It isn’t just dust from disturbed soil and eroded roads. It’s ash from volcanoes and wildfires; crystalline silica from construction sites; pollutants from oil, gas, and mining operations; and much more.
Particles break into dust by impact, combustion, crushing, and grinding. The dust is released during loading, dumping, and transporting, or, by the wind, and can be carried for miles by air currents. And once it’s airborne, it can reduce visibility enough to cause vehicle and workplace accidents, wear and tear on vehicles, damage to equipment, and serious health problems.
Fugitive dust is categorized by size, mainly because of the health effects associated with each size. PM10 fugitive dust usually consists of soil particles with a median diameter of about 10 microns, like silicon, aluminum, calcium, and iron. It’s considered inhalable dust, and it can be trapped in the nose, throat, and upper respiratory tract. It may irritate eyes, ears, and nasal passages and may injure mucous membranes and skin.
PM2.5 fugitive dust is known as respirable dust because it can easily be inhaled into the lungs. It includes gases like nitrogen oxides, sulfur dioxide, volatile organic compounds (VOCs), and ammonia, and it generally does more harm than the larger PM10 dust. It can cause or contribute to nose and throat irritation, bronchitis, asthma, and cardiovascular and lung disease.
On work sites, doing the work differently or clearing vegetation from only the immediate work area may generate less dust. Building wind breaks or wind screens, covering stockpiles with tarps, or wetting disturbed areas down with water may limit the amount that blows around. When the work is finished, exposed areas can be covered with vegetation, mulch, gravel, or landscaping rock. On roads from work sites, cattle guards can clean mud from tires.
On unpaved roads, vehicle tires cause ruts and potholes, creating more dust and possibly leading to the loss
of the road itself. Lowering the speed limit reduces the amount of dust that’s stirred up because it reduces the wind speed at ground level. Adding larger particles like gravel and aggregate increases the road’s surface strength.
But some situations need longer-lasting solutions, and a wide variety of products can help. Some hold dust onto the surface by attracting moisture. Some bind dust particles together; some seal the surface; and some cover it up. The advantages, disadvantages, and lifespan of each depend on the soil, weather, condition of the road, and kind and amount of traffic.
They all perform best when the road is well prepared, which ensures that water won’t settle into depressions or be absorbed into the soil. Blading, which shaves off high spots and fills in low ones; grading, which smoothes the surface; and crowning, which gives the surface a rounded A-shape, should all be done first. Roads should be compacted, too, but whether that’s done before or after the application depends on the product.
Timing is important. Most prep work shouldn’t be done if it might rain or freeze within 48 hours. A road that’s too wet can develop ruts, depressions, and potholes before the product is even applied. A road that’s too dry can lose fine material, which leads to a lack of cohesion in the surface materials, and eventually causes problems such as washboarding.
Substances That Reduce Dust
Chloride salts work in two ways. They’re hygroscopic, so they attract moisture from the air and the roadbed, especially at night when the relative humidity is higher. And they’re deliquescent, so they also resist evaporation, keeping the surface wetter for longer in hot, dry weather.
Calcium chloride and magnesium chloride are the most effective. They result in a very hard road surface in dry weather, but can wash away when it rains. Both are corrosive if they don’t contain an additive to inhibit corrosion, toxic to plants, and irritating to human skin.
Many organic and synthetic compounds bind dust particles together. Both kinds are effective on roads and disturbed ground.
Organic compounds include byproducts from the refining of vegetable oils such as soybean, canola, cottonseed, and linseed. They penetrate road surfaces and are less likely to wash away in the rain than chloride salts, but they’re water soluble, so their effect also may be destroyed by heavy rain. They leave a residue, so follow-up applications use a reduced amount.
Lignin sulfonate, or tree sap, is a byproduct of the pulp-and-paper industry. Lignin binds cellulose fibers together to give trees and plants their firmness. As a dust suppressant and soil stabilizer, it’s produced and extracted from the sugar side of the distillation process. It’s effective in dry weather, but is also water soluble. For soil stabilization, it can be incorporated into the top few inches of the soil. When used with high amounts of clay, it permits reshaping and additional compaction.
Tall oil pitch is also a byproduct of the pulp-and-paper industry, but it’s different from lignosulfonates. When the oil is distilled, the lightest products, the sugars, are drawn off first, then the gums, which are heavier. The heaviest, the pitch, remains at the bottom. Tall oil pitch isn’t water soluble once it’s cured. It leaves a residue, too, so follow-up applications use a reduced amount.
Polymer emulsions are synthetic compounds that bind dust particles together. They are byproducts of the adhesive manufacturing process and are composed of polymers that stick to more particles and bridge greater gaps between particles than other products. A light coating can be used as a dust suppressant. Blended with the top few inches of surface material, polymers form a strong crust and stabilize the soil.
Petroleum-based products both bind surface particles and seal road surfaces. These include petroleum resins, emulsified asphalt, emulsified oils, and mineral oils. They may not retain their resilience in dry conditions, and the oil may be toxic. These also leave a residue, so follow-up applications use a reduced amount.
