ENVIRONMENTAL POLICY ANALYSIS
AIR EMISSIONS Reservoir Sediment Cores Show U.S. Lead Declines EDWARD CALLENDER U.S. Geological Survey 432 National Center Reston, VA 20192 PETER C. VAN METRE U.S. Geological Survey 8011 Cameron Rd. Austin, TX 78754
As a result of the Clean Air Act, lead (Pb) emissions to the atmosphere have been greatly reduced since the mid-1970s. As part of its National Water Quality Assessment, the U.S. Geological Survey has been using paleolimnological techniques to assess past trends in hydrophobic contaminants. In urban-suburban environments, reservoir sediment cores show prominent peaks in Pb distributions that correlate well with the rise and fall of leaded gasoline. However, Pb concentrations in sediments are approximately double those of baseline values prior to the 1950s and 1960s. It is apparent that significant concentrations of anthropogenic Pb still exist in soils and aquatic sediments and that it will take many years to reduce these concentrations to prepollution values, even if there are no new sources of Pb pollution.
Natural sources and anthropogenic contamination contribute lead (Pb) to the environment, but the anthropogenic sources are much more important than the natural ones. Lead from anthropogenic sources is approximately 30 times the total load from natural sources, and the bulk of Pb contamination arises from atmospheric emissions (1, 2). In 1983, for instance, anthropogenic Pb sources were approximately 332 kilotons (kton) per year (yr) and natural sources about 12 kton/yr. The major concern about Pb in the environment is that it is highly toxic to humans and other animals. Because human activity contributes so much Pb to the environment, efforts to reduce anthropogenic sources should have a significant effect on levels of Pb in the environment. Environmental legislation designed to reduce anthropogenic Pb to the environment, specifically the Clean Air Act, are discussed in this article. The atmosphere has been contaminated with Pb since 4500 B.P., when smelting technology was developed in southwest Asia. There have been two periods when Pb contamination of the atmosphere has reached maximum levels. The first occurred about 2000 B.R, when the Romans mined large quantities of Pb ore (3); the second happened during the postindustrial revolution with the advent of the automobile and the use of leaded fuels (4-6). The Clean Air Act was passed in 1970 to set air quality standards and reduce emissions of various contaminants. With respect to Pb, the Clean Air Act has reduced emissions from all sources to the atmosphere by 98% since 1972. This has been accomplished primarily by eliminating Pb from gasoline and placing controls on specific industrial sources (7). As part of its role as regulator of anthropogenic inputs to the environment, EPA has conducted an exhaustive study of national air pollutant emission trends (8). In 1970, automobiles produced 182 kton/yr of Pb emissions; all other industrial sources contributed 37 kton/yr. In 1980, some eight years after u n leaded gasoline was introduced, automobiles produced 67 kton Pb/yr and all other industrial sources emitted about 8 kton/yr. By 1992, some 20 years after the ban on Pb additives in gasoline, automobiles emitted 2 kton/yr of Pb to the atmosphere, and other industrial sources contributed 3 kton/yr. Associated with the dramatic decline in Pb emissions was an equally dramatic decline in ambient Pb concentrations: 1.5 micrograms (ug) of Pb per meter (m) 3 in 1975 to 0.07 ug Pb/m 3 in 1990 (9). EPA reports (7) that since 1978, the average bloodlead levels in children have declined by 75%. These improvements have occurred even though the economy has grown by 90% since 1970, the population increased by 27%, a n d t h e n u m b e r of motor-
4 2 4 A • VOL. 31, NO. 9, 1997/ ENVIRONMENTAL SCIENCE & TECHNOLOGY/ NEWS
0013-936X/97/0931-424A$14.00/0 © 1997 American Chemical Society
FIGURE 1
Sites of intensive U.S. aquatic lead studies The yellow areas represent U.S. Geological Survey National Water Quality Assessment study basins. Intensive studies of lead levels in sediments were made in four of these areas (red), including the enlarged section of Georgia depicted at right.
vehicle miles driven has increased by 110%. Because motor-vehicle emissions were the major source of Pb contamination in the atmosphere, and because wet and dry deposition deposited much of this Pb on the land surface, an immediate source of Pb pollution has been greatly reduced. The Pb pollution "recovery phase" that is a result of significant reductions in Pb emissions to the environment in North America is described here in detail. Although many studies of metal distributions in sediment cores from natural lakes were conducted during the era of acid rain research (1975-85), only the "pollution phase," or the rise in Pb levels caused by atmospheric pollution was documented beginning in the 1940s. Very few of these studies were done at a time (post-1970s) or at sites (lakes with moderate sedimentation rates) where the results of controlling Pb emissions could be recorded in aquatic sediments. We have analyzed sediment cores from several reservoirs throughout the Midwest and Southeast (Figure 1). The data presented here are part of a large study by the U.S. Geological Survey's National Water Quality Assessment Program (NAWQA) to define historical trends in our nation's water quality, using dated sediment cores. Our data show that the recovery phase began in the late 1970s and continues until the present. However, current Pb levels in aquatic sediments from urban watersheds are approximately twice the levels in sediments deposited before the pollution phase began, a result suggesting that even today there is a slow addition of anthropogenic Pb from watershed soils. Reservoir sediment cores as environmental archives Marine and lacustrine sediments often record natural and anthropogenic events that occur in drainage basins or are forced upon the aquatic system (10).
