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

Aim 2: Screening Level Risk Assessment to Assess the Impacts of Occupational and Residential Exposure to Arsenic and Lead within Glass Beads

CSEM Development for Arsenic and Lead Glass Bead Occupational and Residential Exposures

The conceptual site exposure model was developed to assess the workflow of beads within their product lifecycle, perform a human health exposure assessment based on bead workflow, develop exposure assessment models to evaluate screening level concentrations in glass beads that are protective of human health, and identify existing sources of data for use within the developed modeling. CSEM development was based on field observation of human interactions with the glass beads during manufacturing, transportation, storage, application, and disposal of old marking residues. All field observations were conducted in an arid environment during warm, dry, and low wind conditions (< 15 mi/h). Although observations were made in an arid environment, this assessment considers a range of environmental conditions. Additional information used in this assessment was obtained through interviews with individuals involved in the glass manufacturing and highway marking industries.

Based on the completed exposure assessment, the proposed risk assessment model focused on three specific exposure scenarios:

·         Scenario 1—Worker: roadway marking crew employee exposed through incidental ingestion, dermal contact, and inhalation of fugitive dust emissions.

·         Scenario 2—Adult Resident: resident living in close proximity to an active bead storage yard or on top of a former storage yard exposed through ingestion of contaminated drinking water, incidental ingestion, dermal contact, and inhalation of fugitive dust emissions.

·         Scenario 3—Child Resident: resident living in close proximity to an active bead storage yard or on top of a former storage yard exposed through ingestion of contaminated drinking water, incidental ingestion, dermal contact, and inhalation of fugitive dust emissions.

The proposed modeling framework focuses on developing quantitative measures to evaluate the screening level concentrations of arsenic and lead in glass beads that result in an increased risk. The quantitative assessment requires calculation of two components: 1) the level of metal uptake or air concentration as a function of each individual exposure route, and 2) the permissible level of exposure (screening level) due to either cancerous or non-cancerous end points considering the combined intake from the multiple routes of exposure affecting a single receptor. All calculations are based on EPA guidance documents addressing human exposure to soil, water, air, and food.^([4]) The contaminant exposures calculated for each receptor, environmental medium, and pathway combination are the basis for estimating the potential risk or hazard to exposed individuals.

The full list of equations used to determine the intake of metals applicable to every exposure scenario and the calculations used to evaluate the cancer and non-cancerous human health screening levels for direct contact, inhalation, and ingestion of bead-impacted groundwater are found in section 2. The proposed model (presented in section 2) was externally peer-reviewed prior to use of the model to predict screening level concentrations of arsenic and lead found in section 3.

Arsenic and Lead Concentrations in Mixed Glass Bead/Soil Samples Taken From a Glass Bead Storage and Transfer Facility

During the CSEM model development, the mass percentage of beads in soil at a storage facility and the resulting arsenic and lead content within mixed glass bead/soil samples was determined to be an important parameter. Initial assumptions used in the model included the assumption that all of the soil in a bead storage facility was composed of spilled beads with a concentration equivalent to the highest measured arsenic and lead content. To arrive at a more realistic conservative assessment, five samples of soil from a bead storage facility where beads had been stored and transferred for more than 20 years were collected and analyzed for bead mass content. The resulting arsenic and lead concentration in the mixed sample were determined. A control soil sample selected from a nearby vacant lot was used for comparison.

Method

Surface soil samples were collected, weighed, and then sieved on U.S. Sieve #30, #40, #50, and #80. The fraction of soil retained on each sieve was weighed and kept separately. An inclined plane made of a strip of Plexi-Glass® and a light table was used to manually separate glass beads, based on roundness, color, and translucence, from a representative portion of each fraction of soil. The beads separated from each fraction were then weighed. A portion of the original, non-sieved sample was digested following the PNNL KOH method followed by ICP-MS analysis of the digested solutions according to EPA Method 6020A.

Findings

Table 4 lists the average glass bead content in site soil samples collected from a bead storage and transfer facility (along with a nearby vacant field). The difference in the content of glass beads is owing to different sampling locations within the facility. The glass bead soil content varied from 19 percent to a maximum of 78 percent, with an average bead to soil content of 42 percent. Reportable levels of arsenic were observed in only two of the five site samples. However, lead was observed in all samples and in the control at concentrations ranging from 11 ppm to 120 ppm. The concentrations observed in the mixed bead samples are in excess of the background concentration by 33 percent to 400 percent. While it may be reasonable to assume that elevated arsenic and lead content in the storage facility samples are associated with the presence of glass beads, the glass bead content in field site soil sample does not correlate with the metal content in site soil samples. Based on the historical use at this site, the initial assumption that site soils would have higher concentrations of both arsenic and lead does not appear to be valid.

