When Ft. Carson, Colorado Springs, Colo., began considering options for a landfill cover, it wanted something that would protect the environment, human health and taxpayer's wallets. After working with Englewood, Colo.-based Earth Tech, an environmental consulting firm, Ft. Carson opted for a alternative to conventional landfill cover designs - an evapotranspiration (ET) cover.
The ET cover, which was the first Resource Conservation and Recovery Act (RCRA) subtitle C alternative cover to be approved in the state of Colorado, was specifically designed to meet the needs of the 15-acre Ft. Carson landfill site.
A landfill cover's purpose is to isolate underlying waste from precipitation and drainage. It is a hydrologic control to reduce leachate production and the potential for waste constituents movement into ground or surface water.
The Ft. Carson ET cover's construction consists of a 4-foot thick soil vegetated with a combination of native grasses. The ET cover's performance depends on many variables including climatic conditions, soil and vegetation characteristics, and cover thickness.
In arid and semi-arid climates, such as Colorado's, an ET soil cover promotes soil water loss from precipitation through evapotranspiration, which is a combination of the processes of evaporation and transpiration. Transpiration is the water loss from transporting soil water through plant roots and from the plant foliage during photosynthesis. Plant transpiration helps remove the soil water that has infiltrated into the soil and is beyond the influence of surface evaporation.
The project required hydrologic, soil productivity and geotechnical engineering analysis. A numerical water balance model was used to assess the ET cover performance based on climatic conditions, soil hydraulic properties, vegetation characteristics and cover thickness.
The cover's soil thickness provides sufficient water storage capacity to inhibit drainage during the plant dormant period in late fall and winter. During the plant growing period, spring through late fall, stored soil water is removed from the cover by evapotranspiration.
At a relatively low cost, the cover soil was obtained from a nearby quarry. Biosolids, used to fertilize the vegetative layer of the cover, were available for free from the installation's sewage treatment plant. Native warm and cool season prairie grasses were selected for the vegetative cover because they provide an extended growing period, tolerate adverse environmental conditions, provide cover for erosion control and develop a root system capable of removing deep soil water.
After more than three years of perfecting the design to meet the Colorado Department of Public Health and Environment's requirements, the ET cover accomplished Ft. Carson's goals. The ET cover's cost was approximately $70,000 per acre or a total of $1.05 million dollars for the 15-acre site. Reportedly, conventional covers can exceed $200,000 dollars per acre.