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Post-fire Treatment Effectiveness for Hillslope Stabilization

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Tackifiers, or soil binding agents, are mixed with fiber, seeds, and/or fertilizer for use in hydromulching and hydro-seeding; however, tackifiers may be used alone as a surface soil treatment. Soil binders are applied by putting them into solution and spraying them on the soil or by spreading solid granulated particles on the soil where they can dissolve in rain and/or overland flow.

Chemical soil binders are often classified by their source with natural or organic binders, such as guar and starches, being derived from plant materials and synthetic binders, such as such as PAM, acrylic polymers and copolymers, being derived from petroleum products. While guar tackifiers have been used in hydromulch mixes applied as post-fire treatments, polyacrylamide (PAM) is the only soil binder that has been used alone as a post-fire hillslope stabilization treatment.

There are some environmental and health concerns associated with the use of some categories of PAM. However, negative environmental impacts have not been documented when the anionic PAMs (the class of PAMs used for soil erosion and infiltration management) are applied at recommended concentrations and rates.

Performance Characteristics of PAM and Other Polymers

Longevity–Longevity of soil binders is generally expressed in months, not years. Guar and starches, which degrade through biological decomposition, have short (1 to 3 mo) longevity ratings, while PAM, which photo-degrades, has a moderate (3 to 12 mo) longevity rating. Given that post-fire hillslope stabilization treatment effectiveness is needed for at least two to three years while re-vegetation occurs, the rapid degradation of PAM and other soil binders is a draw-back to their use in post-fire treatment.

Soil type and cation ion availability–PAM has a high affinity for clay mineral surfaces and once adsorbed is not easily removed. Generally, PAM is less easily adsorbed onto coarse-textured soils and organic matter tends to interfere with the adsorption process. Rough (2007) reported that PAM preferentially bound to ash over mineral soil when used after the 2002 Schoonover Fire in Colorado.

Adsorption of anionic PAMs to mineral surfaces, which carry predominately negative charges, is aided by an abundance of divalent cations such as Ca2+ (calcium ions) in the solution. Consequently, PAM is often applied with gypsum (CaSO4-2H2O) as a source for the Ca2+ ions.

Viscosity and infiltration rate–Much of the research on the use of PAM in agricultural irrigation have reported increases in infiltration rates which generally have been attributed to the effect of PAM on stabilizing soil surface structure and preventing the formation of surface seals. However, if the soil structure has already deteriorated (as is often the case in areas of high soil burn severity) or if the soil is sandy (larger particles, less structured soil), PAM.s tendency to increase the viscosity of the infiltrating water may reduce rather than increase infiltration. Obviously, any reduction in infiltration would be detrimental in post-fire stabilization treatments and is a significant disadvantage to using PAM in burned areas.

Effectiveness of PAM and Other Polymers

The effectiveness of PAM has been documented for use in agricultural irrigation and in disturbed but not burned areas; however, only a few of these studies involve the types of soil and water control needed in post-fire hillslope stabilization. In the post-fire studies that have measured the effectiveness of PAM in reducing post-fire runoff and/or erosion, the results have shown no treatment effect or have not been conclusive.

The information on this web page has been excerpted from the following publication:

Robichaud, Peter R.; Ashmun, Louise E.; Sims, Bruce D. 2010. Post-fire treatment effectiveness for hillslope stabilization. Gen Tech. Rep. RMRS-GTR-240. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 62 p.

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