Environ. Sci. Technol. 1997, 31, 2193-2197
200-Year Record of Metals in Lake Sediments and Natural Background Concentrations H . R . V O N G U N T E N , * ,† M . S T U R M , ‡ A N D R . N . M O S E R §,| Paul Scherrer Institut, PSI, CH-5232 Villigen PSI, Switzerland, and Swiss Federal Institute for Water Research, EAWAG, CH-8600 Du ¨ bendorf, Switzerland, and Laboratorium fu ¨r Radiochemie, Universita¨t Bern, CH-3000 Bern-9, Switzerland
Lake sediments conserve important information about past conditions of a lake and its environments. We present analyses of calcium, manganese, iron, copper, zinc, cadmium, mercury, and lead in dated (210Pb, 137Cs, varve-chronology) sediment cores from Zu¨ richsee (Lake Zurich), Switzerland, covering a time span of the last 200 years and an additional time interval between 13 500 and 15 000 years before the present time. The concentrations of these elements varied very little in pre-anthropogenic sediments and represent geochemical background concentrations. With the beginning of industrialization in the early 19th century and the corresponding growth of population, significant changes occurred in the concentrations of the investigated metals. Copper, zinc, and cadmium increased until about 1960 and decreased sharply afterwards. The increase correlates to the growth of local industrial productivity (and population growth), whereas the decrease after about 1960 was caused by (i) the introduction of sewage treatment plants, (ii) more stringent legal restrictions for releases of pollution to the environment, and (iii) a growing public awareness toward environmental conservation. Contrary to world-wide observations, the increase of lead by automobile exhausts was not evident in the sediments of Zu¨ richsee. Here, its concentrations remained almost constant between 1900 and 1975. Then they began to decrease due to the reduced use of leaded fuel. Thus, the lead input to the lake by automobile exhausts was nearly compensated by (i) continuously decreasing industrial releases to the environment and (ii) the mentioned improvements of environmental conservation techniques. Despite the considerable improvements in recent times, the present-day concentrations of the investigated heavy elements in sediments of Zu¨ richsee are still much higher than their natural background values.
Introduction Lake sediments conserve valuable historic information on past conditions of lakes and their environments. Dated sediment samples thus allow studies of past natural and anthropogenic environmental conditions and changes. Sedimenting particles that reach the lake floor are continuously * Author for correspondence: phone: +41 56 310 2407; fax: +41 56 310 21 99; e-mail:
[email protected]. † Paul Scherrer Institut. ‡ Swiss Federal Institute for Water Research. § Universita ¨ t Bern. | Present address: Oberstrasse 29, CH-3550 Langnau, Switzerland.
S0013-936X(96)00616-5 CCC: $14.00
1997 American Chemical Society
covered by succeeding sediment layers. Therefore, the sediments form, after a relatively short time, a ‘closed system’ that no longer exchanges with components of the free water column. When closed system conditions are reached, each sediment layer represents an archive for environmental conditions corresponding to a certain period in the past. Zu ¨richsee is well suited for investigations of anthropogenic changes, since it receives major inputs from agriculture, industrial, and domestic discharges and from atmospheric fallout. The allochtonous flux is small because the particle load of the main inflowing River Linth is mostly deposited upstream in Walensee and in the upper part of Zu¨richsee (Obersee), which today is separated by a dam from the lower part (Untersee; Figure 1). Major changes in the anthropogenic behavior occurred at the beginning of the 19th century when industries began to develop and expand. The early local industrialization in the Zu ¨ richsee region was dominated by a rapidly growing textile industry and their chemical and mechanical suppliers. These developments induced in this region the first major manmade contamination of the environment. Among the pollutants were heavy metals (for instance, copper, zinc, cadmium, lead) that resulted from the expanding industrial activities and the related population growth. Polluting heavy metals play an important role because they are poisonous at trace concentrations already and are indicators for human activities of a certain time period. In the following, we present metal data that were obtained from dated sediment cores of Zu ¨ richsee and relate these results to early and later local industrial activities and to geochemical background concentrations.
