The Basics of Landfills

WHAT IS A LANDFILL?

A secure landfill is a carefully engineered
depression in the ground (or built on top of the ground,
resembling a football stadium) into which wastes are put. The
aim is to avoid any hydraulic [water-related] connection between
the wastes and the surrounding environment, particularly
groundwater. Basically, a landfill is a bathtub in the ground; a
double-lined landfill is one bathtub inside another. Bathtubs
leak two ways: out the bottom or over the top.

WHAT IS THE COMPOSITION OF A LANDFILL?

There are four critical
elements in a secure landfill: a bottom liner, a leachate
collection system, a cover, and the natural hydrogeologic
setting. The natural setting can be selected to minimize the
possibility of wastes escaping to groundwater beneath a landfill.
The three other elements must be engineered. Each of these
elements is critical to success.

THE NATURAL HYDROGEOLOGIC SETTING:

You want the geology to do
two contradictory things for you. To prevent the wastes from
escaping, you want rocks as tight (waterproof) as possible. Yet
if leakage occurs, you want the geology to be as simple as
possible so you can easily predict where the wastes will go.
Then you can put down wells and capture the escaped wastes by
pumping. Fractured bedrock is highly undesirable beneath a
landfill because the wastes cannot be located if they escape.
Mines and quarries should be avoided because they frequently
contact the groundwater.

WHAT IS A BOTTOM LINER?

It may be one or more layers of clay or
a synthetic flexible membrane (or a combination of these). The
liner effectively creates a bathtub in the ground. If the bottom
liner fails, wastes will migrate directly into the environment.
There are three types of liners: clay, plastic, and composite.

WHAT IS WRONG WITH A CLAY LINER?

Natural clay is often fractured
and cracked. A mechanism called diffusion will move organic
chemicals like benzene through a three-foot thick clay landfill
liner in approximately five years. Some chemicals can degrade
clay.

WHAT IS WRONG WITH A PLASTIC LINER?

The very best landfill
liners today are made of a tough plastic film called high density
polyethylene (HDPE). A number of household chemicals will
degrade HDPE, permeating it (passing though it), making it lose
its strength, softening it, or making it become brittle and
crack. Not only will household chemicals, such as moth balls,
degrade HDPE, but much more benign things can cause it to develop
stress cracks, such as, margarine, vinegar, ethyl alcohol
(booze), shoe polish, peppermint oil, to name a few.

WHAT IS WRONG WITH COMPOSITE LINERS?

A Composite liner is a
single liner made of two parts, a plastic liner and compacted
soil (usually clay soil). Reports show that all plastic liners
(also called Flexible Membrane Liners, or FMLs) will have some
leaks. It is important to realize that all materials used as
liners are at least slightly permeable to liquids or gases and a
certain amount of permeation through liners should be expected.
Additional leakage results from defects such as cracks, holes,
and faulty seams. Studies show that a 10-acre landfill will have
a leak rate somewhere between 0.2 and 10 gallons per day.

WHAT IS A LEACHATE COLLECTION SYSTEM?

Leachate is water that
gets badly contaminated by contacting wastes. It seeps to the
bottom of a landfill and is collected by a system of pipes. The
bottom of the landfill is sloped; pipes laid along the bottom
capture contaminated water and other fluid (leachate) as they
accumulate. The pumped leachate is treated at a wastewater
treatment plant (and the solids removed from the leachate during
this step are returned to the landfill, or are sent to some other
landfill). If leachate collection pipes clog up and leachate
remains in the landfill, fluids can build up in the bathtub. The
resulting liquid pressure becomes the main force driving waste
out the bottom of the landfill when the bottom liner fails.

WHAT ARE SOME OF THE PROBLEMS WITH LEACHATE COLLECTION SYSTEMS?

Leachate collection systems can clog up in less than a decade.
They fail in several known ways:

1. they clog up from silt or
   mud;
2. they can clog up because of growth of microorganisms in
   the pipes;
3. they can clog up because of a chemical reaction
   leading to the precipitation of minerals in the pipes; or
4. the
   pipes become weakened by chemical attack (acids, solvents,
   oxidizing agents, or corrosion) and may then be crushed by the
   tons of garbage piled on them.

WHAT IS A COVER?

A cover or cap is an umbrella over the landfill
to keep water out (to prevent leachate formation). It will
generally consist of several sloped layers: clay or membrane
liner (to prevent rain from intruding), overlain by a very
permeable layer of sandy or gravelly soil (to promote rain
runoff), overlain by topsoil in which vegetation can root (to
stabilize the underlying layers of the cover). If the cover
(cap) is not maintained, rain will enter the landfill resulting
in buildup of leachate to the point where the bathtub overflows
its sides and wastes enter the environment.

WHAT ARE THE PROBLEMS WITH COVERS?

Covers are vulnerable to
attack from at least seven sources:

1. Erosion by natural
   weathering (rain, hail, snow, freeze-thaw cycles, and wind)
2. Vegetation, such as shrubs and trees that continually compete
   with grasses for available space, sending down roots that will
   relentlessly seek to penetrate the cover;
3. Burrowing or soil-
   dwelling mammals (woodchucks, mice, moles, voles), reptiles
   (snakes, tortoises), insects (ants, beetles), and worms will
   present constant threats to the integrity of the cover;
4. Sunlight (if any of these other natural agents should succeed in
   uncovering a portion of the umbrella) will dry out clay
   (permitting cracks to develop), or destroy membrane liners
   through the action of ultraviolet radiation;
5. Subsidence–an
   uneven cave-in of the cap caused by settling of wastes or organic
   decay of wastes, or by loss of liquids from landfilled drums–can
   result in cracks in clay or tears in membrane liners, or result
   in ponding on the surface, which can make a clay cap mushy or can
   subject the cap to freeze-thaw pressures;
6. Rubber tires, which
   “float” upward in a landfill; and
7. Human activities of many
   kinds.

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