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.
In such areas, pollution due to local urbanization seems to have overwhelmed regionwide declines in air pollution, says Edward T. Furlong of the agency’s lab in Lakewood, Colo. With Peter C. Van Metre and Barbara J. Mahler of USGS in Austin, Texas, Furlong has also uncovered a new trend in the mix of PAHs. Since the 1970s, the researchers find, the ones that predominate have changed from types indicative of fuel spills and seeps to those more typical of auto exhaust, tires, soot, and road asphalt.
Sprawl with its associated traffic appears to foster deposition of PAHs. For instance, the USGS team found that development around one site corresponded to a doubling in PAH accumulation, although the population there had increased only 5 percent. This suggests that the amount people drive can play a bigger role in PAHs pollution than population growth does, the scientists note in the Oct. 1 Environmental Science and Technology. They also found signs that commuters from outside an area contribute greatly to increasing PAHs in local waters.
The USGS researchers report that at least 6 of the 10 reservoirs were polluted with PAHs exceeding Canada’s estimates of the sediment concentrations expected to poison aquatic life.
The Environmental Protection Agency plans to define such thresholds some time next year.
The new study found concentrations of three PAHs in one reservoir that exceeded the Canadian thresholds 12- to 24-fold. These data not only indicate that sprawl can increase PAHs pollution in local waters, the scientists conclude, but also that it already poses “an ecological concern.”