Experimental Section A 200 m long sediment core (denoted in the following as the long core) was recovered in 1980 with piston-and-punch corers at the deepest part (136 m depth) in the middle of Zu ¨ richsee (1, Figure 1). The core sampled 152 m of unconsolidated sediments and penetrated 49 m into the Molasses bedrock (1, 2). In the present work, we analyzed only the upper ∼25% of the core to a sediment depth of ∼ 40 m. At the same location three short cores (lengths ∼0.7 m) were recovered in 1978 and 1990 (3) with a gravity corer. Based on varve-chronology (anomal layers) in the sediments of the long core, Zhao et al. (4) established early sedimentation rates and corresponding sediment ages. The varves resulted from annual freezing and thawing of Zu ¨richsee during and at the end of the last ice age. Each varve consists of a coarser lamina that is topped by a layer of clay-sized particles that settled during winter when turbulence in the lake was reduced by the ice sheet at the water surface. Zhao et al. (4) assigned an annual sedimentation rate of about 3 mm up to a core depth of 7 m. Correcting for the increasing sediment densities with depth, they arrived at a sediment age of about 13 500 years at 7 m core depth. At about 12 m core depth, the sedimentation rate increased drastically (∼60 mm/year) due to a more excessive material transport by rivers as result of the accelerated melting of glaciers at the end of the last glaciation. The age of the samples at 40 m depth is estimated to be about 15 000 years (4). The short cores were dissected (see Table 1) and were dated with 210Pb (5-7). 210Pb was measured by R-spectroscopy of 210Po in radioactive equilibrium with 210Pb (8) and with the 47-keV γ-ray line of 210Pb (9). In addition, two 137Cs activity peaks indicated the years 1963 (nuclear tests in the atmosphere) and 1986 (Chernobyl reactor accident; 10). Based on these methods, the sedimentation rate for the short cores was calculated to be about 2.9 mm/year during the last 100
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FIGURE 1. (Top) Map of Switzerland with insert. (Bottom) Expanded view of the insert showing Zu1 richsee and environments. Also indicated is the coring position (sampling) and the location of the chemical plant. years, which is in agreement with the estimated sedimentation rate for the long core (up to 7 m depth). The 0.3-g aliquots of the dissected core samples were dissolved in HNO3/HClO4/HF. Results of the analyzed aliquots between the sediment surface and a depth of 52.5 cm are listed in Table 1. Between 7.8 m and 40 m sediment depth (long core), 18 samples were analyzed (Table 1) in order to obtain values for the geochemical background concentrations. The concentrations of Ca, Mn, Fe, Cu, Zn, Cd, Hg, and Pb were measured by AES and/or ICP-OES (3). The cores of 1978 and 1980 were analyzed at EAWAG, that of 1990 was analyzed at PSI. No systematic differences are observed between the analyses of the two laboratories.
Results and Discussion In Figure 1, we present a schematic map of Zu ¨ richsee and its environments and indicate the approximate location of the sediment sampling point. Also shown is the site of a major chemical plant. This plant since 1818 was the supplier of the local textile industry that dominated the industrial development in this region. Thus, the chemical plant has directly and indirectly contributed to the metal input to Zu ¨ richsee. The measured concentrations of the analyzed metals in the short cores of Zu ¨ richsee sediments are shown in Table 1 and in Figures 2-4. In Table 1, we present the data for the heavy metals copper, zinc, cadmium, mercury, and lead together with sample
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depths and allocated sample ages. The uncertainties in the ages are estimated from the age determinations of three different cores. The errors in the ages are smallest in the vicinity of the well-defined 137Cs radioactivity peaks (Chernobyl accident and fallout of nuclear tests in the atmosphere). Dating with 210Pb (half-life 22 years) is only applicable to about 1880; however, with an increasing uncertainty with depth. Older sediment ages are estimated under the assumption of a constant sedimentation rate of about 3 mm/year. This approximate rate may have been influenced by human developments in the lake basin. The metal results of Table 1 allow a direct comparison of the concentrations in different sediment cores at three depths (4.5, 7.4, and 10.7 cm) and show a considerable variability. The variability results mainly from local inhomogeneities in the sediments, from disturbances during the coring operation, and to some extent from the uncertainties of the sample ages. However, in the present investigation changes and trends in temporal concentration are more important than the absolute values. The observed temporal changes are generally much larger than the variability of the double determinations and show increases and decreases in the metal concentrations very clearly. For the double determinations, mean values are plotted in the figures. In Figure 2, we present the measured concentrations of calcium and of the redox-sensitive elements iron and manganese versus sample age. Between 1800 and 1960, the