Hydromulches form a protective coating over disturbed surfaces. They can be made of wood chips, shredded paper, or ground newsprint, and may also contain a tackifier such as gypsum, guar gum, or a polymer and other additives. These are mixed with water and often spread with a hydroseeding machine.
The following examples show three successful applications of three different products.
Gravel Roads in Colorado and Arizona
In 1972, Rifle, CO, was the center of attention as the artists Christo and his wife, Jeanne-Claude, finished their Valley Curtain project: 142,000 square feet of orange fabric tied to posts along the Grand Hogback Mountain Range. It took more than two years to complete the project-and only 28 hours before a 60-mile-an-hour gale blew it down.
With its massive mountain peaks, steep valleys, and high plateaus, this part of the country can get very gusty winds. And although snowfall can be heavy, summers are dry. These conditions, together with thousands of miles of unpaved roads, are the reason Randy Parsons, operations manager at GMCO Corp. in Rifle, has been busy overseeing dust suppression on unpaved roads since the mid-1980s, both in Colorado and Arizona.
The company maintains roads for federal, state, and local governments, as well as for private owners. They’re mostly residential roads, Parsons says. Some have mine traffic.
He uses DustGard, from North American Salt in Overland Park, KS, a liquid magnesium chloride that comes from minerals in the Great Salt Lake in Utah. It both provides environmentally friendly dust control and prevents soil erosion.
“DustGard hardens road surfaces,” Parsons says. It’s designed to penetrate the soil and bind fine dust particles to create firm, smooth road surfaces. It reduces the need to maintain roads, as well as the costs to maintain and clean vehicles and equipment.
Because magnesium chloride works by attracting water molecules from the air and the road surface, Parsons usually applies it in the spring and early summer, before the weather gets too hot and dry.
Usually the owner of the road takes care of the prep work, Parsons says, which includes blading, grading, crowning, compacting, and pre-wetting the road. Pre-wetting is crucial: It opens the surface and draws the magnesium chloride in to give the road greater structural integrity.
“Sometimes they get water from hydrants, but counties usually draw it from creeks or irrigation ditches,” he says. “Water rights can be an issue.”
Then Parsons’ crew comes in. They apply DustGard, which comes ready to apply, usually by truck, but occasionally by hose. It penetrates about 2 inches. Instead of applying it at the usual rate of half a gallon per square yard, they generally do two applications of a quarter of a gallon each, about 45 minutes apart.
“Two applications minimizes runoff,” he says.
Most roads need a new application every year, Parsons says, but residential and very low-traffic areas need one only every three years or so. The frequency depends on a number of factors, including the amount and type of traffic and the weather. Although rain washes it off, the damage is mitigated by having the road properly crowned, he says.
“It’s a very simple procedure,” he says. “DustGard is the best value for the dollar.”
Santa Maria Valley Restoration Project
For 80 years, beginning in the late 1880s, hundreds of oil wells dotted the sand dunes in the Santa Maria Valley in central California. As the companies pumped oil and natural gas out of the ground, it was acceptable and common practice for them to dump the unwanted-and toxic-waste products, including water, drilling mud, and oily soil, into large pits as deep as 10 to 15 feet. When the wells were capped, the companies covered these pits, or sumps, with soil or sand, in compliance with the environmental regulations of the day. Later, they began removing the sumps voluntarily.
Chevron has been removing its sumps in the Santa Maria Valley, which is now a residential and agricultural area, for the last 10 years.
“The project is the restoration of a former oil and natural gas field,” says Josh Friddell, the project manager who oversees Chevron’s contractors. “The oil field operations need to be restored. It’s our due diligence to make the land the same as before.”
It’s a very straightforward process, he says. Workers dig down to get the hydrocarbon-impacted sand and haul it away, then they backfill the pits with clean sand. “The project is winding down as we’re moving forward,” he says. “We’re taking out more of the bigger areas we can get to and leaving some of the smaller areas.”
The only problem facing the crews is the mountains of clean sand they create while they’re digging out the sumps, which can be exposed to the wind for two days before all the impacted sand is removed and the clean sand placed back into the pits.
Crews typically use water for dust control on the roads, Friddell says. On the stockpiles, however, they use Soil-Sement, an acrylic polymer emulsion from Midwest Industrial Supply in Canton, OH, as well as other polymers. It acts as a dust suppressant on the stockpiles of clean soil as well as a vapor barrier to keep the hydrocarbon-impacted soil from becoming airborne.
Soil-Sement is a good alternative to tarps and plastic liners, Friddell says. “It’s so easy to apply. There are a lot of safety issues with tarps. They’re big and bulky, and they take a lot of manpower. They also cause a lot of stress injuries. You can easily do the same thing by using a polymer.”
According to Gonzalo Garcia, an environmental specialist with Chevron Environmental Management, “Polymers are effective barriers for wind-blown dust and to keep VOCs down. They’re very effective and easy to apply.”
