PM10 Dust Control Options
There are many alternatives to help control dust.
Below are some of the more common options for dust control.
Reducing Numbers of Vehicles
Vehicles traveling on unpaved roads stir up dust. Reducing the numbers of vehicles using the roads can reduce dust. This can be done voluntarily, such as encouraging walking, or by the local government imposing restrictions on weight or vehicle type, and limiting access to dirt roads by motorized traffic. For example, the City of Kotzebue prohibits anyone under 14 years old from using a snow machine or all-terrain vehicle unless with an adult on the same machine (Kotzebue, 2006).
Reducing Vehicle Speed
Fast moving vehicles stir up dust as well. Studies show PM10 emissions go up with vehicle speed. A reduction in speed from 40 miles per hour (mph) to 20 mph reduces dust emissions by 65% (Succarieh, 1992). Road postings and enforcement, drainage channels, and speed bumps can reduce speeds. Effectiveness of speed limits depends on the availability of law enforcement and community willingness to abide by the rules. Speed bumps and drainage only reduce dust emissions within a short distance of the road modification.
Correcting Road Design Problems
The long term performance of a roadway dust suppressant depends on the road shedding water and providing a smooth driving surface. Water puddles will form on a road surface without good drainage. Water floats fine particles from the soil beneath the road and distributes it across the roadway surface with passing traffic. Standing water next to a roadway may saturate the road bed, resulting in potholes. When fine particles are washed away larger particles are left unanchored. The larger particles are pushed to the side of the road resulting in loss of material and an eventual need for expensive road resurfacing.
Increasing Moisture Content
Moisture in the surface of dirt roads causes particles to stick together. The moisture content of dirt roads can be increased either through spreading water or application of salts which attract water.
Though water is available in almost all Alaskan communities, methods to move water to unpaved roads is a problem in smaller communities. Larger communities might have water trucks to take advantage of local water supplies. Smaller communities may not have such equipment. When water can be applied, it only provides a short term reduction in dust. Regular, light watering is better than less frequent, heavy watering (Bolander, 1999).
The application of deliquescent salts to road surfaces can control road dust. A deliquescent salt, like calcium or magnesium chloride, absorbs water from the air. Soils treated with these salts have a higher water content than untreated soils. Slippery wet roads and vehicle corrosion are disadvantages of salt application. Also, rainfall eventually removes salts from the roadway. The practical duration of a salt application is no more than one year (Morgan, 2005). Sodium chloride, table salt, also attracts water and has been tested in a number of studies. Sodium chlordide is not as effective as calcium or magnesium chloride.
Calcium chloride has been tested and used as a dust control palliative in Alaska over several years. Locations include Kotzebue, Teck Cominco's Red Dog Mine, and Haines. Calcium chloride seems to reduce visible generation of dust and remains effective through the summer season without problems or environmental impacts (Baxter, 2001).
Binding Particles Together
This group of dust palliatives includes chemicals which bind fine particles together or onto larger particles. These chemicals fall into several groups, such as petroleum-based, organic nonpetroleum, electrochemical stabilizers, and synthetic polymers.
Petroleum-based Binders include emulsified asphalts, cutback asphalt, and Bunker C. These agents coat particles with a thin layer of asphalt increasing particle mass and decreasing the chance of becoming airborne. Emulsified asphalt is a mix of asphalt and water which penetrates road surface dirt. This works well when the asphalt is mixed into the top inch or two of road surface with a grader. These products can contaminate waterways due to runoff and are generally not considered any more.
Organic Nonpetroleum Dust Suppressants include lignosulfonates, and resins. Lignosulfonates result from the manufacture of paper when lignin is extracted from wood. Lignin is a natural polymer and can bind soil particles together. Lignin occurs in solution with sodium, calcium, ammonium, or magnesium bisulphate. Resins made from combining lignosulfonates and additives can neutralize adverse effects. Lignosulfonates are water soluble and can move out of, or deeper into, a roadway surface with rainfall. These products corrode aluminum unless calcium carbonate is present. Lignosulfonates last six months and work best with fine dusts having a high plasticity in dry environments such as clay particles (CPWA, 2005). Glacial tills have low plasticity. Lignosulfonates may be of limited value in controlling dust in Alaska.
Electrochemical Stabilizers include sulphonated petroleum, ionic stabilizers, and bentonite. These products neutralize soils that attract water and allow bonds to form between particles. Electrochemical stabilizers need to be worked into the road surface, requiring equipment that may not be available in remote rural communities.
