By Janet Raloff
A new study links the traffic associated with urban sprawl to an unexpectedly large rain of air pollutants entering local waters. Scientists with the U.S. Geological Survey studied carcinogens known as polycyclic aromatic hydrocarbons (PAHs) that had collected in reservoirs in six states. Neighborhoods around some of the 10 sites in the study were long-established and compact; others, young and sprawling. By dating sediment from several depths, the scientists could figure out when PAHs had been deposited in the reservoirs.
In most U.S. waters, PAHs peaked by the 1970s, then waned for a while. The new data show that by the 1980s, PAHs deposition was again climbing. For reservoirs in some of the watersheds studied, however, PAHs never peaked and continue to climb—even though airborne concentrations in these regions probably fell during the recent past.