LANDFILL: Digging for Gold

Kim A. O'Connell

By digging a little deeper, a Virginia landfill may hold the secret to longevity. The Southeastern Public Service Authority (SPSA), Chesapeake, Va., says it has increased its regional landfill's capacity by constructing below the groundwater level.

Unlike typical landfills in which liners are normally built above bedrock, the SPSA landfill's fifth cell features an “inward gradient” design, where a double-composite liner is constructed below the groundwater table. The SPSA says this allows for more airspace, increasing capacity by a minimum of 10 years. Additionally, the authority says the 60 percent capacity increase only cost $300,000 more than a typical system.

In operation since 1985, the landfill receives about 2,000 tons of waste per day at an 833-acre site, which contains 151 acres of permitted landfill area. The landfill also features gas recovery and a power plant, and a leachate collection and aeration system.

When the fifth cell opened in 2000, the SPSA was looking for an environmentally sound way to increase capacity while maintaining cost-efficiency.

The landfill's original four cells were built atop the ground because the landfill is located in Virginia's Tidewater region. But the authority paid millions to bring in daily cover soil. Cell V, on the other hand, was placed below the water table, and its inward gradient design yielded 1.5 million cubic yards of soil, while adding more capacity on the same footprint. With an average cost of $7 per yard to truck in soil, the excavated soil's value was approximately $10 million.

The inward gradient cell also produced substantial savings in cost per ton of capacity vs. more conventional construction designs, according to Joe Readling, vice president for HDR Engineering, Omaha, Neb., which provided landfill design and construction administration. “The landfill economics are improved considerably by gaining additional airspace through significant excavation prior to liner placement, while at the same time affording a higher level of environmental protection,” he says.

Groundwater contamination concerns also have been reduced. According to Readling, even the best liner systems can expect seven to nine 1-centimeter holes per acre. The SPSA's liner system features higher than required standards — including a synthetic clay layer and two different polyethylene liners — to prepare for potential leakage. However, the SPSA believes its inward gradient landfill is more environmentally sound because the groundwater pressure below the water table prevents leachate from flowing out.

In landfills built above the water table, this pressure does not exist and leachate more readily seeps into the groundwater if the liner system tears or holes develop, Readling says. But in the SPSA's fifth cell, if holes appear, groundwater is forced inward and is treated by the leachate management system.

Readling adds that the cell currently does not have enough garbage to act as a ballast to weigh down the liner and prevent groundwater from seeping in, so groundwater is being pumped out of the cell. HDR anticipates that the landfill eventually will stop pumping groundwater, especially if it remains uncontaminated.

In the worst case scenario, if groundwater becomes contaminated and has to continually be pumped out, HDR estimates treatment costs to be a relatively low $70,000 per year.

The SPSA's cell design has been awarded the Silver Spring, Md.-based Solid Waste Association of North America's (SWANA) Gold Award in landfill management, reflecting “the highest levels of achievement in solid waste programs across North America,” according to SWANA Executive Director John Skinner. And several other landfills in Virginia and elsewhere are considering or have constructed inward gradient landfill cells.

The design has widespread applicability, but each site should consider federal and state regulations, and individual site geology, Readling says.

The SPSA site had a relatively shallow geologic layer that was highly impermeable, he explains. But another site might not have such a layer, so it might require a costly dewatering system that would need to be operated long into the site's post-closure phase.

“Our long-range management of water is very minor,” Readling says. “We don't [need] active pumping and dewatering systems into the long term.”

The inward gradient landfill appears to prove that greater environmental protection does not have to be cost-prohibitive. “This is one of the few methods,” Readling says, “where you've got greater financial efficiency and greater environmental protection at once.”