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E-mail:

P.C. Van Metre, USGS, Austin, Texas
pcvanmet@usgs.gov

Getting the Lead Out - Positive Effects of the Clean Air Act

Lead concentrations have declined significantly in several urban lakes and reservoirs across the country, an encouraging sign that the switch to unleaded gasoline and the Clean Air Act of 1990 have produced positive effects on the Nation's water resources.

Lead concentrations declined despite significant increases in both population and the number of motor vehicles during the same period. Lead concentrations in water sediments declined as much as 70 percent since the 1970's and 1980's. While this is good news, lead concentrations still remain almost twice as high as the baseline levels of the 1950's and 1960's. Clearly, large amounts of lead still exist in soils and in lake sediments. It will take many years to reduce these concentrations to the levels experienced before the rise of leaded gasoline, even if there are no new sources of lead introduced into the environment.

USGS hydrologists collected core samples of lake-bottom sediment from lakes and reservoirs in four areas of the country, representing urban and suburban lakes and reservoirs that drain agricultural watersheds. They used gravity-type coring devices and grab baskets to collect samples of the lake-bottom sediments. In the same way that tree rings can provide insight to historical climatic conditions, the chemistry of slices of the sediment contained in these cores can provide historical water-quality information.

Because lakes and reservoirs efficiently trap sediments and heavy-metal concentrations, such as lead, from rivers and streams, the accumulated sediments can provide a valuable historical record of lead in the lake's drainage basin. Accumulated lead in sediments can become a source of future water pollution if the sediments are disturbed, reintroduced into the water column, and transported elsewhere in the watershed.

Scientists can look at the lead concentrations recorded in the core and determine whether or not there has been a decrease. The decrease can be seen in the more recent sediment deposits in the upper part of the core compared to the baseline concentrations in the oldest sediments in the bottom of the core.

Radiochemical dating is used to determine the time represented by specific points along the length of the core. The "dated" historical record of the sediment cores contains a distinct "signature" of the water quality in the lake and its drainage basin over time. Analysis of this signature can determine the time frame when leaded gasoline was being used in the basin. By tracing the history of the lead concentrations in the core, USGS hydrologists can tell if there has been a change in the percentage of lead present in sediments deposited in more recent times, as compared to the earlier sediments.

The lake study is part of the USGS National Water Quality Assessment Program, which is the first comprehensive, ongoing study of trends in the quality of the Nation's surface- and ground-water resources. Looking at trends can improve understanding of the influence of human activities on water-quality conditions. Trends can also indicate the effectiveness of environmental regulations, like the Clean Air Act. Another important function of information on water-quality trends is to signal a warning of additional degradation of water quality in the future, for which additional study or action might be taken.

Lead (Pb) distributions in urban and suburban sediments and peak levels of lead accumulation in selected reservoir sediments. Note recent declines due to switch to unleaded gas.

Where the Rubber Meets the Road --
A Source for Zinc in the Environment?

Zinc is a heavy metal that can be toxic. An intriguing finding that came out of the USGS reservoir coring study was the perplexing pattern of the zinc curve. While lead showed a reassuring and rather dramatic drop in its presence in the environment, strongly correlated to the removal of lead from gasoline, the picture for zinc is quite different.

Take, for example, White Rock Lake, a 2-square-mile reservoir in Dallas, Texas, with a drainage area of 100 square miles. The watershed was mostly agricultural before 1950 or so. Since then, it has undergone urban development, which now dominates land use. Sediment is washed off the land surface into streams during rain storms. Much of it is deposited in White Rock Lake. Over time, sediments have accumulated to form a thick layer on the bottom of the lake more than 8 feet thick in the deepest part of the lake.

Cores from these sediments and other similar reservoir bodies show that, in general, lead and zinc respond similarly to environmental disturbances. Data from White Rock Lake and other suburban-urban reservoir sediments, however, show that while lead declines in modern sediments, zinc either increases or stays elevated in concentration to the present time. The zinc anomaly suggests a continued, as yet unknown, contaminant source of zinc to the environment.

What might be the source for the zinc? One hypothesis that USGS will test is whether rubber tires are the source. The simple fact is that automobile use has outstripped population

growth by more than four times. Manufactured rubber contains zinc, which means that each time the tire runs over the road, it leaves a residue of zinc that can run off into streams and enter ground-water systems.

Distribution of sedimentary lead (Pb) and sedimentary zinc (Zn) in a sediment core from White Rock Lake, Dallas, Texas.