Soil-Sement can be diluted with either fresh or seawater for both soil stabilization and erosion control. When it bonds with the soil, it creates a crust that reduces PM10 and PM2.5 fugitive dust emissions. It also resists ultraviolet light and water. It’s environmentally safe, nontoxic, noncorrosive, and nonflammable, and it doesn’t pollute groundwater, disturb vegetation, or change the alkalinity or acidity of soil. It can last for three to six months, depending on soil type, climate, and type and amount of traffic.
For the Santa Maria Valley project, crews spread the polymer with hoses or from trucks in a concentration of 0.28 gallon per square yard, Friddell says, typically with their own water. They use just enough to create a thin crust that makes a temporary barrier against the wind. It might leave a little bit of white residue, Garcia says, but it’s hardly noticeable.
Soil-Sement penetrates into the sand as far as the water does, Friddell says. Because they don’t apply it in a heavy-enough volume to provide a thicker crust, it breaks up easily, especially when crews add to the piles or start filling the holes.
“You pile the sand up, hit it with Soil-Sement, and in two days it’s gone,” says Garcia. “Soil-Sement and other polymers get us where we want to be.”
Canada’s Red Hill Valley Trail
The Niagara Escarpment bisects the city of Hamilton, ON, separating it into “the mountain” to the south and “the old city” to the north, some 330 feet below. Red Hill Valley stretches between the foot of the escarpment to Hamilton Harbour, which drains into Lake Ontario.
Red Hill Valley consists of mostly residential areas, with some industrial and commercial ones, says Nello Violin, one of the two senior program coordinators working for the Parks and Roads section of the Public Works Department in Hamilton. He does maintenance operations in the valley on roads; a recreation facility; Red Hill Creek, which runs through the valley; and the 6-mile trail system, which is popular with hikers, joggers, cyclists, and people walking their dogs.
The main trail begins at the top of the escarpment and follows the creek for about 4 miles. It ends just south of the Queen Elizabeth Highway and the harbor. “Some areas are rather steep,” Violin says. “Some parts approach 30%.”
Part of the trail had been paved with tar and chip in the past, but the surface was so badly eroded that it wasn’t safe to use. Last summer, workers repaired a portion of it with Entac, a tall oil pitch made by Enssolutions Ltd. (formerly Entac Emulsion Products) in Hamilton.
“We’re using Entac experimentally, as a replacement for the tar and chip,” Violin says. “We did about 300 meters [just under a fifth of a mile] where erosion is a great concern. Even the tar and chip was washing out.”
Entac is a distillation of crude tall oil (adapted from the Swedish word for pine oil), and is usually recovered from pinewood in the pulp and paper manufacturing process, according to Enssolutions. Entac is a mixture of resins, including the same resin in chewing gum and the same fatty acids in the vegetable oils that are used as binders for dust control.
The city chose Entac for a number of reasons. “It’s a completely natural material,” Violin says. “Our public works department has just approved a new strategic plan, and one of the priorities is to be a leader in environmental stewardship. This fits in quite nicely in our taking a leadership role in the community.”
In addition to being environmentally friendly, it’s organic, nontoxic, noncorrosive and nonflammable, according to the company.
It’s also very simple to apply, Violin says. Unlike asphalt, tar, and similar products, it doesn’t need a “hot box” to heat up; there’s no clean up; and crews can use smaller equipment, which not only makes it easier to work on the steep slopes, but also is an environmental benefit.
Crews prepared the surface by laying down aggregate, which leveled the trail, brought it back to its original width, and gave the emulsion enough particles to which to adhere. Then they graded it.
Enssolutions provided the pulverizer, a tractor that tows machinery with a series of teeth. It ground up the aggregate, “fluffing” it, Violin says, and at the same time, injected Entac, which penetrated the surface to a depth of 4 to 6 inches.
Because tall oil pitch doesn’t flow in its raw state, Enssolutions premixes Entac with clean water and trucks it in ready to spread. Entac is available in customized formulations, depending on the need and the conditions. Once the water in the emulsion evaporates, the pitch hardens and becomes insoluble to water.
Crews let the first coat cure for the rest of the day. The next day, they applied a second coat with a hose to seal the surface. On the third day, the city used its own compactor, which has a metal drum, and rolled the surface to compact the soil. This is something they’d do differently another time, Violin says.
“One of the ideas we had was that it would be better to use rubber-tired rollers than a metal drum. They’d give better traction.”
This section of the trail has fairly heavy pedestrian traffic, especially now that it’s been improved, Violin says. Maintenance pickup trucks have been driving on it, but it won’t get much wear and tear from use. Entac shouldn’t be affected by weather, including ice and snow.
“I’m expecting it to last somewhere between two and three years, based on the pulverizing and compacting,” he says. “We’ll be monitoring it. As it stands now, if I can get two to three years, it will be very cost-effective.”
