Figure 5. Correlation of concentrations of different metals to concentration of iron in suspended sediments indicates which samples might have an anthropogenic contribution. A linear relation between iron and the other metals is shown by a dashed line. For most metals, spring and surface-water sediments plot on that line, indicating that the concentration of the individual metals is proportional to that of iron, and thus probably reflects the natural geochemistry of the sediments. A few outliers plot above the line, suggesting an additional, anthropogenic source of the metal for those samples. In contrast, for lead and zinc, two metals with a strong urban source signal, most samples from the two urbanized surface-water sites (Barton Above and Williamson) plot in a cluster above the line, as indicated by the dashed ovals, while suspended sediments from the springs and Barton 71 plot on the line.