Environ. Sci. Technol. 199 1 25 1619- 1627 ~
Increases in the Polychlorinated Dibenzo-p-dioxin and -furan Content of Soils and Vegetation since the 1840s Lars-Owe K/elier,t Kevin C. Jones,*mt A. E. Johnston,§ and Christoffer Rappe*gt Institute of Environmental Chemistry, University of UmeA, S-90 187 UmeA, Sweden, Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, LA1 4YQ, U.K., and AFRC Institute for Arable Crops Research, Rothamsted Experimental Station, Harpenden, Herts AL5 2JQ, U.K. ~~
W Archived soil samples (0-23 cm, plough layer depth)
collected from the same semirural plot in southeast England between 1846 and 1986 have been analyzed for polychlorinated (tetra to octa) dibenzo-p-dioxinsand -furans (PCDD/Fs). Atmospheric deposition will have been the major source of PCDD/Fs to the site over this time. PCDD/Fs were present in all the samples, and concentrations started to increase around the turn of the century, rising from 31 to 92 ng of CPCDD/Fs (kg of soil)-’ between 1893 and 1986. Unwashed bulked herbage samples from 1960-1970 and the 1980s contained 96 and 85 ng of CPCDD/F kg-’ respectively, compared to 12 ng of CPCDD/F kg-l in a sample from 1880-1990. Average CPCDD/F net rates of increase in the soil over the last century were calculated as ca. 190 ng m-2 year-’. It is suggested that the increases in soil and herbage PCDD/Fs observed this century at Rothamsted are representative of those likely for agricultural systems in many industrialized regions. The possible changing sources of PCDD/Fs to the environment are discussed in the context of the concentration trends and congener-specific observations.
Introduction Polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) are ubiquitous in the contemporary environment, albeit at, very low concentrations (1-4). They are formed during various chemical and industrial manufacturing processes, and by combustion or organic materials (5, 6 ) . Atmospheric transport and deposition processes then result in their widespread dispersal through the environment (7-9). What is still unclear at present, however, is the relative importance of the various anthropogenic sources into the environment and the possible contribution of natural inputs (10). This issue needs to be addressed, because it has important implications for the management and control of PCDD/F releases into the environment. In an initial attempt at source apportionment in the United Kingdom, the Department of the Environment estimated the relative contribution to tetra-CDDs released into the atmosphere from various combustion sources (1). Based on very limited data, their report implicated municipal incinerators, various coal-burning activities (domestic, power station, and industrial plants), and vehicle exhausts as being of major importance nationally. These findings are generally supported by those of the Swedish National Environment Protection Board (2). However, this approach can be supplemented by examining evidence for temporal trends in the environmental burden of PCDD/Fs and relating this to known changes in the sources of release. Some workers have undertaken retrospective analysis of archived samples [sewage sludges, human tissues (11-14)l or dated sediment cores (7, 8, 15-18) to address this issue, but these studies have so far ‘University of Umeh Lancaster University. g Rothamsted Experimental Station.
*
00 13-936X19 110925-1619$02.50/0
failed to adequately resolve the issue of natural versus anthropogenic sources. For example, temporal trends in PCDD/Fs have been studied by analysis of sediment cores in the United States and Europe to see whether the historical input of these compounds into the environment could more clearly define the mechanism of their formation. Hites and co-workers reported that PCDD/F concentrations in dated sediment cores have increased greatly since 1940 in the Great Lakes and in Lake Siskiwit, a remote site on Isle Royale in Lake Superior, which would only have received inputs via the atmosphere (8,15). The trend was almost identical in cores from three Swiss lakes (16). Combustion is implicated as the source of PCDD/Fs to the atmosphere by their occurrence in the Siskiwit Lake core and by comparison of the congener distribution pattern observed in the cores with that for known contemporary sources, such as municipal incinerators and coal-fired power stations. Hites and colleagues argued that the historical increase is similar to known trends in the production, use, and disposal of chlorinated organic compounds. However, the sediment records were ambiguous about the occurrence of PCDD/Fs prior to -1940; some samples from before this time gave detectable values, but were not significantly greater than the laboratory background to be unequivocal. Despite this uncertainty over whether natural combustion sources generate PCDD/Fs, the studies with lake cores provide strong evidence that anthropogenic sources have substantially increased their release into the environment (18). The studies on preserved human tissues also provide a somewhat ambiguous picture. Schecter et al. (121,for example, analyzed human adipose over 400 years old for tetra- to hexa-CDD/F congeners and had problems because they were working at the limits of their detection. They commented that “they were reluctant to draw firm conclusions”, but that the congeners could not be clearly detected in the ancient samples, and if they were present, the levels were significantly lower than in modern samples. In a subsequent study (14) these workers reported that on reanalysis the ancient tissues showed the presence of low ppt levels of hepta- and octa-CDD after blank contamination was carefully eliminated. The levels of PCDD/F were well below those reported in modern human tissues, leading the authors to conclude that the main contemporary source of PCDD/Fs is anthropogenic. In this paper we describe changes due to atmospheric deposition in the PCDD/F content of soils and herbage collected from the same experimental plots between the mid-1800s and the present. This approach has been used previously to examine the temporal trends of heavy metals and polynuclear aromatic hydrocarbons (PAHs) (19-26). All the samples originate from an experiment that has been under continuous cultivation since 1843 at the Rothamsted Experimental Station, a semirural location in southeast England. One plot on each of the experiments (i.e., the “control”) has not received any additions of soil fertilizers or amendments. Therefore, by comparison of the chemical composition of archived soils or vegetation with that of