TABLE 1. Results of Investigated Sediment Cores from Zu1 richsee sediment depth (cm)
calendar year (error, y)
0.3 0.6 0.9 1.5 1.8 2.4 3.6 3.9 4.5 5.1 5.4 5.7 6.2 6.5 6.8 7.4 8.2 8.6 9.5 10.1 10.7 11.5 11.8 13.1 15.0 15.3 17.5 19.5 20.3 20.6 22.3 24.5 27.5 28.0 29.7 32.5 40.5 47.0 52.5 780-4000
1989 ((2) 1988 ((2) 1987 ((1)a 1985 ((1)a 1984 ((2) 1982 ((2) 1978 ((2) 1977 ((2) 1975 ((2) 1973 ((2) 1972 ((2) 1971 ((2) 1969 ((2) 1968 ((2) 1967 ((2) 1965 ((2) 1962 ((1)b 1960 ((1)b 1957 ((2) 1955 ((2) 1953 ((3) 1950 ((3) 1949 ((3) 1945 ((4) 1936 ((4) 1935 ((5) 1930 ((5) 1925 ((6) 1921 ((6) 1920 ((7) 1918 ((7) 1910 ((8) 1902 ((8) 1900 ((8) 1896 ((9) 1886 ((10) 1839 ((15) 1819 ((20) 1801 ((20) 13500 BPc 15000 BPc
Cu (µg/g) core 1990 core 1978 48 45 42 47 38 44 49 46
Zn (µg/g) core 1990 core 1978 224 235 182 218 197 258 259
37 35 37
46
273
51 61 78
232 275 300
102
375 475
52 65*
675 587
67
300 450*
56 55
47
350* 235 170
106
150* 135 77
6.25 13.4
3.2 4.0
0.7
6.5 7.7*
0.7
10.9
6.2* 4.0 2.4
1.0 0.6
4.5 4.7
142 130 105* 138
202* 165 150 150* 117* 82* 60* 55* 55* 50* 84 ( 12* (n ) 18)d
0.4
ndc
120
205
0.3
5.4
111 375 202 167*
1.7 2.1 2.5
19.1
134 250
57* 62 67 65* 77* 57* 32* 27* 20* 22* 23 ( 1.5* (n ) 18)d
107 117* 125
270 55 ndc 47*
2.5
2.6
116
347
53
144 104
Hg (µg/g) core 1978
2.4
116
375 429
97 112 105 130 137
476 65* 52 65
Cd (µg/g) core 1990 core 1978 1.1 1.0 0.75 1.0 0.8 1.0 1.5
109
398
67 56
69 68 57 72 62 86 108
308 55 55
Pb (µg/g) core 1990 core 1978
4.5 2.0 1.2*
0.7 0.5
1.3 115* 122 112 112* 112* 115* 17* 40* 12* 10* 19 ( 5* (n ) 18)d
1.7* 1.0 1.0 1.0* 1.0* 0.5* 0.2* 0.2* 0.2* 0.2* ndc
0.6 0.5 0.7 0.4
a Based on 137Cs peak from Chernobyl accident. b Based on 137Cs peak from bomb fallout. c Abbreviations: nd, not determined; BP, before the present; *, data from long sediment core, see text. d Number of samples between 7.8 and 40 m sediment depth.
FIGURE 2. Concentrations (mg/g of sediment) of calcium (9), manganese (2), and iron (b) in dated sediment samples from Zu1 richsee. Analytical errors (5% (1σ). concentrations of Ca increased in the lake sediments by more than a factor of 2. This increase is the result of an increasing eutrophication of Zu¨richsee due to regional population growth and human activities in the catchment area (e.g., by sewage and extensive use of fertilizers). These nutrients enhanced bioproductivity in the lake and, herewith, related biogeochemical processes, like calcite formation and precipita-
tion. After 1960, the condition of Zu ¨ richsee has improved considerably. The improvement resulted mainly from the operation of sewage treatment plants and has led to a small decrease in the Ca concentrations. However, despite this improvement, its present mean concentration of 247.8 ( 18.9 mg/g of sediment (12 samples) is still much higher than the Ca concentration at the beginning of the 19th century (107 mg/g of sediment) and the mean natural concentration for Ca of 131.8 ( 18.5 mg/g of sediment, as determined from 18 samples at depths between 7 and 40 m (age 13 500-15 000 years before the present). The simultaneous increase of manganese and calcium (Figure 2) points to a manganese/calcite coprecipitation phenomenon. The variability of the sediment concentrations of manganese is probably produced by random changes of the redox potential at the bottom of the lake that determine the solubility of manganese compounds. The short time changes in the redox potential are mainly the result of varying weather and temperature conditions that influence bioproductivity (e.g., the growth of algae) and related biogeochemical processes (e.g., oxidation of organic matter). After 1960, the improved and less eutrophic conditions in the lake have led to an increase in the redox potential with a related general increase in the concentrations of manganese in the sediments. The mean present day concentrations of Mn (2.20 ( 0.92 mg/g of sediment; number of samples n ) 17) are much
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FIGURE 3. Concentrations (µg/g of sediment) of zinc (1) and lead (b) in dated sediment samples from Zu1 richsee. Analytical errors (5-10% (1σ). For additional information see Table 1.
