4. Road Dust Suppression: Effect on Unpaved Road Safety and the Environment
4.1 Introduction
The dust cloud formed when vehicles use an unpaved road can impair the visibility of motorists, leading to accidents and other road hazards. The fine abrasive particles can also increase the wear and tear on the moving parts of vehicle using the unpaved road, resulting in higher road-user cost (Colorado Transpr. Info. Center, #3, 1989). The loose surface aggregate also can be dangerous to pedestrians and oncoming vehicles passing each other as the loose coarse aggregate can easily be turned into a projectile. This happens when the vehicle tires pick and throw the loose aggregate.
Although unpaved roads provide a cheap transportation route for vehicles, the fugitive dust generated by vehicular activity contributes significantly to the particulate loading in the atmosphere. According to air pollution studies, nearly 34 percent of the particulate matter in the atmosphere originates from unpaved roads nationwide, making unpaved roads one of the major man-made sources of fugitive dust (Barnard and Stewart, 1992).
Concern for unpaved road user comfort and safety as well as atmospheric air pollution are some of the reasons for unpaved road dust suppression. This section of the report evaluates the effect, if any, the use of dust suppressants (additives) has on the safety of unpaved road users and the environment. The issue of environmental impact is important because the additives commonly used, lignin, CaCl2, and MgCl2, may contain environmentally unacceptable contaminants such as chlorides, heavy metals, and organic compounds that are currently regulated by the U.S. Environment Protection Agency (EPA).
4.2 Safety Concerns
Unpaved aggregate surfaced roads treated with lignin or chloride compounds have demonstrated a significant reduction in the dust cloud formed when vehicles use the unpaved road (Sanders, et al., 1997; Hoover, et al., 1981; Squier, 1974). As a result, a driver's visibility is drastically improved. For those that have driven treated versus untreated roads, there is a significant improvement in driving comfort as washboarding, corrugations, and ruts are reduced on the treated roads.
Vehicle braking (stopping) on untreated unpaved road especially those with degraded surface with loose unstable aggregate can be tricky if not dangerous. Sudden braking at higher speeds can cause a vehicle to skid out of control as the loose surface material is not able to provide a firm gripping surface for the vehicle tires. Vehicle braking distance becomes longer and accidents such as a vehicle skidding into roadside ditches can occur.
Although accident data on unpaved roads that were studied were not available in this research, it would be worthwhile to accumulate such data in a future unpaved road safety study. Understanding the relationship between an unpaved road maintenance history and the number of accidents that occur on the road, considering factors such a driving speed, traffic volume, among others, should be an objective of such a study. There may be a legal issue if researchers know a specific treatment may cause more slipperiness than another.
Applying additives stabilizes the unpaved road surface by binding the road surface material together to form a firm driving surface. Unfortunately, treated unpaved roads have been noted to be slippery during wet weather conditions (Conversation, 1995). For example, during the Spring of 1995, the Larimer County Road & Bridges Department reconstructed and upgraded one of the county roads - the Stove Prairie road (CR 27). The reconstruction included blading, shoulder reclamation, reshaping of ditchlines and the addition of about 6 inches fresh virgin road surface material from the Horton Pit (one of the two gravel borrow pits operated by Larimer County). The road was treated with MgCl2 using the mixed-in-place dust suppressant application method to stabilize the road surface and control dust. The physical characteristics of the Horton Pit material are given in Table 2.1.
According to county road officials (Conversation, 1995), shortly after the construction of the MgCl2 treated road (without the road surface curing) the area received nearly 4 to 5 inches of rain over a two-day period. The county received numerous complaints from nearly all residents in the area about the poor driving conditions on the MgCl2-treated road. They complained that the road was unacceptably slippery, resulting in some accidents. Their assertion was that the slipperiness was caused by the MgCl2 additive.
Review of recent research done at the University of Wyoming (DeCastro, et al., 1996) to analyze the behavior of road soil-additive interaction provided some explanation as to why the MgCl2-treated road became very slippery under the wet conditions. In the study, cohesionless road soil material samples were augmented with clay at varying clay content (0%, 4%, and 8%). The test sections were then treated with CaCl2, lignin and no treatment for a total of six test sections. Only CaCl2 was used because it has the same transport and soil-additive characteristics as MgCl2. The test sections were compacted at a 2 percent slope as designated by AASHTO (1993) for construction of unpaved roads.