A serious limitation for most sediment cores from natural lakes is that the sedimentation rate is generally too slow to provide the proper time resolution needed to discern modern pollution trends (3050 years). Sedimentation rates for most natural lakes vary from 0.05 to 10 millimeters (mm)/yr. Manmade reservoirs and river impoundments, however, exhibit sedimentation rates that range from 10 to 200 mm/yr. Therefore, a significant quantity of sediment is deposited annually in reservoirs, and sediment cores can be sampled easily to provide this annual record. Ideally, if the pollution problem is modern, a yearly record is desirable to identify trends. For reservoirs to be a good medium for detecting trends in anthropogenic hydrophobic constituents, several conditions need to be met. First, there should be continuous deposition at the sampling site over the life of the reservoir. Reservoirs are generally divided into three zones: the riverine zone, which is most influenced by the river and has complex sedimentation with periodic resuspension; the lacustrine zone, which is characterized by slower, more constant sedimentation consisting predominantly of finegrained material; and the transition zone, which is between the riverine and the lacustrine zones. Sediment cores usually are collected from the lacustrine zone, where the continuous-deposition criterion generally is met and where the prereservoir surface can be reached by general sampling methods. The second condition is that deposited sediments should show no evidence of chemical diagenesis, that is, mobilization of chemical constituents after deposition of sediments. As demonstrated in previous geochemical investigations of reservoirs in South Dakota and Colorado (11,12), rapidly deposited sediments retain their initial chemical "signal" because of the lack of time (caused by rapid burial) needed to establish new equilibrium conditions. VOL.31, NO. 9, 1997/ENVIRONMENTAL SCIENCE & TECHNOLOGY/NEWS " 4 2 5 A
FIGURE 2
nological interpretation of natural and anthropogenic change within the drainage basin.
Lead distributions in urban-suburban and rural sediments
What do reservoir sediment cores tell us?
Anthropogenic Pb concentrations (|ig/g) in reservoir sediment cores represent the difference between the peak value and the baseline value before 1960.
TABLE 1
Peak levels of lead accumulation in selected reservoir sediments from the Midwest and the Southeast Accumulation rates (ug/cm2/yr) Peak corrected for focusing Year(s)
Lake
Location
Setting
Peak
White Rock Coralville Harding Blackshear Sid. Lanier Anne
Dallas, Tex. E.Iowa Atlanta, Ga. S.E.Georgia N.Georgia Reston, Va.
Urban Agricultural Urban Agricultural Rural Suburban
63 27 12.3 2.2 200 32 14 4.5 6.5 1.3 27 5.5
1977 1980-90 1970-75 1972-84 1963-73 1980
The third condition is that the chemical quality of reservoir bottom sediments should be related to the water quality of the influent river(s). Essentially, reservoirs are flood-control and sedimentcatchment basins. Reservoirs that reflect two primary water quality problems—urban and suburban development and agricultural practices—were chosen for this study. Some reservoirs record both inputs, because an urban area resides near the head of a basin that is predominantly agricultural. As a result of their large drainage-area-to-surface-area ratio of recovery and rapid sedimentation rates, reservoir sediments are dominated byfluvialinputs; and direct atmospheric inputs may be substantially diluted by the fluvial signal. However, most of the original inputs of Pb occurred through the atmospheric route. Reservoirs integrate the dynamic fluvial signal in a way that allows for the systematic, chro4 2 6 A • VOL. 31, NO. 9, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
The details of sediment coring techniques—site selection and core sampling—and chemical analyses are presented elsewhere {13). The Pb concentrations reported here represent the total Pb obtained using microwave digestion with nitric and hydrofluoric acids. Cores were dated using a combination of 137Cs dating (a byproduct of nuclear weapons testing) and the depth of the prereservoir surface defining the date of impoundment. Four dates can be identified in cores from reservoirs constructed before 1952. These are calibrated using the reservoir impoundment date, the first occurrence of detectable 137 Cs in 1952, the peak 137Cs activity in 1963, and the date of core collection. The lacustrine sediments in cores from all six reservoirs are characterized by uniform, fine-grained sediments, and there is no visible evidence of bioturbation in any of the cores. All cores contained >99% silt- and clay-sized particles, and the median clay-particle content varied from 73% to 87%. Because these sediments are relatively homogeneous, Pb concentrations were not normalized before interpretation of trends. (If the chemical composition of major rock-forming minerals, e.g., aluminum, varies considerably, trace-metal concentrations would be divided by aluminum