FIGURE 4. Concentrations (µg/g of sediment) of copper (9), cadmium (b), and mercury (2) in dated sediment samples from Zu1 richsee. Analytical errors (5% for Cu, (10% for Cd, and (20% for Hg (1σ). For additional information see Table 1. higher than the Mn concentrations around the year 1800 (0.65 mg/g of sediment) and the geochemical background of 0.60 ( 0.03 mg/g of sediment (18 samples, long core). In contrast to calcium and manganese, the sediment concentrations of iron in Zu ¨ richsee decreased by roughly a factor 2 (13.8 ( 2.0 mg/g of sediment, n ) 17) in recent times (Figure 2) as compared to geochemical background concentrations of 30.3 ( 2.7 mg/g of sediment (n ) 18, long core). This is a side effect of the eutrophication of Zu ¨ richsee. The increase in organic matter in the sediments and the increased precipitation of calcium diluted the concentrations of iron. We present the concentrations of the trace elements zinc and lead (Figure 3) and copper, cadmium, and mercury (Figure 4) as a function of sediment age. Chemical compounds at trace levels are adsorbed to the surfaces of particles and are carried to the bottom of the lake. It has been shown that sedimenting particles are very efficient carriers for trace elements (11). With the beginning ‘industrial revolution’ in the early 19th century, a significant increase in the sediment concentrations of most of the investigated metals is observed until about 1960. For Zn, Cd, and Pb, the increase is more than a factor of 10. The increase of Zn and Cd was most distinct after World War II. Both elements entered the lake sediments mainly from wet and dry atmospheric fallout. Zinc and cadmium were extensively used in galvanic and plastic industries, respectively. In contrast to Zn and Cd, the concentrations of Cu and Pb remained essentially constant from 1920 to 1960 and from 1920 to 1975, respectively. The very similar pattern of Cu and Pb between 1920 and 1960 points to related sources. The
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significant increase of the sediment concentrations of cadmium (Figure 4) started somewhat later than that of the other elements. The reason for this different behavior of cadmium may be explained by a later beginning of a significant use of cadmium by local industries. The data for Hg are rather scarce, but suggest a decrease in the sediments after 1960. An increased pollution by heavy metals is generally observed all over the world (12, 13) and is described by many publications on the sediments of lakes and ponds (14-19) and of bogs (20, 21). It is also reported from the ice of Arctic, Antarctic, and Alpine glaciers (22-25). It is of interest to point to the fact that the concentrations of lead in Zu ¨ richsee sediments increased significantly long before leaded fuel was extensively used in automobiles in Europe. This shows that local industries were the main sources of lead pollution until about 1950. This result is in agreement with recent high-resolution measurements of lead in sediments of Lake Constance by Wessels et al. (19), who concluded that the early increase of lead concentrations in Lake Constance was caused by local emissions of regional industries and not by long-distance transport of lead in particulate form (e.g., as dust from coal-burning industries). However, in contrast to the measurements from many other locations in the world (13), no significant further concentration increases due to the use of lead tetraethyl are observed in the sediments of Zu ¨ richsee after 1950. As indicated above, the sediment concentrations of lead remained rather constant between about 1920 and 1975 (Figure 3). We assume that continuous and significant reductions of the industrial lead releases to the lake and improvements of the environmental conditions (see below) must have nearly compensated the growing lead output from automobile exhausts to the atmosphere. Since several sources (e.g., effluents, runoffs from the environment, atmospheric fallout) contribute to the sediment concentrations, the situation in lakes is more complicated than for instance in ice cores (2225) or bogs (20, 21) where atmospheric inputs are very dominant. The concentrations of the investigated heavy metals (except Pb) in Zu ¨richsee sediments started to decrease sharply in the 1960s (Figures 3 and 4). The decrease is due to several effects (beginning operation of sewage treatment plants, changes in industrial products and methods, more stringent legal regulations, growing public awareness of environmental problems, etc.). However, none of the investigated heavy metals has yet reached the concentrations at around 1800 or the pre-industrial natural backgrounds. They are still factors of 