LANDFILL: Top Off the Heap

Kim A. O'Connell

In the wonderful world of Oz, it probably never rains. But in the land of Kansas, annual rainfall can exceed more than 29 inches. For this reason, the state approved its first capillary landfill cover, which has proven effective in keeping McPherson Area Solid Waste Utility, McPherson, Kan.'s disposal site dry.

In 2001, McPherson County was grappling with the relatively high cost of building and maintaining the prescribed final cover for its closed county landfill. As an alternative, officials looked at capillary barrier covers, which typically are used in dry climates to stop water infiltration at landfills. The covers were untested in wet environments, but this did not deter the county.

Using a VS2D1 computer model developed by the U.S. Geological Survey, Reston, Va., landfill officials simulated soil conditions, infiltration and evapotranspiration data, various cover configurations, such as layer thickness and material composition, and several storm scenarios, including modeling the wettest year in history reoccurring every year for 30 years.

“The model output is provided both in data and graphic form,” says Jack Chappelle, president of Engineering Solutions & Design Inc., Overland Park, Kan., which developed the capillary system for McPherson. “This output can be presented to indicate how the cover functions over any period of time.” And the graphic presentations were used to prove to the Kansas Department of Health and Environment, Topeka, that the cover would operate properly, even during storms.

Capillary barrier covers operate based on the capillary pressure in soils. Capillary pressure is inversely proportional to the soil's pore size, meaning the smaller the pore, the higher the pressure. To create the necessary pressure, a capillary barrier is composed of a soil mixture of clays and loams, underlain by fine and coarse sand layers. A top vegetative soil layer is installed to establish plant growth and absorb moisture. This then allows evapotranspiration to remove the moisture.

“As long as the downward force of the percolating flow is less than the capillary force of the soils comprising the cover, the capillary break system will stop infiltrating water,” Chappelle explains. “With the added sloping of the cover, the percolating water will tend to migrate along the slope where resistance is less.”

Specifically at the McPherson landfill, the materials required for the cover were available in the proper quantity, and the site was sloped adequately to allow for proper drainage, Chappelle says. So McPherson installed its capillary cover by late 2001 using four types of soils. The vegetative layer was made of onsite top soil; the fine sand layer was composed of sand from a gravel pit near Salina, Kan.; the coarse sand layer was obtained from an operation in Wichita, Kan.; and the clay for the bottom layer was onsite. The fertile soils allowed the vegetative cover to be established quickly.

“Vegetation consumes the water that enters the soil,” Chappelle says. “An established vegetative cover results in a greater consumption of water by the vegetation. The vegetation's root systems hold water in the soil, which allows for added plant growth. An established vegetative cover significantly reduces water percolating vertically.”

Because this was the first trial of a capillary cover system in Kansas, state regulators also required that two lysimeters be installed to monitor any leakage through the cover. So far, none has occurred. There were several storms in 2002 and only minor erosion occurred, which was easily repaired, Chappelle says. Vegetation also has grown in steadily.

A capillary system can cost less than a recommended cover because the capillary cover uses natural processes such as vegetation for evapotranspiration, said Ray Finley, a principal investigator with Sandia National Laboratories, Albuquerque, N.M., in a recent report on capillary covers. Also, the capillary cover retains more water than undisturbed topsoil, for the sole purpose of encouraging plant growth and limiting erosion.

“Although [it has] a relatively simple configuration, a capillary barrier should result in a long-lived, easily constructed and low-cost barrier in comparison with many conventional cover systems,” Finley says.

The original estimate for the capillary cover was approximately $1.3 million, which was less than McPherson's prescribed cover estimate of approximately $1.6 million. Actual cost for construction was $904,638; maintenance cost is estimated to be between $3,500 and $8,000 per year, depending on the amount of cleanup and repair after storm, Chappelle says.

Now that the system has been tested in a wetter environment, the McPherson County project widens the applicability of capillary barrier covers nationwide. “The value of this type of cover is based on its constituents, the materials available, the ability to properly maintain the site and specific site conditions,” Chappelle says. “If all of the elements that comprise the cover are in place, [a capillary] cover can be a very reliable, maintainable and durable alternative.”