In the foothills of the Sierra Nevada Mountains, serpentine rock has been mined extensively and widely used as a material for many types of unpaved surfaces, including parking lots, driveways, roads, and apparently even some school playgrounds. When vehicles are driven over unpaved roads surfaced with asbestos-containing serpentine material asbestos fibers are released into the atmosphere as part of the resultant dust cloud. Thus persons near the roadway, especially on the downwind side, are exposed to elevated ambient concentration of asbestos. The goal of the present study was to quantify asbestos concentrations downwind of these roadways and relate the concentrations to vehicle traffic, road surface materials, and meteorological and climatological conditions.
After reviewing the occurrence of serpentine-covered unpaved roads in various parts of California and visiting roads throughout the State, it was found that the locale most suitable for study was in the vicinity of Oakdale in eastern Stanislaus County. After gaining permission from landowners, four sites were selected for field experiments. At each site, a network of four to five asbestos monitoring stations was established as well as a meteorological station for measuring wind speed and direction. During five to eight one-hour test runs at each site, traffic was simulated on the road by repeated van trips while air samples were taken and meteorological conditions were monitored. Bulk samples of the road surface material were also taken for analysis of bulk asbestos content, silt content, and moisture content. Air samples were analyzed for asbestos using both optical and electron microscopes for two size ranges: all structures and structures > 5 Ám.
The EPA model that consists of the Copeland road dust emission model and Gaussian line source equation was evaluated by comparing measured asbestos concentrations with concentrations predicted by the model for the test conditions. The EPA model was found to be good only to estimate an order of magnitude of downwind concentrations. The structure of the model was found to be generally adequate, but the inclusion of both short temporal and long-term average parameters in the model appeared to decrease the accuracy of model estimates. Residual analysis of model-predicted concentrations less measured concentrations revealed that the model tends to overestimate asbestos concentrations at lower vehicle speeds and the model's performance is skewed with respect to model's site parameters such as moisture, silt, and asbestos contents.
A modified roadside asbestos model called CALSCRAM was developed by rectifying some of the defects found in the EPA model. The new model, which was calibrated over the range of 14% to 18% bulk asbestos content, was found to reduce the EPA model prediction errors by 76%. It is capable of predicting both short-term and long-term average asbestos concentrations and has a feature that accounts for the effect of a finite road segment on downwind concentrations.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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