NATIONAL-SCALE TRENDS IN METALS AND HYDROPHOBIC ORGANIC CONTAMINANTS IN URBAN AND REFERENCE LAKE SEDIMENTS, 1970 TO 2001

Major shifts in environmental policy, urban land use, population distribution, and vehicle use have occurred in the United States over the past three decades. To assess the effects of these environmental changes on the water quality of streams and lakes, the U.S. Geological Survey collected and analyzed sediment cores for historical trends in pollutants from 42 lakes and reservoirs across the United States. Watershed land use for the sampled lakes ranged from undeveloped forest or grassland to dense urban. Core samples were analyzed for major and trace elements, polycyclic aromatic hydrocarbons (PAHs), and chlorinated hydrocarbons (organochlorine pesticides and polychlorinated biphenyls), and contaminant trends from the core profiles were tested for statistical significance from 1970 to the tops of the cores (1996–2001). Statistically-significant trends were mostly downward (decreasing concentrations over time) for Cd, Cr, Ni, Pb, PCBs, and total DDT. Approximately equal numbers of upward and downward trends occurred for Cu, Hg, and chlordane. Significant trends in zinc and PAHs were mostly upward, especially in urban areas. Urban enrichment for metals, as indicated by the ratio of median concentration in the 1990s in dense urban lakes to reference lakes, ranged from a factor of 2.0 (Cr) to 5.4 (Pb). Enrichment for organic compounds ranged from 2.6 for total DDT to greater than 20 for chlordane and PCBs. PAHs were twice as likely to exceed the probable effect concentrations (PEC) from consensus-based sediment quality guidelines in sediments deposited in the 1990s as they were in sediments deposited from 1965 to 1975, whereas DDT and PCBs were half as likely. The combination of upward trends in zinc and PAH concentrations coupled with the strong association of both with urban settings, suggests these pollutants could eventually surpass lead and the chlorinated hydrocarbons in potential toxicity to aquatic biota in urban streams and lakes.