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#125 - Clay Landfill Liners Leak In Ways That Surprise Landfill Designers, 17-Apr-1989

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

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

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

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

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


Descriptor terms: studies; landfilling; clay; landfill liners; leaks;
organic chemicals; ontario, canada; groundwater; benzene;