Fate and Transport of Metals and Particulates within the Roadside Environment – A Review - page 2


Factors influencing contaminant quantity and quality

The quantity and quality of traffic-generated contaminants are influenced by functional, physical, and regional factors. Functional influences include traffic volume and density (Daines and Motto, 1970; Ward et al., 1977; Wheeler and Rolfe, 1979; Fakayode and Olu-Owolabi, 2003), and traffic type (commuter, commercial, industrial, construction, and/or agricultural traffic). Physical influences include pavement type and condition (Pagotto et al., 2000; Dean et al., 2005), operation and maintenance practices (Ball et al., 1998), and the presence of rumble strips which may assist in dislodging metal laden debris from vehicles through increased vibration. Regional regulations, such as the composition of gasoline, emissions standards, catalytic converter requirements, acceleration/deceleration zones, preference of traffic circles over stop lights, and brake pad or tire material composition can all affect the quantity and quality of traffic generated contaminants contributing to any given section of roadway. For example, the elimination of tetra ethyl lead from gasoline in some areas has resulted in significant reduction in lead loadings (Helmers et al., 1995; Legret and Pagotto, 1999; Turer et al., 2001), illustrating how aggressive regulatory control can substantially reduce environmental impact. However, as metal migration is generally reported as a relatively slow process, current and historically generated metals contaminants are most likely still present not far from where they were first deposited.


Transport of contaminants to roadside environments

Once metal and particulates are generated and introduced to the contaminant plume associated with a transportation corridor, they are susceptible to macro-, meso-, and micro-scale transport mechanisms to the roadside environment. Macro-scale mechanisms are a function of regional meteorology and land use. Meso-scale mechanisms include localized traffic conditions and physical roadway geometry. Micro-scale transport mechanisms from road surfaces and within natural environments are highly dynamic and complex functions of physical, chemical and temporal characteristics. These transport mechanisms are discussed in detail below.

Macro-scale mechanisms

Regional precipitation patterns

Regional meteorology affects the quantity, frequency and duration of runoff events. These factors strongly influence contaminant concentrations. Contaminant-laden particulates, aerosols, and ions are scoured from the atmosphere and flushed from roadway surfaces by precipitation as runoff. Long antecedent dry periods facilitate accumulation of contaminants on road surfaces, resulting in a correlation with higher concentrations of some contaminants in stormwater runoff (Hewitt and Rashed, 1992). Long duration storms have the overall result of producing dilute runoff, as the finite contaminant load is dispersed within the runoff volume. Long duration storms also contribute to pavement wear, as wet weather road wear is two to six times greater than dry weather road wear (Backstrom et al., 2003). High intensity storms result in high runoff velocities, increasing sediment transport of particulates (Deletic, 1999).

Regional wind patterns and land use

Regional meteorology also affects prevailing wind speed and direction, which influences the distance and direction particles are transported (Piron-Frenet et al., 1994; Ylaranta, 1995). Some conditions may be mutually affective. For example, the combination of the agricultural activities of eastern Washington and the regional Palouse Loess soil type, coupled with the prevalence of the blustery winds of the high plains, succeeds in dusting the area with fine organic rich soil (Horner and Mar, 1984), to which metal species from roadways may readily adsorb. Regional atmospheric chemistry, either degraded by anthropogenic pollution or simply influence by a nearby marine environment, will alter rainwater quality, and will thus affect the partitioning of metals to particulates in runoff (Glenn III et al., 2001).

Seasonal variability

Seasonal variability in northern climates causes precipitation in the form of snow, necessitating the use of deicing salts on roadway surfaces, some of which contribute directly to impaired runoff quality as chloride and cyanide ions (Novotny et al., 1998; Marsalek, 2003). In some locations sand or gravel is used to increase traction during winter seasons. Not only do these materials contribute to the particulate load to roadside environments (Oberts, 1986), but they have a secondary degradative mechanism since sand and gravel assist in abrading road surfaces and create more particulates. Studded tires that are widely used in some northern climates during winter months also increase the particulate load from roadways due to increased winter wear, while summer road surface wear is minimal in comparison (Backstrom et al., 2003).