Screening Level Assessment of Arsenic and Lead Concentrations in Glass Beads Used in Pavement Markings

Bead-Specific Exposure Parameters

Parameters that are bead specific reflect exposure mechanisms that depend on the properties of the beads under evaluation. The following subsections describe bead-specific parameters used in the assessment, including the concentration of the beads in the bead/soil source material, the concentration of metals in beads, and the leaching rate of metals from the glass beads.

Mass of Beads in Bead/Soil Source

The quantity of beads present in the environment as a result of bead loss during storage and application is an important parameter in the risk evaluation. The fraction of beads in the source material affects the magnitude of the exposure to humans. The following section presents estimates of bead loss, as well as field measurements of the bead fraction observed at an active bead storage facility.

Table 21. Toxicity data for glass bead assessment.

NA = not applicable

Notes: Toxicity data from the EPA⁽²¹⁾

EPC—exposure point concentration in bead/soil assuming 42-percent beads in the source.

Arsenic concentrations of 54 and 62 ppm are the arithmetic average and 95-percent upper confidence limit (UCL95%), respectively.

Table 22 provides available data for estimating the bead concentrations in environmental media for various bead loss scenarios. The estimates presented in table 22 are based on the bead workflow elements and release potential outlined in table 17. The release potential values have been converted from English to SI units for use in the model equations.

The estimated bead concentration in the soil of a storage yard assumes that an area of 4,050 m² is used to store and transfer beads and that the beads are mixed into the soil to a depth of 150 mm. If approximately 4.5 kg were spilled daily for 250 days each year, a total of 1,125 kg of beads would be mixed into the soil annually, resulting in a bead concentration in soil of approximately 1,500 ppm after 1 year.

Bead loss to the environment may also occur during application of the beads onto pavement markings. During application, bead loss is expressed as a percentage of the total beads applied. The total bead load applied is a regulated quantity in some states. Table 23 presents an estimate of bead mass loss rate in kg/km, based on typical pavement marking paint and bead application rates. The application scenarios are shown in English units, with the final bead mass loss rate converted to SI units. Actual bead mass loss depends on the number and width of the pavement markings, operational characteristics of the application equipment, speed of application, and other factors.

The bead load estimate above is based on a four-lane undivided highway with a central turning lane, and the following 6-inch lines:

·         Left shoulder—continuous.

·         Lane 1 and 2 divider—dashed.

·         Left double line for central turn lane—continuous/dashed.

·         Right double line for central turn lane—continuous/solid.

·         Lane 3 and 4 divider—dashed.

·         Right shoulder—continuous.

Based on worker interviews and field observations, typical bead loss percentages are assumed to be 15 percent of the applied load. With a total estimated bead application rate of 1,150 lb/mi (see table 17), the resulting 15 percent bead loss rate is approximately 49 kg/km. The quantity of beads lost to the roadside can be used to provide an estimate of soil concentrations at residential locations near roads. Assuming that all beads are blown to one side of the road, and that they are mixed into the soil to a depth of 50 mm and to a width beside the road of 2 m, then the total mixing soil volume is 100 m³/km. For a typical silty loam soil with a density of approximately 1.28 g/cc or 1.28 × 10⁶ g/m³ or 1,280 kg/m³, the mixing mass of soil for each kilometer is 128,000 kg. The resulting bead concentration in the roadside soil would be 49 kg of beads in 128,000 kg of soil or 385 ppm. 

Table 22. Estimates of bead loading to soil per year.

Notes: Assumes storage yard area is approximately 1 acre or 4,050 m².

Assumes area affected for the roadside scenario is approximately 2 m wide, approximately 50 mm deep and 1 km long.

Soil density is that for silty loam (1,280 kg/m³).

Bead mass for the storage yard assumes 4.5 kg per day over 250 days each year, or an annual release of 1,125 kg.

Bead mass for the roadside during application assumes 1,150 lb/mi application rate for six continuous 6-inch-wide lines with a bead loss of 15 percent the release is 175 lb/mi.

Bead mass for line degradation assumes 25 percent of the bead load on the line (85 percent of total applied load) is released in a year, or approximately 69 kg/yr per kilometer of roadway.

Bead mass for the total roadside is sum of the releases from the original line application and line degradation, per kilometer of road, assuming both occur during the same year.

Table 23. Estimated glass bead application and loss rate for line applications.

Notes: All quantities are estimates based on data from several State transportation departments. Site-specific information is used where available to refine the estimates.

In general, lines are replaced when reflectivity has dropped to 25 percent of the original value, which is assumed to occur within 3 years within the model calculations. Therefore, during 1 year, approximately 25 percent of the original line load will be released to the environment. For the example presented in table 22, the total applied load is 324 kg/km, with 275 kg/km adhering to the lines and 49 kg/km lost to the roadside initially. An additional loss of approximately 69 kg/km may occur over a period of a year through line degradation.

Field investigations have measured the fraction of beads in soil at one bead storage yard to provide perspective on the estimated bead concentrations (table 15). The storage yard has been used for at least 20 years and indicated a range of bead fractions (by weight) of 20 percent to approximately 78 percent and averaging 42 percent, or 420,000 mg/kg. The field measurements indicate that the estimated bead loss calculation presented in table 22 may be an underestimate by approximately an order of magnitude. However, it is not clear to what depth the beads were mixed into the soil during field measurements, which could account for the discrepancy.

In actuality, the mass balance of contaminants in the beads has not been included in the modeling methodology. The methodology currently proposes that the total mass of metals in the released beads is available for both direct contact and migration to groundwater individually. In reality, the bead mass lost to leaching would no longer be available for ingestion, dermal contact, or inhalation, thereby reducing the exposure concentration of metals for the direct contact receptor. Similarly, bead mass removed from the marking crew storage yards by wind scour or ingestion would no longer be subject to leaching. These conservative assumptions provide an additional margin of protectiveness in the calculations.