity materials, such as compacted clay barriers, geosynthetic clay liners (GCLs) or flexible membrane liners (FMLs), which also are referred to as geomembranes (GM). This design philosophy often is referred to as a “raincoat” barrier or “umbrella” approach. Barrier covers have been shown to lose their impermeable qualities over time because of the influence of climate variations on the integrity of the liner system.
One alternative is to design landfill covers using a water balance approach to exploit the water storage capacity of finer textured, organic-rich soils and the water removal capability of vegetation. This type of cover is referred to as an evapotranspiration (ET) cover or a water balance cover.
Landfill ET covers also can be described as vegetated soil landfill covers with the primary purpose of controlling infiltration of precipitation into the waste zone through water balance mechanisms, including evaporation and photosynthesis, instead of the resistive mechanism employed by conventional barrier covers, according to a research paper by William H. Albright and others in 2010.
The variables that can be manipulated during the design of ET covers are vegetation, soil properties and thickness of soil layers. An appropriate combination of these variables is needed based on the local climate to assure an appropriate design, according to a research paper by Mark Ankeny and others in 2000. Therefore, the key in designing ET covers is to provide enough soil to store water during vegetation’s dormant months so that the stored water can be released during the plants’ growing season.
These types of covers have been widely used, especially in arid and semiarid regions, according to the 2010 Albright paper. In general, the layers of ET covers are designed with soils that can support plants with highly distributed root systems and a great deal of organic content. Roots and vegetation also have been known to be suitable media for a natural bacteria called methanotrophs, which are known to oxidize landfill gases—including methane—to carbon dioxide because they possess the methane mono-oxygenase enzyme, which enables them to use methane as a source of energy and a major carbon source.
Several previous studies on landfill methane oxidation in soil have demonstrated its ability as a mechanism to reduce methane emissions from landfill surfaces. However, the capacity of landfill cover soil to oxidize methane depends on both the physical and chemical properties of landfill cover materials, such as soil type, moisture content, density and organic and nutrient content. Additionally, environmental conditions such as temperature and precipitation can affect landfill cover soils’ ability to oxidize methane.
Methane oxidation levels also have been reported to depend on the magnitude of methane loading from the waste mass into the soil profile. Because all these variables exist, mathematical models have been developed to estimate methane oxidation in landfill soil covers by simulating water, heat and gas transport, as well as biological oxidation in a variety of climates.
One such model was developed by a research team from the Florida A&M University-Florida State University College of Engineering: the Landfill Surface Emissions Model (LandSEM). LandSEM combines water and heat flow with gas transport and oxidation in the cover soil profiles under any climatic conditions and under different methane-loading conditions. The latest version of the model was bundled via a graphical user interface. The new LandSEM model comprises of four major modules:
- a climate module that generates daily minimum and maximum air temperature along with daily rainfall based on the landfill’s location;
- a soil property module that generates soil characteristics, such as saturated hydraulic conductivity, porosity and methane oxidation capacity, based on a built-in database of soil property data;
- a soil-water content and temperature simulation module that considers the landfill’s location, soil texture and the climate data developed in the first two steps and then predicts the daily soil moisture and temperature at any depth of the soil cover profile for an average climatic year; and
- a core computational module that simulates the concentration and flux of the gas components, including methane, in the landfill cover soil profile.
Previous research used LandSEM to develop a technique to estimate the extent of methane biological oxidation in any given cover soil profile installed on landfills across the climatic zones of the Mediterranean basin, ET covers under the different climates in California and ET covers, referred to as phyto-covers, to be constructed in different ecozones of Australia. In all these studies, LandSEM was used to develop correlations between the percent of methane oxidation and methane escaping the gas collection system into the bottom of the soil profile under different microclimatic conditions, factoring in daily precipitation, mean air temperature and barometric pressure.
Using bio-oxidation in cap-and-trade context
Previous research divided the Mediterranean region into four subclimates or subregions. To estimate methane oxidation for ET covers at dumpsites in the Mediterranean basin, 17 sites in or near large cities were selected. The 17 locations were assumed to be representative of regional microclimates within each of the subregions of the Mediterranean basin. That research reported that a 1-meter-thick ET cover (if constructed on a landfill in Tunis, Tunisia) could generate an $80,000 carbon credit during its first year. The estimated carbon credit decreases to $40,000 after 46 years. This is very encouraging in terms of motivating developing countries to close their waste dumps using soil caps.
Previous research reported that the potential of ET covers to reduce the fugitive release of methane to the atmosphere should encourage the use of ET covers in Australia and elsewhere. That research used LandSEM to develop methane oxidation estimates for abatement of a typical 5-acre landfill site with 500,000 tons of mixed waste. The gross methane loading flux into the bottom of the ET cover was estimated with the National Greenhouse and Energy Reporting solid waste emissions calculator to be 110 grams per cubic meter per day.
Based on calculations using a range of Australian Carbon Credit Units, the installation of an ET cover at this particular landfill could lead to an abatement of between approximately AU$56,000 and AU$114,000 per year and an approximate present market value of AU$76,000 per year.