(Conservation Currents,
Northern Virginia Soil and Water Conservation District, March
2003)
Anionic polyacrylamides. It was on
the tip of your tongue, right?
Fairfax County is looking at innovative products
aimed at protecting streams and other natural resources. To
reduce erosion and control sedimentation on construction sites,
the County is evaluating anionic polyacrylamides (PAM).
Anionic PAM can be used in addition to, not in lieu of, currently
required erosion and sediment control practices. It is a temporary
soil-binding agent that reduces soil erosion.
Polyacrylamide is a type of polymer. It is
hard to define polymer in a sentence or two. Polymers are everywhere—in
furniture, toys, spandex clothing, shoes, eyeglasses, contact
lenses...and even your skin. They can be natural like cellulose
(cotton, wood) or synthetic like nylon or rubber. The word comes
from the Greek “poly,” meaning “many,” and
“mer” which comes merous, meaning “parts.”
Typically, the word polymer is used when talking
about molecules that have several thousand or more, sometimes
millions, of atoms. References to polymers usually are about
linear polymers where the atoms are arranged, more or less,
in a long chain. The size and shape of these giant chain-like
molecules causes them to act in ways in which small molecules
don’t. Intermolecular forces affect all molecules, large
and small. But with polymers, these forces are greatly compounded;
the bigger the molecule, the greater the intermolecular force.
That’s one reason why polymers make very strong materials.
Anionic PAM products reduce erosion and
sedimentation by targeting the smallest soil particles, such
as fine silts and clays, which are difficult or impossible to
control using conventional erosion and sediment control measures.
Silt fences and sediment basins trap only sands and course silts.
In a recent three-year efficacy study* on construction sites,
cited by the Virginia Department of Conservation and Recreation,
anionic PAM provided up to 70% reduction in sediment runoff
and even better results when combined with conventional mulching
and seeding measures.
Anionic PAM increases aggregation of the finer
particles to improve soil stability and prevent soil detachment
in the first place. It also decreases the settling time of particles
that have become suspended in water, which aids in their retention
and deposition on the site.
A major selling point for anionic PAM is
that it is environmentally benign. Providing that the product
is prepared and applied properly, it is harmless to fish, wildlife,
and plants. The breakdown products of anionic PAM are carbon
dioxide, water, and ammonia.
Anionic PAM is NOT the same as cationic
PAM, which is toxic to plant roots and animals because it binds
to them. Cationic PAM should not be used as an erosion and sediment
control measure on construction sites.
There are some disadvantages to using
anionic PAM.
- Materials are soil type-specific so a contractor cannot
use leftover material at another site or bulk order for multiple
sites.
- Using it requires site-specific testing that may take several
days to complete.
- Overuse may clog soils, thereby decreasing infiltration.
- It is not effective when applied to pure sand or gravel
with no fine silts or clays, nor when applied over snow cover.
* Mostaghimi, K.A. et al. 2000. A Comparative
Investigation of the Effectiveness of Polyacrylamide for Erosion
Control in Urban Areas. Biological Systems Engineering Department,
Virginia Tech, Blacksburg, VA. Unpublished report prepared for
Virginia Department of Conservation and Recreation.
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