concentrations [normalized] in order to compensate for variations in geologic source.) Figure 2 shows the distribution of sedimentary Pb concentrations versus age for cores from urban and suburban reservoirs: White Rock Lake in Dallas, Tex.; Lake Harding, downstream from Atlanta; Lake Anne in Reston, Va., west of Washington, D.C.; and one rural reservoir, Lake Sidney Lanier, northwest of Atlanta. For the three urban and suburban sites, there was a steep rise in Pb concentration beginning about 1950 for Lake Harding, about 1960 for White Rock Lake, and 1963 for Lake Anne. Also, there is an equally steep decline in Pb concentrations after the peak Pb value in 1972 (Lake Harding), 1977 (White Rock Lake), and 1980 (Lake Anne). These dates are based on the assumption that sediment rates were constant from 1963 (major 137Cs peak) to the 1990s, the time the cores were collected. Finally, current Pb concentrations for all three reservoir sediments are approximately double the value of "pre-urbanization" (White Rock Lake and Lake Harding) or early reservoir (Lake Anne) concentrations. The origin of the earlier, somewhat smaller, Pb peak (about 1960) in the Lake Harding core probably relates to local anthropogenic activities in the Atlanta metropolitan area that may predate the more ubiquitous Pb pollution exemplified by the other urban and suburban cores. Figure 2 also shows the distribution of sedimentary Pb concentrations versus age for a core from rural Georgia. Lake Sidney Lanier is located in northwestern Georgia, about 90 kilometers (km) north of Atlanta, and drains a rural watershed. The time trend of this sedimentary Pb concentration is somewhat different in shape from the urban and suburban distributions. Although the sedimentary Pb data for Lake Sidney Lanier do form a symmetrical peak, it is broad
in shape. We also have data for reservoirs draining agricultural watersheds in eastern Iowa (Coralville Lake) and southeastern Georgia (Lake Blackshear). Neither of these distributions has the symmetrical increase and decrease in Pb concentrations that bracket the maximum values. In fact, Pb data for Coralville Lake, which has a high sedimentation rate (4.7 centimeters (cm)/yr), present no peak at all. Present-day Pb concentrations for all three agricultural and rural reservoir sediments are approximately equal to sedimentary values deposited during early stages of reservoir development before 1960. The sedimentary Pb distributions at urban, suburban, and rural reservoir sites strongly suggest that the significant reduction in gasoline Pb emissions to the environment has resulted in a recovery phase that mimics the time distribution of Pb in the atmosphere (2, 8). U.S. leaded gasoline consumption peaked in 1972, whereas more local urban sources peaked at different times in the 1970s. In the mid1970s, Pb concentrations in ambient air were about 1.5 ug/m3; by 1990, the concentrations were only 0.07 ug/m 3 (9). The decline in sedimentary Pb concentrations from urban and suburban reservoirs correlates well with this decline in atmospheric Pb (Figure 3). These reservoirs occupy watersheds that drain substantial areas of impervious landscape (streets, highways, parking lots) and contaminated soil. Reservoirs draining agricultural and rural watersheds have lower peak concentrations and a broader shaped peak because they receive sediment that contains smaller amounts of contaminated airborne particulates that mix with the ambient, uncontaminated bedrock and soil. Thus, sedimentary Pb distributions from these reservoirs tend to be more irregular and lower in anthropogenic concentration.
Origin of lead distribution Why are sedimentary Pb distributions so different for the urban and suburban versus rural reservoir sites? Several environmental factors govern these distributions. The most important is proximity to major sources of Pb contamination. In the Northern Hemisphere during the time of maximum tetraethyl Pb use, atmospheric Pb concentrations varied between 2.3 and 3.3 ug/m 3 for urban sites and averaged about 0.5 pg/m3 for rural sites {14). All three urban and suburban reservoirs occupy watersheds that receive major anthropogenic inputs. For Pb, these inputs were primarily atmospheric in origin and a direct result of automobile emissions from the 1960s to the 1980s. Urban and suburban areas have the highest concentration of automobiles and thus the greatest amount of automobile emissions to the local atmosphere, and city size has a direct bearing on the amount of Pb in urban soils [15, 16). Table 1 presents anthropogenic sedimentary Pb accumulation rates for the reservoirs discussed above. Sedimentary Pb accumulation rates are calculated from dry density estimates (gram (g)/cm3), linear sedimentation rate data (cm/yr), and the Pb concentration of each core interval. The anthropogenic Pb accumulation rates are derived by subtracting baseline Pb values (sedimentary concentrations before the 1960s) from the total Pb concentrations at peak value (Figure 2).
