Tips of the Cap

Addressing erosion and waste stability in capping a rural landfill.

The Population of Kewaunee County, Wis., may be small, but its 21,000 residents are no strangers to environmental issues and regulations.

The county's nuclear power plant has operated since the 1960s. The county also is the site of the state's first wind farm, a 23-megawatt facility with 213-foot-high towers that was built in the late 1990s to capture winds near the shores of Lake Michigan. In addition, the county is home to a number of large dairy cow operations, which present numerous liquid and solid waste issues.

In late 2007, however, the county's environmental focus turned to capping a five-and-a-half-acre portion of its 25-year-old landfill that contains 140,000 tons of waste. This latest capping venture is a far cry from the two feet of clay cover that the state required when the first phase of the landfill was closed 20 years ago.

Today's Capping Regulations

The recently completed final cover system consists of two feet of compacted clay, a 40 mil linear low density polyethylene (LLDPE) geomembrane, a geosynthetic drainage layer, a 30-inch rooting zone and a six-inch layer of topsoil, says Dale Jandrain, highway commissioner of Kewaunee County.

In addition, perforated piping was installed along the base of the cap to remove seepage collected in the drainage layer. A separate piping system was installed to collect methane gas, which will be conveyed to a planned blower and flare.

Two new technologies, recently approved by the state, have been integrated into the new cap design. These include a new surface water runoff erosion control concept and seepage control that uses a geosynthetic drainage composite instead of sand. In 2008, the landfill will add leachate recirculation to meet the state's newest regulation.

Controlling Surface Runoff

A major concern in developing the current project was controlling erosion from surface runoff on the 65-foot-high, 15-degree landfill slope, says Michael Ruetten, principal engineer for STS Consultants of Green Bay, Wis. To mitigate the runoff, a series of terraces were designed to direct rain and snow melt to an 8-foot-wide down-slope flume constructed with 2-foot-high beams.

The interior side slopes of the berms and the bottom of the flume will be covered with Recyclex TRM, a turf reinforcement matting that allows seeded grass to grow through it. As a result, the fiber matrix becomes an integral part of the turf. The product is manufactured by American Excelsior of Arlington, Texas, and is made of 100 percent recycled post-consumer green soda bottles.

The Kewaunee down-slope flume is designed to accommodate water traveling at a velocity of 13 feet per second, and will offer greater variability in capacity than a typical rip rap channel or drainage pipe, Ruetten says.

“With pipe, you can't design for the maximum storm. It would be so oversized as to be impractical,” he says. “Our flume is designed for a 25-year storm, but if two 25-year storms occur back to back, it has the capacity to handle the water.”

Interface friction testing between project geosynthetics and soils was conducted to determine what impact dozers and other construction equipment would have on cover stability during construction and over the long term.

Stabilizing the Waste Mass

Kewaunee County currently is preparing to cap the landfill's seventh and eighth stages later this year. Chapter NR514 of the Wisconsin Administrative Code requires that all new landfill expansions include a plan to stabilize the waste mass, primarily through leachate recirculation. This involves wetting the garbage to enhance microbial activity and accelerate decomposition.

James Zellmer, the state's Department of Natural Resources representative in the Kewaunee County project, explains that leachate recirculation offers several benefits, including increased gas production, better compaction and more airspace to accommodate more waste, and a reduction or even the temporary elimination of the need to haul leachate to sewage treatment plants.

In 2006, Kewaunee County hauled more than 1.6 million gallons of leachate to a municipal treatment plant at a cost of $32,000. This included processing and transportation costs, which are compounded by the rising cost of fuel.

While there are no conclusive findings on the long-term impact of leachate recirculation, Zellmer notes that peak methane production is generally reached a couple of years after the landfill closes. From that point, production steadily falls for about 40 years, after which there is little, if any, gas present. This is expected to reduce the landfill operator's period of financial responsibility for the site.

Michael Dovichi, president of Earth Science & Technology, Algoma, Wis., currently is planning the leachate recirculation system for the seventh- and eighth-stage caps. He notes that the accelerated methane production facilitated by leachate recirculation necessitates a methane handling system with a blower and flare.
Joe Hastreiter is a freelance writer based in Kewaunee, Wis.