Synthetic Polymer Products include polyvinyl acrylics and acetates. They bind soil particles and form a semi-rigid film on the road. These products are either liquids or powders that are mixed with water. Products are applied in liquid form and require drying. Temperatures during the curing should not approach freezing. Traffic should be diverted from treated areas until drying which can take 12 to 24 hours. Synthetic polymer products have been used for dust control and improved soil strength on a number of airfields in Northern Canada and Alaska.
Covering Unpaved Road Surface Soils With Gravel
Applying gravel to a dirt road surface can reduce dust. Gravel provides a hard surface protecting soils from vehicle wheels. Gravel does not reduce the strength of airflows behind vehicles and this can blow loose soil particles into the air. Without a road base of crushed aggregate, traffic will push down surface gravel into the road surface, especially when the road is wet. If the road surface does not have enough fine material to cement the surface gravel in place, traffic will push the gravel away from the driving lanes. To be effective over a long period of time, new gravel must be anchored to the road surface. This is done through incorporating gravel with aggregate mixes or soil adhesives.
If gravel is lost from being pressed into soils beneath the road, then the use of geotextile fabrics may be necessary. These fabrics are constructed of polymer threads with very high tensile strength, and are available in designs that either form water barriers or allow water, but not fine soil, to migrate through ( Hopkins, 1989).
Sealing Unpaved Roads With Pavement or Other Impermeable Materials
Paving is the most effective, and expensive, method to control dust from unpaved roads. Asphalt and Portland concrete provide durable and effective surfaces that prevent the break down of soil surfaces. Paving is expensive. Except for roadways carrying more than 250 vehicles per day, the use of paving to control dust emissions may not be cost-effective (Bolander, 1999). In the past few years, roads in Kotzebue carrying more than 500 vehicles per day have been paved (Hadley, 2006). Thin pavements, like chip seals, have been applied to roads in southern Alaska but can fall apart during breakup.
Fiberglass plates, used in cold climate oilfields, provide temporary road surfaces. These interlocking plates are manufactured in 14 feet by 8 feet by 2 inch sections. The plates can carry very heavy loads over short distances without the need to construct structural roadbeds. This is good for areas like northwestern Alaska where the supply of construction aggregate is limited. The plates are expensive at about $2,000 per plate, but appear to have a very long lifespan. (Compositech, 2005)
Baxter, R., 2001, Arrest That Fugitive Dust, Erosion Control, March/April 2002, http://www.forester.net/ecm_0203_arrest.html
Bolander, P. and A. Yamada, 1999, Dust Palliative Selection and Application Guide, U.S. Department of Agriculture Forest Service, http://www.dot.state.ak.us/stwddes/research/assets/pdf/dust_sag.pdf, accessed on December 20, 2005
Compositech, 2005, http://www.composite-tech.com/products/durabase.html, accessed on December 14, 2005, and telephone communication with Dennis Swarthout, President, on December 15, 2005
CPWA, 2005, Dust Control for Unpaved Roads, A Best Practice by the National Guide to Sustainable Municipal Infrastructure, Canadian Public Works Association.
Hadley, J., 2006, Telephone communication with Jeff Hadley, City of Kotzebue, January 10, 2006
Hopkins, J., 1989, Geotextile Selection and Installation Manual for Rural Unpaved Roads , prepared for U.S. Federal Highway Administration by Oklahoma State University
Kotzebue, 2006, City website, http://kotzpdweb.tripod.com/city/index.html, accessed on January 11, 2006
Morgan, R.J., V.R. Schaefer, and R.S. Sharma, 2005, Determination and Evaluation of Alternative Methods for Managing and Controlling Highway-Related Dust, Phase II - Demonstration Project, Iowa State University, http://publications.iowa.gov/archive/00002804/01/tr506.pdf, accessed on December 23, 2005
Sierra Research, January 2006, (Draft) Alaska Rural Dust Control Alternatives, prepared for the Alaska Department of Environmental Conservation. The Appendices are large and are not included with the linked document.
Succarieh, M., 1992, Control of Dust Emissions From Unpaved Roads, prepared for Alaska Cooperative Transportation and Public Facilities Research Program by the University of Alaska Fairbanks, http://www.dot.state.ak.us/stwddes/research/assets/pdf/fhwa_ak_rd_92_05.pdf, accessed on December 20, 2005