Table 4. Mean glass bead content (by weight) and mean ± standard deviation total arsenic and lead (ppm) in site soil samples.

ppm = parts per million

BDL = below detection limit (< 0.07 µg/g for arsenic, < 0.001 µg/g for lead)

 — indicates only one viable data point

Arsenic and Lead Screening Levels Resulting in Minimum Risk From Residential and Occupational Exposures to the Beads

Reasonably conservative screening levels for protection of human health risk from glass bead exposures were determined for each evaluated exposure scenario identified during field investigations of bead workflow. Exposure pathways included in the model were incidental ingestion of beads, incidental inhalation of beads, and ingestion of bead contaminated groundwater. The potential for leaching of arsenic to groundwater was evaluated using laboratory-generated characterization data. Lead and arsenic toxicity data used in the risk evaluation are from the Risk Assessment Information System maintained by the Oak Ridge National Laboratory.

The developed model indicates that while the majority of risk is associated with the ingestion pathway, current concentrations in the beads are within EPA’s acceptable risk range of E-6 to E‑4. Therefore, there is a low likelihood of adverse human health effects due to exposure to beads released to the environment. Table 5 presents the bead screening levels for arsenic and lead based on the evaluated exposure scenarios for all combined pathways. The presented results assume a source that is 42-percent beads diluted with soil (based on field-analyzed samples at a bead storage facility). Screening levels are generated based on both carcinogenic and non-carcinogenic effects, with the lower value selected as the final screening level. The recommended screening levels were determine to be 220 ppm for arsenic based on the child resident scenario and 580 ppm for lead based on the worker scenario.

Table 5. Screening levels for arsenic and lead from each scenario.

ppm = parts per million

NA = not applicable

Note: Bolded values represent the most conservative screening levels and are the recommended screening levels for arsenic and lead in glass beads.

Guidance to Support Decisionmaking

The determined screening levels for arsenic and lead are above the current maximum content of 200 ppm arsenic and 200 ppm lead adopted in legislation. Therefore, the existing legislation is determined to be protective of human health when all currently available data are considered. In addition, based on laboratory and field sample characterization completed in this study, the mean arsenic and lead concentrations observed in the beads were below the 200 ppm limit currently adopted in legislation. Therefore, the proposed limit should not present a hurdle to using existing beads already in the commercial markets for pavement-marking purposes.

Although current risk levels are minimal, field observations of bead workflow processes did identify easy-to-implement practices that would further reduce exposure. In the occupational setting, employees were observed handling the beads without gloves or masks. Concern was raised during the visits and during the model peer-review process that employees might be exposed to high levels of silica from bead dust that could lead to silicosis. Wearing gloves and respirators to protect against potential silica exposures would have the added benefit of reducing exposure to arsenic and lead.

The model also predicted potential concern regarding the impact of bead storage facilities on residential groundwater due to leaching of arsenic and lead from the beads that may occur within some climates. Current practices of bead storage prevent rainfall from leaching arsenic and lead from stored beads to groundwater. Efforts to reduce bead spillage during transfer would also reduce the likelihood that bead-contaminated media could affect groundwater. Existing locations with long-term histories of bead (and or cullet) storage and transfer may present a challenge to groundwater where shallow groundwater tables are present.

Although application of the beads does result in bead loss to the surrounding environment, long line applications in which bead loss may reach up to 30 percent under poor application practices or conditions does not appear to present a risk to human health or the environment. During long line applications (roadway center and edge line markings), bead loss occurs over a long distance and the beads quickly scatter. Long line application is also performed using bead drop equipment in a manner that does not expose employees to the lost beads. Short line applications (cross walks and intersections), however, do result in greater worker exposure and higher concentrations of spilled beads accessible to the general public. Efforts should be made to reduce excess bead loss during short line applications. Employees putting down beads should wear gloves to reduce exposure, and beads should be dropped so that the majority land on the binder (paint, thermoplastic, or epoxy). In particular, efforts should be made to reduce excess bead loss in short line applications in locations with curbs and gutters because of the potential slipping hazard.

Line removal presents a separate set of potential risks. To minimize exposure to arsenic and lead from glass beads during marking removals, employees should wear gloves, eye protection, and respirators if they are performing removal techniques that generate dust. Grinding, sand-blasting, or water blasting systems used to remove the lines should be equipped with vacuum recovery systems to reduce dust removal. Additional investigations into dust exposures during marking removal are advised.

As a final comment, arsenic and lead in glass beads may be a minor concern for environmental health and safety compared with other components in pavement-marking systems. A thorough review of the risk posed by residential and occupational exposures to components in other marking systems is advised to alleviate potential concerns for environmental and worker safety.