Michael Fickes

Ten years after the U.S. Environmental Protection Agency (EPA) issued its Subtitle D guidelines, landfill engineers and state regulators finally seem to be growing comfortable with liner, cap and cover standards and alternatives.

Problems, of course, still arise, especially in active states building new landfills and expanding old ones across a range of sites with different geologies and climates.

Nevertheless, according to Allen Geswein, an environmental engineer with the EPA's Office of Solid Waste, the EPA has issued full approvals of state liner, cap and cover regulations in 39 states and Puerto Rico. Another seven states have received partial approvals.

As this decade-long approval process has progressed, operators across the country have been capping landfills at a tremendous rate. The EPA estimates that there were 6,000 landfills in the United States in 1988. Today, fewer than 3,000 are operating, so more than half have been closed and capped.

At the same time, no more than a couple hundred landfills, either new facilities or expansions, receive permits every year.

The EPA does not track information on the most active states, according to Geswein. "Our role is limited to issuing national minimum standards," he explains. "Because all permitting is done at the local level, we don't have the data on [active states]."

However, engineering firms that have nationwide office networks can provide an outline of the most active states based on their business activity.

Up and Down the East Coast In the Northeast, landfill activity is relatively limited, and many states hew to the Subtitle D line. For example, Massachusetts has more or less adopted Subtitle D and generally does not allow alternatives, says Bruce Haskell, a principal with Cambridge, Mass.-based Camp Dresser & McKee (CDM). Liners must be two feet of clay with a 60 mil layer of HDPE, while caps must have 18 inches of clay with a 40 mil membrane.

Some operators have installed geosynthetic clay liners (GCL) on piggybacking sideslopes, according to Haskell. "Beyond that, Massachusetts has adopted a fairly strict view of Subtitle D guidelines," he says. "Regulators will allow some alternatives, but these are mostly material substitutions. Connecticut has adopted Sub D almost verbatim, although it allows a little more flexibility than Massachusetts.

"In Rhode Island, caps and liners aren't an issue," he continues. "There is only one landfill there, and it is operated by a state authority. Delaware is similar. There are only three landfills in Delaware, and each is run by a regional governmental authority."

On the other hand, North and South Carolina seem to be humming along with plenty of new construction. "The offices serving those states are constantly busy today," says Abdul Mulla Saleh, an associate in CDM's Tampa, Fla., office. "We have just completed two new landfills there and started a new one."

According to Saleh, North and South Carolina regulators tend to stick close to Subtitle D's guidelines for liners: Two feet of clay with a geomembrane on top, thanks to the availability of clay.

Liners, of course, determine the specifications for caps. "We cap either with clay or geomembrane, depending on what is least expensive," Saleh says. "The regulators normally will approve either of these methods."

Florida also stands out among the most active landfill states, he says. Conditions in Florida are different from those in the Carolinas, which has led engineers and regulators to develop variations on Subtitle D themes. "For the most part, Florida follows federal guidelines, but the regulators allow some different thicknesses in the soil and head components of the liner," says Fred Sebesta, senior project manager in HDR Engineering's Tampa, Fla., office.

The Florida Department of Environmental Protection (FDEP) has approved nine liner soil and head designs of varying thicknesses, according to Sebesta. "With tighter soils or thicker liners, the regulations allow deeper heads," he says. For example, if a landfill uses soil with a conductivity of 1 X 10 (-7) or less, the liner may consist of a 2-foot layer of soil with a 1-inch hydraulic head or a 211/42-half foot soil layer with a hydraulic head of 6 inches, and so on up to nine different thickness combinations.

All of these alternatives stray from the Subtitle D guide of a 3-foot soil layer. The reason? Clay can be hard to come by in Florida."In most cases, we use a composite liner, although you sometimes see a double synthetic liner with a fabric or net detection layer in between," Sebesta says. "In Alabama where the climate and lack of clay make conditions similar to those in Florida, we were involved with one landfill that used a GCL in place of the clay layer." The benefit of this method was low cost, compared to shipping clay over long distances.

