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

Concentration of Metals in Glass Beads and Leaching Potential

The evaluation of long-term exposures requires an understanding of the long-term releases from beads to the environment. Most significantly, metals may migrate from glass beads through interactions with precipitation and migrate to the groundwater. The modeling of bead leaching over a period of time similar to the long-term exposure duration of the receptors provides the most representative assessment of risk.

In the environment, beads are incorporated into the soil matrix, and any interactions with infiltrating water would be complex and include partitioning of contaminants among the beads, soil, and water in the pore spaces of the soil/bead matrix. As contaminants in beads go into solution, they may interact with soil particles and adsorb to the surface. Over time, contaminants may repeat this process, with the general effect of retarding the movement of some portion of the contaminants from freely moving with the infiltrating water through the soil column and into the underlying groundwater system. In addition, some portion of the contamination will remain in the soil matrix and never reach the groundwater. For calculation of groundwater protection screening levels, it is conservatively assumed that contaminants leaching from beads are not retained or retarded by the soil matrix, and instead travel unimpeded to a groundwater aquifer that is used for drinking water.

To determine long-term groundwater concentrations, it is important to understand the magnitude and rate of contaminant leaching from beads. The residential receptor is assumed to ingest groundwater over a period of 30 years; therefore, an average concentration over that period of time is most representative of the exposure scenario.

The results of leaching experiments may be used to estimate exposure over a lifetime by assuming that the leachable mass in beads enters the environment during a 1-year period. Experimental results may also determine whether beads in the environment longer than 1 year are inert and are not a source of further contamination to groundwater.

Calculating the groundwater concentration requires an estimate of either the mass of contamination entering the aquifer, which can be based on the mass of contaminants that can be leached, or the concentration of contaminants in the leachate. These estimates can be made through general assumptions regarding the leaching potential of beads or through experimental analysis. For the development of groundwater screening levels, the mass of contamination leaving the beads and entering groundwater on an annual basis may be calculated from the experimental data, which are discussed in the following sections.

An additional consideration is the physical location of the bead contamination within the bead structure. If contamination is coating the bead surface, it may leach quickly, leading to high initial water concentrations that quickly decline. Contamination within the bead matrix may provide lower initial water concentrations that persist longer. Continuing laboratory studies may provide data to clarify leaching dynamics.

The leaching rates used in the evaluation are necessary to describe the following processes: leaching rate of contaminants from glass beads to groundwater, leaching rate of contaminants to soil, and leaching of contaminants in a mixture of soil and beads to groundwater. Two older studies are available for determining the leaching rates of metals from glass beads: the TAMU study published in 2011 and NJIT/RU study.^{([22], [23])} The TAMU study evaluated the leachability of metals from glass beads present in an up-flow column system as a function of pH, short-term exposures to high-intensity ultraviolet light, short-term exposures to high temperatures, and particle size. The NJIT/RU study also evaluated leaching in batch reactors as a function of pH and particle size. In addition, the NJIT/RU study evaluated leaching using the simulated precipitation leaching procedure and toxicity characterization leachate procedure, and a long-term leaching experiment conducted over a period of 160 days.

Section 2 of this report presents findings of the most recent study conducted on a large sample of beads from across the United States to provide a more representative study of beads actually in use on roads. Laboratory characterization of the State transportation department provided beads indicated a total arsenic concentrations ranging from 11 to 82 ppm with an overall average of 54 ppm. Total lead ranged from 3 to 199 ppm, with an overall average of 71 ppm. Statistical analysis of these datasets with EPA software is summarized in table 24 for arsenic and lead.^([24]) The 95-percent upper confidence intervals for arsenic and lead are 62.14 and 118.6 ppm, respectively. Estimates of human health risk are presented for exposure to both the mean and 95‑percent upper confidence limit (UCL95%) concentrations in source materials for comparison.

Table 24. Summary of statistics for bead analysis.

Notes: All concentrations in ppm.

UCL95% is the 95-percent upper confidence limit on the mean concentration.

Statistics are based on the average concentrations in glass beads (table 7).

The EPA software ProUCL version 4.1, was used to generate the statistics.⁽²⁴⁾

The potential leaching of metals was evaluated by comparing the total metals analysis to the extractable metals in laboratory experiments. Similarly, the bioaccessible fraction of metals was determined by comparing total and bioaccessible measurements. Extractable arsenic was not detectable in all samples and was similarly not detectable in the bioaccessibility analysis. Extractable lead was measured at up to 3.4 percent of the total measurement, and bioaccessible concentration was up to 16.4 percent of the total (see table 20). The time series analysis of column leaching studies indicated that lead concentrations were all below detection, and arsenic leachate concentrations were below detection within 48 hours (table 25 and table 26). The laboratory data indicate that there is a low likelihood of significant leaching of arsenic and lead from pavement-marking beads.

Table 25. Summary of TTI/TAMU leaching studies for arsenic—December 2012.

Table 26. Summary of TTI/TAMU leaching studies for lead—December 2012.