2-3 above the levels shown in Table 1 (long core). Also, the variability in the concentrations of the more recent samples is much larger than in the pre-industrial times (Table 1). During and at the end of the last ice age, the variations were mainly produced by strongly varying water flow rates and discharges of the rivers. For lead, the decrease in the sediment concentrations occurred about 15 years later than for the other investigated elements (Figures 3 and 4). The later decrease of lead was caused by the significant additional contribution of lead due to substantial traffic emissions after 1950 (26) that compensated the improvements achieved by sewage treatment plants and the other effects mentioned above. In Figure 5, we compare the sediment concentrations of copper, zinc, and lead with the growth of industrial development in the region of upper Zu¨richsee. For the lack of relevant metal production data during the early industrial development in this region (i.e., after 1800), we have used the increase of sulfuric acid production (in 1000 t/year) as a measure for the growing industrialization. A chemical plant, situated on the shores of upper Zu ¨ richsee, produced copper sulfate, iron sulfate, and other chemicals but mainly sulfuric acid by the lead chamber process since the early 19th century. The chemicals were used in the rapidly growing textile industry
were of minor importance, in agreement with the results from Lake Constance (19).
Acknowledgments We thank W. Stumm and U. von Gunten for very valuable and stimulating discussions and R. Keil and A. Zwyssig for analytical and technical help. The critical comments of three anonymous reviewers are appreciated. The work was partly supported by the Swiss National Science Foundation and by L + F fonds of EAWAG.
Literature Cited
FIGURE 5. Comparison of the annual production of sulfuric acid (9, in 1000 t/year, ref 27) by a local chemical plant situated on the shores of upper Zu1 richsee (see Figure 1) and sediment concentrations (µg/g of sediment) of copper (2), zinc (1), and lead (b). Some points are interpolated from the data of Table 1. Correlation coefficients (r) between 1820 and 1950 for sulfuric acid and the respective metals are as follows: 0.76 for Cu, 0.99 for Zn, and 0.86 for Pb. Estimated errors in the production data of sulfuric acid are (5-10% (1σ). that dominated the region for many years. We assume that the production data of sulfuric acid can be used as a suitable measure for the early industrial development and related changes to the environment. The lead chamber process is expected to be responsible for part of the early lead contamination of Zu ¨ richsee. This process was changed in 1908 to the catalytic contact method, and this technological improvement might be the main reason for the leveling of the lead concentrations in the lake sediments (see above). However, further measurements at other sampling points are necessary to support these assumptions. We extracted the yearly sulfuric acid production from ref 27. The production data exhibit some uncertainty because the concentrations of the acid were not always clearly indicated; they increased with a higher technical development and with changes in the chemical plant. Furthermore, some of the metal concentrations (Table 1) had to be interpolated to the year given in ref 27. Despite these uncertainties, the production of sulfuric acid allows a correlation with the metal concentrations in sediments of Zu ¨ richsee. The comparisons in Figure 5 show that the concentrations of the three trace metals started to increase with increasing sulfuric acid production. The increase lasted until about 1960 when metals were more and more removed by the operation of sewage treatment plants and other factors mentioned above. The following correlation coefficients (r) were obtained from 1820 to 1950 between sulfuric acid production and the relative metals: Cu, 0.76; Zn, 0.99; Pb, 0.86 (n ) 11). The good correlation (Figure 5) from 1820 to about 1950 between local industrial activities (here represented by the production of sulfuric acid) and the concentrations of copper, zinc, and lead in Zu ¨richsee sediments demonstrates that local industrial developments and the related growth of the regional population influenced directly environmental changes in this time period. Therefore, the early man-made pollution was mainly of local origin, and long-range transport phenomena
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Received for review July 15, 1996. Revised manuscript received March 24, 1997. Accepted April 14, 1997.X ES960616H X
Abstract published in Advance ACS Abstracts, June 15, 1997.
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