The test sections were then subjected to the same rainfall conditions using the University of Wyoming's rain stimulator. The rainfall study measured the surface erosion, runoff, solute concentration, and infiltration properties of the test sections. The results indicated that the samples with clay had lower infiltration rates and higher runoffs than those without clay. The lignin-treated test sections produced the highest runoff and the infiltration rates. The salt-treated test sections reduced infiltration to a rate about midway between that of the lignin and the untreated test sections.
Decastro et al., (1996) rain infiltration and runoff study supports the following explanation for treated road slipperiness:
- The addition of clay to road material was found to significantly reduce the infiltration rate of the treated road regardless of the additive type. So for the Larimer County road in question, the high clay fraction of the road material used (Horton Pit Gravel -Table 2.1) combined with the MgCl2 additive reduced the infiltration rate of the treated road. As a result, the use of the road at the onset of the rain created ruts as vehicle tires made deep tracks in the uncured soft surface. The formed ruts consequently impeded the flow of surface runoff into the roadside drainage ditch and held the water on the road surface. The clayey soil in the presence of the double dose of lubricant, the MgCl2 and water, became more plastic as more vehicles used the road and more rain showers occurred. The resulting road surface therefore provided no traction for the safe control of vehicles. The slippery conditions would have existed even if Lignin was used instead of MgCl2.
- In dry weather conditions salt-treated roads give an illusion of slipperiness especially under humid conditions when enough moisture has been attracted from the atmosphere to make the treated road surface wet. If enough road fines are present in the road material mix, the clay, salt, and moisture can indeed affect stopping (braking) distances of vehicles traveling at higher speeds. In the case of lignin-treated roads, the lignin is known to form a hard crust on the top of the road surface as shown in Figure 4.1. The crust was measured to be about 1/4 inch (6 mm) in the case of the University of Wyoming study. This crust however, makes lignin-treated road surfaces very smooth. The smoothness of the road surface, unlike paved roads, can affect stopping distances on the lignin-treated road.
Figure 4.1 Lignin Crust Formed during Curing
Although vehicle stopping distance tests were not performed in this research study, future studies can include stopping (braking) distance tests on treated and untreated unpaved road under both wet and dry driving conditions. ASTM (E 503/E 503M) "Standard Test Methods for Measurement of Skid Resistance on Paved Surfaces Using a Passenger Vehicle Diagonal Braking Technique" can serve as guide for such studies.
4.3 Environmental Impact
Very little quantitative information currently exists on the environmental impact from the use of dust suppressants (additives). Although it is obvious that fugitive dust emissions from treated unpaved roads are significantly less than those of untreated roads (Sanders, et al., 1997), the real issue of the amounts and size distribution of the road dust particles with and without dust treatment is unknown. This may be particularly important as particles smaller than the PM10 (10 μm) constitute a health hazard because they can damage lungs (Gottschalk, 1994). The quality of runoff from treated roads and its impact on groundwater and surface water resources are not fully known either. Experience with deicers in the eastern part of the United States has demonstrated the tremendous impact deicers have on nearby water quality. As a result there is concern that the water quality and other environmental impacts are sufficiently high that the use of dust suppressants may become regulated in the future.
The dust suppressants, especially the chloride compounds, are the same compounds used for winter season road deicing because of their freezing-point-lowering properties. Addo and Sanders (1995), stated a few reasons why very little direct information exist on the environmental impact of dust suppressants. "The reason for the lack of research in this area may be attributed to the following:
- Dust suppressing is mostly done on gravel-and-earth-surfaced roads which happen to be low-volume secondary roads located primarily in rural areas, and thus can easily be ignored.
- Some of the most commonly used compounds for dust suppressing, namely; CaCl2, MgCl2, and NaCl, are the same compounds used for road deicing. As such, the results of environmental impact studies on the effect of road deicers can be extrapolated for dust suppressants.