A number of existing landfills in Florida feature vertical slurry walls keyed into natural clay under the site. A slurry wall is a trench filled with a mixture of bentonite and water. When a landfill is encircled by the slurry, water moves off into the surrounding soils and hardens to form a clay cut-off wall.

"With a slurry wall, you get into a long-term pumping situation," Sebesta says. "You have to pump out the leachate inside the wall to ensure that any migration across the wall is inward."

Slurry walls are no longer in fashion for new landfill construction in Florida, where most landfills now appear on top of the groundwater table.

Florida's capping regulations stick close to Subtitle D guidelines that favor cap permeability equal to or less than the liner. "In most cases, we're seeing a 40 mil synthetic cap with 2 feet of cover soil above and 18 inches of intermediate cover below," Sebesta says. "Vendors have told me that in some cases, operators are self-installing a layer of overlapping GCL on top."

Sebesta reports an increasing interest in geonet cap components. Geonets are HDPE materials used to provide drainage and ensure that the protective layer of soil over the liner does not become saturated, which could cause it to slide off and down the slope. "The net sits above the flexible membrane and carries moisture out from under the soil top," Sebesta explains. "Without a net, you have to repair erosion continually."

Sebesta estimates, however, that geonets cost about 30 cents per square foot, which can significantly increase the installation cost.

The Arid West In the more arid western states, most liner designs tend to favor alternatives to Subtitle D, according to Richard Sprague, vice president and national director for HDR's landfill services. Based in HDR's Denver office, Sprague has handled landfill designs in 25 states, from the Carolinas to California. "In the early 1990s, many permit applications were filed using EPA's standard design," he says. "But that has changed, at least in California, Colorado, Idaho and Texas, where I've been working for the past three years. In these states, most designs tend to use alternates. I can't think of a landfill that I have worked on in the last three years that has used a standard liner."

In arid states, less precipitation generates less leachate than Subtitle D guidelines anticipate. "In addition, there are technical constraints related to Subtitle D guidelines in arid states," Sprague notes. "Where the climate is normal or wetter than normal, 2 feet of compacted clay will work fine. But in a dry state, clay may dry up and crack over time.

"As a result, some western states, such as Utah, have been looking at alternatives such as GCLs that hold the clay in place so it can't pull apart as it cracks," he says. "In addition to Utah, we've worked on landfills using this technique in Idaho, Nebraska and Texas."

Sprague reports landfills in other states are being permitted with more clay than Subtitle D's guidelines call for and no geomembrane as well as two geomembrane layers with a geotextile in between, to allow water to move between the two liners. One Utah landfill boasts a completely natural liner consisting of 1,200 feet of shale, which is probably better than what could be engineered.

Regulators in dry states also may prove amenable to alternative capping ideas. "To be honest, we haven't capped a lot of landfills since Subtitle D became final," Sprague says. "We're expecting to begin capping cells opened between 1993 and 1995 soon. I think we're going to see regulators becoming comfortable with alternate cap designs, which nevertheless must have the same performance as the Subtitle D standard."

The biggest advance in capping concepts is capillary-break caps, according to Sprague. "These caps provide a layer of soil on top of gravel," he says. "Plants grow in the soil and store precipitation between growing seasons. The small pores in the soil hold water with more force than gravity, so it can't move down into the gravel unless it becomes saturated.

"You design the soil thickness so that in a wet year, it won't get saturated," he continues. "The thickness of the soil layer depends on the local climate. In Utah, for example, that might mean 18 inches."

In the end, wherever a landfill needs a Subtitle D or alternative liner and cover, cost drives the decision-making process. Liner systems prior to Subtitle D cost around $50,000 per acre, according to Charlie Fiedler, senior project manager with HDR's Dallas office. "Today, a prescriptive liner system can cost between $100,000 and $150,000 per acre," he says. "At the high end, that is a 300 percent increase in the cost of the liner system alone.

"So far, garbage prices have absorbed a lot of this added cost, but somewhere along the way, prices will have to go up, and people will begin to complain," he continues. "That's why alternatives to Subtitle D have become important in many locales, especially those with an absence of clay."