=======================Electronic Edition========================
RACHEL’S HAZARDOUS WASTE NEWS #125
—April 18, 1989—
News and resources for environmental justice.
——
Environmental Research Foundation
P.O. Box 5036, Annapolis, MD 21403
Fax (410) 263-8944; Internet: erf@igc.apc.org
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CLAY LANDFILL LINERS LEAK IN
WAYS THAT SURPRISE LANDFILL DESIGNERS.
Organic chemicals are moving through clay landfill liners much
more rapidly than was previously thought possible, according to a
report published by a team of American and Canadian scientists in
the March, 1989, issue of ENVIRONMENTAL SCIENCE AND TECHNOLOGY, a
journal of the American Chemical Society. This is bad news for
people who have been relying on clay landfill liners to protect
them from dangerous chemicals such as benzene, toluene,
trichloroethylene, and ethylbenzene.
The team investigated a five-year-old landfill in southwestern
Ontario and found a rapid movement of organic chemicals through
tight clay; they then developed mathematical models to help
explain what they had observed. After they developed computer
models that fit well with what they had observed, they used the
computer models to predict the time it would take for benzene and
other organic chemicals to pass through a typical clay landfill
liner. They conclude that a mechanism called diffusion will move
organic chemicals like benzene through a three-foot thick clay
landfill liner in approximately 5 years. Furthermore, they
conclude that diffusion will move large quantities of benzene (or
other dangerous organics) through the liner year after year in a
steady flow.
Obviously, this specific information is important, but what’s
equally important is their general conclusion that engineers
have, up until now, misunderstood the basic principles that
explain how chemicals move through clay. It is a shocking and
embarrassing revelation: up until now, the civil engineering
profession has failed to understand the most fundamental
mechanisms controlling landfill behavior.
There are two basic ways that chemicals move through clay; they
are called advection and diffusion. To understand these, we need
to know a little about clay. Clay is like sand, except that each
individual clay-grain is much smaller than each individual
sand-grain. As a conequence, the spaces between grains of clay
are much smaller than the spaces between sand grains. These small
spaces make it difficult for fluids to pass through clay, which
is why clay has gained a reputation as a “tight” material.
Advection is what you might call “normal” movement of fluids
through soil. The fluids travel through the spaces between grains
of soil as if those spaces were pipes. The more pressure you
apply to the fluids, the faster the fluids will pass through the
pipes. Modern clay-lined landfills are designed and built to
specifications that require that the “pipes” or spaces between
clay grains be very small. This is expressed as an allowable
“permeability” of one ten-millionth of a centimeter per second
(10-7 cm/sec), also expressed as one billionth of a meter per
second (10-9 m/s). A meter is about a yard and a centimeter is a
little less than half an inch.
In addition to being built with clay that passes fluids through
it very slowly, a modern landfill is also designed to prevent the
buildup of fluids inside the landfill. Fluids building up inside
a landfill provide weight that pushes down on the landfill liner,
creating pressure that forces fluids through the “pipes” between
the grains of clay. The theory of advective flow [expressed in
Darcy’s law] says that, if you reduce the pressure, you will
reduce the flow of fluid through the bottom clay liner. To reduce
the pressure, a modern landfill is equipped with a leachate
collection system to prevent fluid buildup. (See RHWN #119 for
more details.)
However, the Canadian-American team emphasizes that advective
flow is not the only means by which fluids flow through the
spaces between grains of clay. The second means by which fluids
move is called “diffusion” or Fickean diffusion after the man who
first explained the phenomenon. All molecules are in constant
motion; this motion is what we call “heat.” Hotter molecules are
moving more rapidly than cooler molecules. Due to the motion of
heat, molecules tend to move from a more concentrated chemical
solution to a less concentrated chemical solution. As a
consequence of this, the concentrated chemicals inside a landfill
tend to move through the bottom clay liner even if there is no
pressure pushing them downward. The random motion of the
molecules causes the chemicals inside the landfill to move
steadily through the clay liner.
The engineers who design landfills have, up until now, simply
ignored diffusion. They have concentrated their efforts entirely
on minimizing advective flow of fluids through clay. Now the
Canadian-American research team has shown that diffusion is an
important mechanism by which substantial quantities of dangerous
chemicals are moving through clay landfill liners. This is not
just a theory; it is based on observations of real landfills
built of nearly-ideal clay. Thus, they stress that their
conclusions are, if anything, optimistic; real landfills made out
of less-ideal clay will leak more rapidly. Furthermore, the
Canadian-American team has pointed out that diffusion will
transport chemicals through a double clay liner EVEN IF THE
LEACHATE COLLECTION SYSTEM BETWEEN THE TWO CLAY LAYERS IS WORKING
PERFECTLY.
They conclude that even a small landfill (2.5 acres) will pollute
groundwater with 42 pounds of benzene per year, year after year;
they note that 42 pounds of benzene is sufficient to contaminate
3.8 billion liters of water (1.004 billion gallons) up to the
allowable drinking water criterion of 0.005 milligrams per liter
(5 parts per billion).
In their report the researchers cite eight prior studies that
have reached similar conclusions about diffusive transport of
organic chemicals through clay.
There are four important lessons to be drawn from this most
recent report, and from the earlier studies that had reached
similar conclusions:
1) Clay-lined landfills will leak, and are leaking, much more
rapidly than their designers assumed they would. Even very thick
deposits of clay will be penetrated much more rapidly that had
previously been assumed. For example, the Ontario site studied by
the Canadian-American team has 130 feet of clay beneath it, yet
they conclude that it will leak in 1000 years or less. Such
leak-rates will only be judged satisfactory by persons who
believe it is morally acceptable to poison future generations to
preserve modern life-styles based on convenience and waste.
2) The designers of clay-lined landfills prior to 1989 have not
understood the physical principles by which chemicals pass
through clay, and they have therefore built landfills that are
not adequate to protect public health and safety. This
information should shake our confidence in the civil engineering
profession that has been giving its stamp of approval to flawed
landfill designs for decades.
3) The nation’s best-informed and best-funded engineers have once
again given the public false assurances and have promoted a
technology that has now proven to be unreliable and dangerous. We
should develop a healthy skepticism about high-tech solutions to
social problems like waste disposal. Today’s “state of the art”
is very likely tomorrow’s disaster.
4) All clay-lined landfills should be stopped from accepting
dangerous chemicals immediately.
Get: Richard L. Johnson, John A. Cherry, and James F. Pankanow.
“Diffusive Contaminant Transport in Natural Clay: A Field Example
and Implications for Clay Lined Waste Disposal Sites.”
ENVIRONMENTAL SCIENCE AND TECHNOLOGY, Vol. 23 (March, 1989), pgs.
340-349.
–Peter Montague, Ph.D.
Descriptor terms: studies; landfilling; clay; liners; leaks;
organic chemicals; cn; ontario; groundwater; benzene;