Long-Term Closure Stops Leaking Leachate

Donna Foster

When contaminated leachate migrated from the Toytown landfill in Pinellas County, Fla., solid waste officials took a less-traveled route to a solution.

The result: an effective, state-of-the-art leachate collection and treatment system that came in under budget and continues to earn admiration and appreciation from industry experts, federal regulators and local taxpayers.

The municipal landfill, situated on 250 acres bordering Interstate 275, ceas-ed accepting waste in 1982 after more than a decade of op-eration. Although the landfill was cap-ped in 1984, the discovery of a leach-ate problem coupled with new federal regulations in 1985 resulted in the need for a comprehensive closure at the Toy-town site.

"Tests revealed that the waste pile was leaking leachate around the site down into the water table on the outside. It's a common problem with older, un-lined landfills," said Mike Rudd, assistant director of operations for the Pinellas County Department of Solid Waste Management.

Prior to the 1985 regulations, landfill liners were not required and groundwater monitoring lacked uniformity, Rudd added.

Compounding the problem was the landfill's location on the Gulf Coast, where the water table is high. Because all of the population's drinking water comes from the area, solid waste management officials needed to find an immediate and efficient solution to the leachate problem.

"Pinellas County officials chose to exceed required standards by in-stalling a state-of-the-art groundwater monitoring and leachate control system at the Toytown landfill site," said Rudd.

The project is the only landfill in Florida that is closed comprehensively, with the installation of both a slurry wall and leachate collection system.

"The Toytown closure is often used as an example for other landfill closures in the state of Florida," said Rudd, who added that Pinellas County officials are pleased because the Environmental Protection Agen-cy also seems to be impressed with the project.

After evaluating the groundwater situation and geology, engineers recommended tying together the clay base of the landfill with a 600,000 square foot clay slurry wall to prevent further migration or contamination. A collection system was in-stalled just inside the slurry wall to intercept, remove and treat contaminated waste water from the waste site.

The gravel-filled interceptor trench provides a positive cutoff down to 16 feet to prevent the migration of contaminated water and protect the a-quifer. The slotted leachate pipe collects contaminated water flowing into the permeable gravel fill and channels it to pumping stations to be monitored and removed for treatment at an onsite treatment plant.

The leachate is then treated and discharged off-site into a municipal sanitary sewer system, where it undergoes further treatment.

Used in combination with the slurry wall, the leachate system lowers the water table, which prevents any leakage through slurry wall breaches. En-gineers chose this design for Toytown because it functions better than dewatering wells, which have a tendency to allow contaminated water through, said Rudd.

Comprehensive Closure "The drain system draws down the water on the inside so that, if there was a failure of the slurry wall, it would be an inward failure," Rudd said. "It would bring clean water from off the site into it, rather than releasing any contaminated water off the site."

Groundwater Control Inc., Jack-sonville, Fla., used a custom-de-signed, one-pass trencher to install 14,000 linear feet of leachate collection lines around the interior of the slurry wall. First, they dug a 16-foot trench.

"Using this method, we were able to lay the pipe at a constant, pre-determined depth. It is almost im-possible to achieve that kind of precision when you work by hand," said Weldon Woodall, the president of Groundwater Control and supervisor on the Toytown project.

Next, a tube running from the top of the trencher fed a custom-design-ed polyethylene drainage pipe down through a boot on the rear of the equipment and into the bottom of the trench.

Gravel backfill material was simultaneously fed from a hopper on the boot, evenly distributing the fill un-der, around and over the leachate pipe.

"Backfilling from above ground is key to the process because it allows us to exclude human contact down in the contaminated area," said Woodall.

He estimates that the one-pass trenching system helped Pinellas County save time and money be-cause very little contaminated soil had to be removed when the trench was dug.

Selling The Idea Establishing an inward gradient by dewatering the waste site through a collection system that includes an under drain is the best answer to a problem like the one at the Toytown landfill, said Rudd.

"For unlined landfills trying to close or for those that have closed and have a contamination problem, I recommend installing a slurry wall for containment if the local regulators will accept it," he said. "This is really the only way to go. Your only other alternative is to dig it up and line it, which is much more costly in the end."

Rudd said that the key to getting this type of collection system ap-proved by a local regulator is to "sell it as an inward gradient design, establishing a gradient monitoring system that is separate from but can include portions of the groundwater monitoring system."

At a cost of $1.4 million, Pinellas County officials delivered the Toy-town landfill closure under budget and within the strict guidelines of both the EPA and the Occupational Safety and Health Administration (OSHA).

Although it sounds expensive initially, it costs less in the long run, according to Rudd.

"It depends on whether you have a cheap outlet for the water fluids you collect. If you have a sanitary sewer nearby, then it's the cheapest route. If you don't have a sanitary sewer, then you will have to spend a lot more. The Toytown project is still the most effective collection system that I've seen," said Rudd.

With the demands of Subtitle D and a focus on improved containment systems, new technology is important to the waste industry. GCLs are one example of an important technological development, although they date back to 1982.

GCLs encase bentonite with geotextiles, or support one pound per square foot of high grade sodium bentonite adhered directly on a geomembrane. A geomembrane-supported GCL is designed to function as a self-sealing geomembrane. The bentonite layer, which swells to several times its original volume when wet, is able to seal leaks in an overlying synthetic liner under confining pressures as low as 27 pounds per square foot or four inches of material. If liquid seeps through the upper geomembrane, the bentonite activates, swells and seals the overlying geomembrane and eliminates potential flow.

Geomembrane backings are made from high density polyethylene (HDPE) or very low density polyethylene (VLDPE). The geomembrane support can be textured, depending upon project specifications and slope requirements, and can vary in thickness from 20 mil (0.5mm) to 80 mil (2.0 mm).

Subtitle D calls for most MSW landfills to install a geomembrane with several feet of compacted, low-permeability soil. A membrane-backed GCL below the geomembrane provides intimate contact. The high-grade sodium bentonite rests directly below the liner. If the upper membrane is flawed, the bentonite swells into the defect to stop the liquids.

GCLs can be used as an alternative to lining systems requiring compacted clay liners (CCLs). When incorporated into a lining system:

* One piece of equipment is needed for installation - the panels are unrolled and deployed dry.

* The confined bentonite loading of one pound per square foot provides the equivalent permeability of more than three feet of 1 x 10-7 cm/sec clay.

* Bentonite's swelling provides intimate contact for composite lining systems.

* GCLs are not adversely affected by freeze/thaw cycles.

* GCLs are not susceptible to dessication cracking.

* Quality control assures a high-quality manufactured product.

* GCLs leave more air space for waste disposal.

GCLs have been installed in different liner systems and used in different environmental applications. The geomembrane-supported GCL can be installed with the bentonite side facing up or facing down.

A geomembrane-supported GCL can be installed as a one-product composite liner to supply the low-permeability soil and geomembrane requirements. In this application, it is made with specified geomembrane and installed with the liner side up. All seams between panels are overlapped and extrusion-welded.