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

Migration Pathways

The exposure scenarios associated with the identified receptors include both direct and indirect pathways. The estimation of exposure via the indirect pathways requires modeling of bead contamination through environmental media. The general modeling methods proposed for use in the bead assessment are based on EPA guidance documents for the development of human health based soil screening levels, as described below.

The two potential migration pathways identified for heavy metals in glass beads used in pavement markings were as follows:

·         Airborne suspension of respirable particulate matter affected by beads and subsequent inhalation by a receptor.

·         Leaching of heavy metals from beads into infiltrating water, migration to a groundwater aquifer, withdrawal of potable water through a residential well, and subsequent ingestion by a receptor.

Developed equations and parameter values for each of these pathways are presented in the following subsections.

Air Pathway

The inhalation exposure pathway is based on suspension of particulate matter that a human could inhale into the bronchial passages of the lungs. Therefore, the assessment of the inhalation exposure requires an estimate of both the amount of material suspended in the air and the fraction of that material that could be taken into the lungs. In general, the inhalation pathway for metals is of less concern than the ingestion pathway; however, details of the inhalation pathway are presented for completeness.^([14])

The suspension of soil into the air could be due to human activity in the area of contamination (e.g., vehicle traffic) or environmental causes (e.g., wind scour). The exposure scenario under consideration is based on the pavement-marking company storage yard as the source of contamination, which is subject to vehicle traffic. However, the vehicle speeds are likely to be low, and traffic would be infrequent, although this variable is difficult to quantify. Therefore, suspension of bead material due to wind scour is likely to be a more consistent and frequent source of airborne contamination and is used as the basis for further model development.

General equations for human health exposure to airborne contamination are based on the air concentration of respirable particles. The size of particles that are considered respirable is based on the deposition velocity of particulates in air and filtering ability of the human respiratory tract.

Calculation of air exposure concentrations for the modeling methods will be based on the amount of respirable material suspended into the air due to wind scour from the soil matrix containing the beads. Models describing the wind scour of soil are based on the concept of a threshold wind speed that can entrain particulates in the air.^([15]) A particle size of 75 µm is considered the limit for wind suspension. For the bead model, it is assumed that the distribution of contamination in the bulk beads is similar to the distribution of contamination in the smaller size fractions. The “unlimited reservoir” wind suspension model assumes that 60 percent of the erodible material would pass a 1-mm screen, which is consistent with standards for Type I and II beads, requiring that > 90 percent be less than 0.85 mm in diameter.^{(15, [16])}

To provide a conservative value for the amount of respirable particulates in the air, the source area is assumed to have no vegetation and no surrounding buildings or trees that might significantly reduce the wind speed at the ground surface. Although only a small portion of the beads (< 5 percent for AASHTO type II beads) are in the respirable range, it is assumed that the distribution of bead sizes approximates the distribution of particle sizes in soil used to develop the wind scour model.⁽¹⁶⁾

Groundwater Pathway

The potential for migration of contaminants out of the glass beads and into groundwater is significant in the evaluation of human health risks. Groundwater ingestion pathways include two elements: 1) the leaching of contaminants out of glass beads and into infiltrating water, and 2) the migration of leachate to the groundwater and subsequent extraction from a residential well, resulting in human exposure by ingestion. The leaching of contaminants depends on the partitioning between the bead matrix and surrounding pore water, and an important parameter in the evaluation of leaching from the solid matrix is the partition coefficient (i.e., the distribution coefficient). The partition coefficient is defined as the ratio of the total metals content in a solid matrix to the metals content in the liquid matrix. Higher partition coefficients indicate that the metal is more likely to be retained in the solid matrix. The partition coefficient can be used to estimate the concentration of contamination in liquid that is in equilibrium with contaminants in soil, which can then be used to determine the impact on groundwater.

The migration of leachate to groundwater and subsequent ingestion through a residential well is highly dependent on site-specific factors related to weather conditions, soil properties, and characteristics of the groundwater aquifer. The relationship between the concentration of a contaminant in leachate and the concentration in groundwater can be expressed as a dilution-attenuation factor (DAF). In general, a high infiltration rate and low groundwater velocity leads to a low DAF, indicative of groundwater concentrations that are more significantly affected by the leachate.

A reasonably limiting DAF is approximately 10 for a low-flow aquifer with a significant infiltration rate. Although a lower groundwater velocity leading to a DAF < 10 is possible, aquifers with a DAF < 10 are less likely to be used for potable water because the capacity for the aquifer to provide a consistent volume of potable water to a household would be reduced. In addition, at very low groundwater velocities, the transport time of contaminants leaching from beads and traveling to a well would approach the exposure duration for a resident of 30 years. The use of a DAF of 10 is considered conservative because DAF values of approximately 4,000 are reasonable for high-flow aquifers with minimal infiltration.

The use of a DAF provides a simplified approach for a screening level assessment and is based on a number of modeling assumptions, including the following:

·         Contamination is uniformly distributed within the source area.

·         The contaminant source is in contact with the surface of the aquifer (no unsaturated zone).

·         Water is withdrawn from the aquifer at the down-gradient edge of the contaminated area from a well screened within the contaminant mixing zone.

The edge of the contaminated area is used as the point of exposure because it represents a location that is significantly affected and feasible for exposure. Groundwater accessed in this area is assumed to mix with leachate to a depth that might be feasible for access to drinking water, yet the dilution is less than at locations further down-gradient from the contaminant source that might also be used for household purposes.

These assumptions are consistent with EPA recommendations for calculation of screening levels protective of residential groundwater use.⁽¹⁴⁾ For calculation of a site-specific DAF, the generic evaluation may be refined based on additional data collection or interpretation.