Trends in Hydrophobic Organic Contaminants in Lake Sediments Across the United States, 1970 to Present

U.S. environmental policy shifted greatly toward water-quality protection beginning about 1970. Conversely, changing demographics and large increases in vehicle use have occurred that could adversely effect water quality. To assess resulting changes in the quality of streams and lakes across the United States, the U.S. Geological Survey collected and analyzed sediment cores from 42 lakes (34 reservoirs and 8 natural lakes). Land use in sampled watersheds ranged from undeveloped (nine sites), to moderate urban (16 sites), to dense urban (17 sites with more than 60 percent urban). Cores were analyzed for polycyclic aromatic hydrocarbons (PAHs) and chlorinated hydrocarbons (organochlorine pesticides and polychlorinated biphenyls) and contaminant trends were tested statistically. Significant trends among the chlorinated hydrocarbons were mostly downward, and significant trends among the PAHs were mostly upward. One-half the 42 lakes, for example, had downward trends in DDE and no lake had an upward trend, whereas 22 lakes had upward trends in benzo(a)pyrene while only three were downward. Most of the downward trends in PAHs were at reference sites and no downward trends in total PAH occurred at dense urban sites. Concentrations varied greatly with intensity of urbanization. Median total PAH at the tops of cores was about 25 times greater among the dense urban sites than the reference sites. Compared to sediment quality guidelines, chlordane was the compound that most often exceeded the probable effect concentration, often by an order of magnitude at dense urban sites. These results suggest that improvements brought about by regulation of chlorinated hydrocarbons since the 1970s are being offset by increases in PAHs in response to urbanization.