- The quantity of dust suppressants used annually, though on the rise, is still small in comparison with the quantity of deicers used annually.
- Unlike deicers that immediately are washed off as snow and ice melts, dust suppressants stay mostly at one place in the road surface."
No experiments were designed and performed to measure any environmental impact from the use of the three additives studied in this research project. However, a review of the effects of road deicing salts on water quality, roadside vegetation, and animal life are presented because of the profound effects salts have the physiology and morphology of plants and animals.
4.3.1 Chloride Compound Additives
The use of salts for road deicing or dust suppressing can contribute substantial amounts of chloride ions to runoff from surface of roads treated with the compounds. The salts (MgCl2 and CaCl2) are very soluble in water and will dissociate as shown in equation 4.1 (Snoeyink and Jenkins, 1980).
MgCl2+Water Mg2++2Cl | (4.1) |
The chloride ion in drinking water is considered a problem when concentrations exceed 250 mg/l and therefore is regulated by the EPA's drinking water standards. The salts, when used on road surfaces, will dissolve during wet weather and be transported into the groundwater through infiltration and/or runoff into surface water bodies. The chloride concentration in the groundwater or surface water depends on several factors including: 1) application rate, 2) composition and type of soil, 3) type, intensity, and amount of precipitation, and 4) the drainage of the road system (Pollock and Toler, 1973). In addition, the chloride concentration in the surface water also depends on the size or flow rate of the water body and the resulting dilution achieved.
In chloride concentration studies carried out in Wisconsin during a winter deicing period, runoff from roadside drainages were analyzed. Schraufnagel (1965), reported up to 10,250 mg/l were measured, while surface runoff downstream from the drainages showed chloride concentrations of only 4.5 mg/l. The significant difference was attributed to dilution. In the same study, measurement in the summer showed up to 16 mg/l in roadside runoff while stream and rivers in the area had chloride concentration ranging from 0.5 to 2 mg/l. In similar work done in Maine, Hutchinson (1966) measured chloride concentration of spring runoff from a culvert that carried runoff from about a mile of Interstate Highway 95. The samples were taken daily over a 60-day period from March through April. The chloride concentration ranged from approximately 40 mg/l to 85 mg/l with a mean value of about 57 mg/l.
The chloride concentration in groundwater along highways has also been studied. The Massachusetts Department of Public Health sampled wells close to several highways and reported chloride concentrations of up to 250 mg/l in most of the samples. This is against a background concentration of 5-15 mg/l in public water supplies (Pollock, 1973). Hutchinson (1966), in a study of the contribution of chloride from deicing, sampled 20 wells in Maine. Of the 20 wells, three that were not close to any road had less than 1.0 mg/l of chloride concentration. The rest which were located close to highways had up to 460 mg/l of chloride in one well sample. This contamination level exceeds the maximum contaminant level (MCL) of 250 mg/l established by the EPA. Most of the highly polluted wells were noted to be hand-dug and shallow while those with low chloride levels were drilled and cased.
Many studies of surface waters contaminated by deicing salt studies have been done (Schraufnagel, 1965; Hutchinson, 1966/67; Demers and Sage, 1989). All these studies indicated that the chloride concentration increased as a result of deicing activities but the levels were still far below the MCL set by EPA. Demers and Sage (1989), analyzed four streams located near a salted highway and measured up to 35 mg/l of chloride concentration. Meanwhile analysis of a sample of one of the streams upstream from the highway had only about 2 mg/l of chloride. Although 2 mg/l to 35 mg/l is a significant increase in concentration, the chloride level is still lower than the MCL of 250 mg/l. Nevertheless, the long-term effect of this exposure is not known.
The EPA has set the maximum chloride concentration in water for domestic use as 250 mg/l. This restriction is base solely on taste and palatability rather than health (Addo and Sanders, 1995). Sawyer (1960) reported that water containing chloride levels as high as 2,000 mg/l has been used without any adverse effect once the human system has adjusted to it. Salty taste in some water can be produced by as little as 100 mg/l while in others as much as 700 mg/l would not affect the taste (Standard Methods; 16th Ed, 1985).
