A GIS INVESTIGATION OF URBANIZATION AND TRACE METAL VARIABILITY IN STREAM SEDIMENTS FROM THE ATLANTA METROPOLITAN REGION

Trace metal inputs to aquatic systems result from a large range of point and non-point sources, many of which are centered in urban environments. Consequently, the land use characteristics of a watershed are determining factors in the types and concentrations of metals present. Adsorbed trace metal concentrations (V, Cr, Ni, Cu, Zn, Pb, Mn) were measured in suspended stream sediments from several watersheds of varying urban land use (0.4-59.7 % urban), within the Atlanta metropolitan region, in order to assess relations to different measures of urbanization, including impervious cover, population density, and road density. Watersheds were delineated with GIS using digital elevation models from the USGS National Elevation Dataset. Percentage of impervious surface cover in each basin was estimated using 1992 Landsat thematic mapper imagery from the USGS-EPA National Land Cover Dataset. Additionally, population and road density were assessed to examine their potential as surrogates for urban land usage.

A Student’s T-test at the 95% confidence level indicated that adsorbed Zn, Pb, Cu, and Cr concentrations were significantly elevated within the urban basin compared to less-developed watersheds. When plotted against the percentage of urbanization in the basin only Pb and Zn were well correlated with urban land use (R2=0.84 and 0.86, respectively). Land use within a 30 m zone on each side of the stream was also evaluated whereby correlations with urban land use were improved for Pb (R2=0.98), Zn (R2=0.97), and Cu (R2=0.87). Plots of metal concentration against population density yielded similar results for Pb (R2=0.97) and Zn (R2=0.88). These findings are consistent with other studies, which have documented that Zn, Pb, and Cu to a lesser extent, are prevalent in urban settings, likely derived from automobile pollution. Zinc and copper are deposited onto roadways by the corrosion of frictional components in automobiles, while lead still persists in urban soils due to previous uses in gasoline. Cu was not well correlated with road density, while Zn was better related to road density within the watershed (R2=0.82) than within the buffer zone only (R2=0.79). Pb was similarly well related to road density in the entire watershed (R2=0.93) and within the buffer zone (R2=0.93).