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

Risk Evaluation Results

The estimate of human health risk from glass bead exposures was based on the residential and worker scenarios. Exposure pathways included in the evaluation were incidental ingestion and inhalation of re-suspended particulates. Although the laboratory data indicate a low likelihood of metal leaching from beads, the groundwater ingestion pathway has been evaluated for completeness.

Toxicity data used in the risk evaluation of arsenic are presented in table 21, and the results of the risk evaluation for beads in soil are presented in table 27. The exposure of receptors to arsenic at the average concentration resulted in risks of 8.8E-7, 2.5E-6, and 5.4E-7 for the adult, child, and worker, respectively. All carcinogenic risks are within the EPA acceptable risk range of E-6 to E-4. The majority of risk is via the ingestion pathway with insignificant risk owing to inhalation. The hazards for arsenic were all below the threshold hazard of 1, indicating a low likelihood of adverse human health effects due to exposure to beads released to the environment. These calculations were based on a source that was 42-percent beads.

Also presented in table 27 are results for exposures at the UCL95% arsenic concentration of 62 ppm for comparison. All risks and hazards are within the acceptable range and are not likely to present a health hazard. These calculations also assumed that the source material was 42‑percent glass beads in the soil.

The potential for leaching of arsenic to groundwater is evaluated in table 28. Laboratory experiments yielded conflicting results regarding the leaching of arsenic and lead from glass beads. The risk evaluation was based on the most conservative leachate concentration observed (speciated arsenic (V) at 15 µg/L). Additional conservatism is included in the calculation because these leachate concentrations were assumed to persist for the duration of the exposures (up to 30 years in the case of the adult resident). The estimated groundwater concentration of 0.63 µg/L is below the Federal Maximum Contaminant Level of 10 µg/L but exceeds the EPA RSL of 0.045 µg/L. The estimated risk associated with arsenic in groundwater was 1.4E-5, which is within the acceptable risk range, even with the inclusion of conservative assumptions in the calculations. The results indicate that adverse human health impacts are unlikely via the groundwater pathway.

Table 27. Summary of glass bead risks for arsenic.

ILCR = incremental lifetime cancer risk

HQ = hazard quotient

ppm = parts per million

Notes: Beads were assumed to represent 42 percent of the bead/soil source.

Arsenic concentrations of 54 and 62 ppm are the arithmetic average and UCL95%, respectively (table 24).

Table 28. Estimated groundwater risk for arsenic.

Notes: Tap water RSL is based on carcinogenic effects to an adult/child at a risk level of 1E-6.

Maximum contaminant level (MCL) is a federally promulgated standard that accounts for economic and human health considerations.

Concentration in leachate is an upper bound based on speciated arsenic analysis of bead sample DC. The majority of samples indicated < 0.7 µg/L of extractable arsenic.

Estimated groundwater concentration assumed that the bead source is diluted to 42 percent of the soil matrix and a DAF of 10 in the aquifer.

Groundwater concentration is calculated as: Leachate Conc (µg/L) × 0.42 × (1/10) = 0.63 µg/L.

Estimated groundwater risk is based on the tap water RSL, and is calculated as:
Tap water RSL (µg/L) × Groundwater Concentration (µg/L)/1E-6.

Table 29 presents the bead screening levels for arsenic based on the exposure scenarios and pathways evaluated. Results are presented assuming a source that is 100-percent beads, as well as a source that is 42-percent beads diluted with uncontaminated soil. Screening levels are generated based on both carcinogenic and non-carcinogenic effects, with the lower value selected as the final screening level. For all scenarios (residents and industrial) the carcinogenic exposure via bead ingestion was the most significant pathway. Screening levels based on non-carcinogenic exposures were approximately an order of magnitude less stringent than those for carcinogenic effects. For an undiluted source, screening levels for the adult, child, and industrial worker were 252, 90, and 421 ppm, respectively. When considering a source that is 42-percent beads, the screening levels are 600, 220, and 1,000 ppm, respectively. The final screening level recommendation of 220 ppm is based on a child exposure to source material that is 42-percent beads.

The evaluation of lead used EPA models for adults and children as presented in table 30 and table 31. The exposures to lead and resulting blood lead levels are compared with the acceptable level of 10 µg of lead per dL of blood. The ALM indicated a blood lead concentration of approximately 1.9 µg/dL, and the ALM calculated screening level was 580 ppm. The IEUBK model for child lead exposure estimated a blood lead level of ~0.6 µg/dL, and the IEUBK calculated screening level was > 1,000 ppm. The adult and child lead modeling indicates that adverse health effects are not likely due to lead exposures. These results are based on the UCL95% concentration of 118 ppm for lead in undiluted beads, which represents a conservative exposure that is unlikely to be exceeded.

Table 29. Summary of glass bead screening levels for arsenic.

Notes: All units are ppm.

Values in bold are the final screening values.

Carcinogenic target incremental lifetime cancer risk (ILCR) was 1.0E-5.

Target HI was 1.0.

Beads were assumed to represent 42 percent of the bead/soil source (table 15).

Table 30. Calculations of adult blood lead concentrations.

NHANES = National Health and Nutrition Examination Survey

— indicates unitless

Notes: Absorption fraction is based on maximum bioaccessible lead measurements for bead group AC.

Source: http://www.epa.gov/superfund/lead/products.htm#alm

.

Table 31. Calculations of adult lead screening levels.

NHANES = National Health and Nutrition Examination Survey

— indicates unitless

Notes: Absorption fraction is based on maximum bioaccessible lead measurements for bead group AC,

Source: http://www.epa.gov/superfund/lead/products.htm#alm