CONCRETE WASH
WATER:
Myths and Realities
Many people have misconceptions
about how concrete wash-water can affect and is affected by
natural water bodies. As a result of this incorrect knowledge,
concrete producers, handlers and finishers may be disposing of
alkaline (and deadly) wastes into street drains or other areas
where the wastes may reach fish-bearing habitat. The following
section discusses some of these misconceptions.
Street
drains go to sewage treatment plants so it is okay to dump toxic
materials into them.
REALITY
The
street drains in most municipalities are designed to convey
storm water from streets and other paved areas to the nearest
watercourse, which is probably also a fish-bearing stream.
If concrete
wash-water is filtered so that the cloudiness is removed, the
wash-water is not toxic to fish and can now be discharged to a
street drain.
REALITY
 Filtering
concrete wash-water will not remove the components that kill
fish (even if the wash-water is crystal clear). They are still
there, dissolved in a form we can not see, and the pH is still
high enough to kill fish in a matter of minutes as is shown in
the graphic to the right. To see the full-size picture,
click on the thumbnail to the right, then use your browser's
"Back" button to return to this page.
The
"buffering capacity" of stream water will make a high
pH effluent non-toxic.
REALITY
"Buffering
capacity" refers to the ability of a water body to resist
large changes in pH when an acidic or alkaline solution is added
to it. Concrete wash-water has so much alkaline material in it
that it overwhelms the buffering capacity of any stream. As a
result, the pH will still remain at levels that are lethal to
fish.
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of page]
Concrete
wash-water can counteract the effects of acid rain so it is good
for creeks.
REALITY
Acid
rain can cause the pH of some streams to decrease below the
range usually found in most freshwaters. The effects of
this input of acid is sometimes naturally counteracted or
neutralized by the alkalinity of the receiving waters.
Concrete wash-water has so much alkaline material, however, that
it would not only neutralize the acid, but overshoot into the
high pH range which is toxic to fish.
The
water in a stream can dilute concrete wash water to a non-toxic
concentration.
REALITY
The
pH of a solution of concrete wash water does not behave as
expected (based on mathematical calculations) when it is
diluted. The graph shows the pH of a solution of Portland cement
in tap water that was repeatedly diluted. The starting
concentration of Portland cement was 7.5 parts per million.
As the graph
above shows, the observed pH actually remains about 1.5 relative
units higher than expected and even after a five hundred-fold
dilution is still at lethal levels.
Fish
hatchery ponds are made of concrete so concrete can't be bad for
fish.
REALITY
Fish
can safely live in concrete ponds when the concrete is
completely set or "cured." It is when water comes into
contact with uncured (or wet) concrete that the constituents of
the Portland cement can dissolve into the water and cause a high
pH level.
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