FIGURE 3
Lead in southeastern and southwestern urban-suburban reservoir cores The atmospheric Pb source function tor the upper Midwest is depicted in red [3S\. This source function represents atmospheric Pb emissions for urbanized areas. Such emissions have been recorded in reservoir cores.
TABLE 2
Deposition of atmospheric lead (Pb) in the terrestrial environment Pb deposition rate (pg/cm'/yr) Environmental setting
Years
Mean
Standard deviation
Urban
1965-75
14.5
±11.1
Suburban Rural
1966-73 1966-75
1.9 0.7
±1.6 ±0.4
References 14,18-28 20,21,25,28-38 20,36,39^15
Corrected peak anthropogenic Pb accumulation rates are derived by dividing the peak anthropogenic Pb accumulation rate by the focusing factor (FF). FF is computed by dividing the total 137Cs activity in a sediment core by the amount of atmospheric 137 Cs delivered to the reservoir water surface {17). In reservoirs, FF is a measure of the quantity of sediment and associated contaminants that are contributed to the reservoir by the drainage basin as opposed to direct fallout on the reservoir water surface. For reservoir lakes with large drainage-area-to-surfacearea ratios (100 to 500) {17), considerable amounts of "contaminants" such as 137Cs and anthropogenic Pb may be contributed from tributary sources. Table 2 presents deposition rates of atmospheric Pb to the terrestrial environment. The data used to construct Table 2 {14-45) were carefully screened from the published literature for the period 1979-87. For the most part, data from recognized experts in trace metal sampling and analyses were used {14,19,20,24,25,29,36,38,40-42). All other data used to construct the mean deposition rates had to fall within the range of the recognized data. VOL.31, NO. 9, 1997 /ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 4 2 7 A
A comparison of the two tables shows that urban reservoirs have corrected peak anthropogenic Pb accumulation rates that are similar to or somewhat greater than atmospheric Pb deposition rates for urban areas. The timing for these peaks (mid-1970s, Table 1) corresponds to the period of maximum Pb levels in air from urban areas (1965-75) (14,25, 46). The Lake Anne value is substantially lower than those of the two urban sites, but its location is about 55 km southwest of Washington, D.C. Suburban areas always have lower Pb concentration levels because of the lower density of emission sources (automobiles) and the smaller percentage of impervious watershed surface from which Pb-contaminated particulates wash rapidly into local water courses. Another important factor in determining anthropogenic Pb distributions in reservoirs may be sedimentation rate. Figure 2 shows that whereas Lake Anne and Lake Sidney Lanier have identical linear sedimentation rates, they have very different anthropogenic Pb concentrations. Anthropogenic Pb and sedimentation rate data from agricultural reservoirs, Corah/ille Lake (7 ug/g, 4.7 cm/yr) and Lake Blackshear (26 ug/g, 1.3 cm/yr), indicate an inverse relationship between sedimentation rate and anthropogenic Pb concentrations. This is because a relatively small anthropogenic atmospheric Pb signal is diluted by a large natural source of sediment containing baseline levels of Pb. For urban and suburban reservoir sediment cores, there is no apparent relationship between linear sedimentation rate and anthropogenic Pb concentration (Figure 2). For these settings, proximity to source is the most important factor. Positive effects of the Clean Air Act There has been a noticeable improvement in our natural environment over the past 20 years. EPA (7), in its 25th Anniversary Report, notes that since 1985 many of the nation's largest cities experienced a 75% reduction in the number of days when the air was considered unhealthy. During die same period, about 60% of the nation's surveyed rivers, lakes, and estuaries were clean enough for basic uses such as fishing and swimming. With respect to Pb, emissions from automobile and industrial sources have declined sharply. These improvements (and more) have been realized even as the economy grew. The findings of this study show that aquatic Pb concentration levels, derived mostly from atmospheric deposition, peaked in the 1970s and declined precipitously thereafter in response to emissions reductions. This is particularly important for the urban and suburban environments where the majority of the U.S. population resides. Other studies of the remote marine environment and ice caps indicate that there has been a concomitant decline in global Pb concentration levels. Concentrations of Pb in corals from the western North Atlantic (47), Pb in surface waters near Bermuda (48), and Pb in central Greenland ice (48) have declined since the late 1970s. It is apparent that significant concentration levels of anthropogenic Pb still exist in soils (15,16) and aquatic sediments and that it will take many years to reduce these concentrations to prepollution values if there are no new sources of Pb pollution. This is because the Pb in soils is moretightlybound (less available to plants) 4 2 8 A • VOL. 31, NO. 9, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
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