Lead concentrations in the sediments of several selected lakes andreservoirs across the country have declined significantly in the lastdecade or more, but are not yet back to the baseline levels of the 1950'sand 60's, according to a new U.S. Geological Survey (USGS) report.
USGS scientists said the declines occurred despite significantincreases in both the population and the number of motor vehicles driven inthe urban drainage basins studied. Although lead concentrations in thesediments declined as much as 70 percent since the 1970's and 80's, theyremain almost twice as high as the baseline levels of the 1950's and 60's.The article, "Reservoir Sediment Cores Show U.S. Lead Declines," by EdwardCallender and Peter Van Metre, which is published in the September issue ofEnvironmental Science and Technology, shows results of the study.
"We purposely picked lakes and reservoirs that were under urbanpressure and likely to be affected by lead contamination," said RobertHirsch, USGS Chief Hydrologist. "The significant declines in lead in theseurban lakes are very encouraging. These declines are a good indicationthat the switch to unleaded gasoline in the late 1970's, coupled withenactment of the Clean Air Act, have produced a positive effect on theNation's water resources."
Previous studies by the USGS show a significant downward trend inlead concentrations in the Nation's streams. The newest sediment studiesare particularly important because lake and reservoir sediments tend to belong-term "traps" that accumulate sediment and associated heavy-metalcontaminants, such as lead. Lead in sediment accumulations can becomesources of future water pollution.
The lake and reservoir study is part of the national synthesis effortof the USGS National Water Quality Assessment Program (NAWQA), which is thefirst comprehensive, ongoing study of trends in the quality of the Nation'ssurface- and ground-water resources.
Hydrologists used gravity-type coring devices and "grab box cores" tocollect samples of the sediments at the bottom of the lakes and reservoirs.>From these samples, scientists can discern a "signature" of the quality ofthe water in the lake or reservoir and its drainage basinover time. Because lakes and reservoirs efficiently trap sediments fromrivers and streams, the accumulated sediments can provide a valuablehistorical record of what has been happening regarding the presence of leadin a particular drainage basin. Heavy metals like lead, for example, tendto accumulate in sediment. Scientists can look at the lead concentrationsin the core and determine the time frame when leaded gasoline was beingused in the basin. Looking at lead concentrations in the older depositedsediments in the core is like moving backward in time to see if there hasbeen a change in the percent of lead present in sediments deposited in morerecent times. Radiochemical dating was used to determine the timerepresented by specific points along the length of the core.
Additional sediment sampling of Lake Anne and Lake Fairfax in Reston,Virginia, and urban-suburban lakes in Minnesota, Colorado, and New Jerseyis planned to look at other contaminants and the effects of urbanization onthe chemistry of sediments in lakes and reservoirs. Significant componentsof the NAWQA Program include consistent methods for sampling and the useof complex "clean" sampling protocols that ensure thevalidity and integrityof the data.
As the Nation's largest water and earth science research andinformation agency, the USGS routinely monitors the quality and quantity ofsurface- and ground-water resources at more than 50,000 sites across thecountry in cooperation with more than 1,200 State, local and other Federalagencies. Visit the USGS at http://www.usgs.gov/ on the World Wide Web.
Materials provided by U.S. Geological Survey. Note: Content may be edited for style and length.
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