Anionic Polyacrylamides for Erosion Control
(Conservation Currents, Northern Virginia Soil and Water Conservation District)
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.