The presence of multivalent cations in water is the cause of water hardness, thus the objection of magnesium and calcium ions in water (Snoeyink and Jenkins, 1980). Hardness of more than 100 mg/l has been noted to cause excessive soap consumption (Phelp, 1984). Water hardness also cause scale in hot water boilers, heaters, pipes and utensils thereby decreasing their useful life.
Animals on the other hand are more tolerant to water with high salinity than humans. In California, water supplies with chloride levels as high as 1,500 mg/l are designated as suitable for livestock and widelife (Mckee and Wolf, 1963). In states such as Colorado and Montana water with chloride concentrations of about 2,000 mg/l are acceptable. In Western Australia the upper safe chloride levels allowed are: 2,860 mg/l for poultry, 4,300 for pigs, 6,400 mg/l for horses, 10,000 mg/l for cattle, and as much as 13,000 mg/l for adult dry sheep (Office of Dept of Ag., 1950). At excessively high levels, chloride is said to affect the health of animals (Heller, 1932; Peirce, 1966). As stated by the National Technical Advisory Committee to the Secretary of Interior (1968), "Salinity may have a two-fold effect on wildlife; a direct one affecting the body processes of the species involved and an indirect one altering the environment making living species perpetuation difficult or impossible." One major problem associated with the use of deicing salt as far as wildlife is concerned is that wildlife are known to have "salt craving" and therefore are attracted to salted highways which can be a traffic hazard to both the animals and motorists. Mountain sheep are seen more often near roads after deicers have been applied particularly on Route 34 in Colorado between Loveland and Estes Park.
As far as plants and vegetation are concerned, the accumulation of salts in the soil adversely affects their physiology and morphology. Allison (1964) stated that salts affect plant growth directly by: 1) increasing the osmotic pressure of the soil solution, 2) altering the plant's mineral nutrition, and 3) accumulating specific ions to toxic concentrations in the plants. Strong (1944), observed that trees along a roadside where CaCl2 was sprayed for dust control were injured as exhibited by leaf scorch. Traaen (1950) also documented injuries to Norway Spruce trees by CaCl2 sprayed to control dust on unpaved roads. He noted that the salt-coated dust particles accumulated on the tree leaves absorbed moisture from the air and the resulting salt solution was in turn absorbed by the leaves.
4.3.2 Lignin Additive
Lignin is considered biodegradable, therefore its presence in the environment can be considered less harmful. Lignin is very water soluble and can be dissolved and washed off into nearby streams and other water bodies under severe rainfall conditions especially when freshly applied. In such a situation, depending on dilution of the lignin, the receiving water body may experience pollution. Pollution from lignin can be measured in terms of biochemical oxygen demand (BOD). BOD is the amount of oxygen required by bacteria to degrade and stabilize organic waste, which are the lignin sugars.
In a typical pulp-paper making process, a ton of bleach sulfite pulp waste (lignin sulfonate) would require approximately 1,000 pounds of oxygen for decomposition as compared to only 0.17 pounds of oxygen to treat a daily human discharge of waste (DeCastro, 1996). Lapinskas, (1989) provided an example of lignin sugar degradation as shown by the following chemical equation:
CnH2n+2+O2 Protein+Water+CO2 | (4.2) |
4.4 Summary
The safety and environmental impact of unpaved roads have always been a concern because the presence of loose road surface gravel can cause damage to vehicle and threaten the safety of motorists as sudden vehicle braking can be difficult. The dust emission from an unpaved road is also noted to contribute significantly to the particulate loading in the atmosphere.
The use of dust suppressants addresses the obvious unpaved road safety and environmental air pollution concerns. By stabilizing the unpaved road surface, road users' driving comfort and safety is improved and by controlling dust emission the atmospheric particulate loading contribution from an unpaved road is significantly reduced. However, the application of dust suppressants has been noted to cause slipperiness on unpaved roads in wet weather conditions. The water-quality effects of the use of dust suppressants are still not entirely known, however the chloride compounds and the lignin additives commonly used contain contaminants such as chlorides, heavy metals and organic compounds that are regulated by the EPA. Although some dust suppressant is washed into the environment after applications, initial research indicates that the quantities are relatively small.