Tire-tread material has a zinc (Zn) content of about 1 wt %. The quantity of tread material lost to road surfaces by abrasion has not been well characterized. Two approaches were used to assess the magnitude of this nonpoint source of Zn in the U.S. for the period 1936-1999. In the first approach, tread-wear rates from the automotive engineering literature were used in conjunction with vehicle distance-driven data from the U.S. Department of Transportation to determine Zn releases. A second approach calculated this source term from the volume of tread lost during lifetime tire wear. These analyses showed that the quantity of Zn released by tire wear in the mid-1990s was of the same magnitude as that released from waste incineration. For 1999, the quantity of Zn released by tire wear in the U.S. is estimated to be 10 000-11 000 metric tons. A specific case study focused on Zn sources and sinks in an urban-suburban watershed (Lake Anne) in the Washington, DC, metropolitan area for a time period of the late 1990s. The atmospheric flux of total Zn (wet deposition) to the watershed was 2 microg/cm2/yr. The flux of Zn to the watershed estimated from tire wear was 42 microg/cm2/yr. The measured accumulation rate of total Zn in age-dated sediment cores from Lake Anne was 27 microg/cm2/yr. These data suggest that tire-wear Zn inputs to urban-suburban watersheds can be significantly greater than atmospheric inputs, although the watershed appears to retain appreciable quantities of vehicular Zn inputs.
Urban settings are a focal point for environmental contamination due to emissions from industrial and municipal activities and the widespread use of motor vehicles. As part of the National Water-Quality Assessment Program of the U.S. Geological Survey, streambedsediment and dated reservoir-sediment samples were collected from the Chattahoochee River Basin and analyzed for total lead (Pb) and zinc (Zn) concentrations. The sampling transect extends from northern Georgia, through Atlanta, to the Gulf of Mexico and reflects a steep gradient in population density from nearly 1000 people/ km 2 in the Atlanta Metropolitan Area to fewer than 50 people/ km 2 in rural areas of southern Georgia and northern Florida. Correlations among population density, traffic density, and total and anthropogenic Pb and Zn concentrations indicate that population density is strongly related to traffic density and is a predictor of Pb and Zn concentrations in the environment derived from anthropogenic activities. Differences in the distributions of total Pb and Zn concentrations along the urban-suburban-rural gradient from Atlanta to the Florida Panhandle are related to temporal and spatial processes. That is, with the removal of leaded gasoline starting in the late 1970s, peak Pb concentrations have decreased to the present. Conversely, increased vehicular usage has kept Zn concentrations elevated in runoff from population centers, which is reflected in the continued enrichment of Zn in aquatic sediments. Sediments from rural areas also contain elevated concentrations of Zn, possibly in response to substantial power plant emissions for the region, as well as vehicular traffic.
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