0 1991 Amerlcan Chemical Society
Environ. Sei. Technol., Vol. 25, No. 9, 1991 1619
recent samples, the significance of atmospheric inputs can be determined. This is the first study to investigate long-term changes in the atmospheric fallout of PCDD/Fs by using archived soils and vegetation as sampling media. There are several advantages to this approach as opposed to using sediment cores. First, the sampling dates are known with certainty and the samples have been undisturbed since collection and preparation for storage. Second, the control plots at Rothamsted will only have received inputs via the atmosphere, whereas the interpretation of some sediment cores may be confused by additional inputs of runoff from the surrounding catchment. Third, the soils and herbage will have received inputs directly from the atmosphere, while material deposited in sediments may have undergone chemical/physical changes during passage through the water column. Fourth, the temporal resolution of sediment core data may be confused by sediment mixing/bioturbation. Two further differences should be realized-the potential for direct atmosphere/soil exchange of PCDD/Fs via the vapor phase prior to sampling, and photolysis of PCDD/Fs adsorbed to particulates at the soil/vegetation surface. Once in storage, however, photolytic and microbial degradation is likely to have been minimal, as the samples were kept air-dried in the dark a t room temperature in sealed glass containers. All soils and vegetation receive inputs of PCDD/Fs from the atmosphere and release some congeners back to the atmosphere by volatilization. The deposition flux will depend on a number of factors, including proximity to point/regional sources, rainfall, meteorological conditions, and vegetation type. Recent surveys of soils have shown significantly higher concentrations of PCDD/Fs in urban areas compared to rural locations (27,29),a trend that had been reported previously for PAHs and PCBs (30,31). It is also known that plant root uptake and translocation of PCDD/Fs are very inefficient processes (32), so that PCDD/Fs associated with the aboveground portions of plants are likely to be primarily derived from deposition. Atmospheric deposition will be the most important source of PCDD/Fs to agroecosystems nationally in the United Kingdom (and elsewhere) (I, 33), and so the trends observed in these soils have general significance for the changing environmental burden of PCDD/Fs in other industrialized countries, notably in Europe and North America. Plant-based foods, meat, and dairy products (all derived from agroecosystems account for a substantial proportion of the human dietary intake of PCDD/Fs (34-37), so atmospheric inputs have important implications for human exposure.
Materials and Methods Location and Soil Samples. Rothamsted (grid reference TL 120137), near Harpenden, is a “semirural” location in western Europe. It lies 42 km north of central London and within 2 km of the major A5, A6, and M1 trunk roads, although the surrounding area is primarily agricultural. All the soils sampled for this study were from the control or “nil treatment” plot of the Broadbalk experiment, which has been under continous winter wheat since 1843 (38). These soils are neutral or slightly calcareous silty clay loams (Batcombe series), containing about 20-30% clay and 0.9-1.1% C, composed primarily of quartz and calcite with smaller amounts of illite, kaolinite, chlorite, and sanidine. All the samples analyzed were taken from the 0-23-cm layer, which is the cultivated plough layer for this arable experiment, except for the first sample (1846),which was taken before the conventional sampling depth was established. Many blocks were excavated to this 1620
Environ. Sci. Technoi., Vol. 25, No. 9, 1991
depth across the plot and bulked in each sampling year. However, it is suspected that the early (1846) sample, taken at a time when ploughs were still drawn by oxen, relates to a depth of about 0-12 cm. All the soils had been air-dried and ground in the same iron pestle and mortar after collection and passed through C2-mm sieves. This procedure is unlikely to contaminate the soils during processing. The samples were subsequently stored in glass jars with cork lids to the present day in a dark room at ambient temperatures. For this study, samples from the years 1846,1856,1893,1914,1944, 1956, 1966, 1980, and 1986 were taken from the archive, transferred to acetone/hexane-rinsed glass jars sealed with similarly rinsed aluminum foil and then ground in an agate Tema mill to produce a fine powder. Stored samples had a moisture content of ca. 8%. Herbage Samples. The herbage samples were also taken from the sample archive, but originated from the Park Grass permanent grassland experiment, rather than the Broadbalk continuous wheat plots. The Park Grass experiment has been in permanent grassland since 1856; herbage is cut twice yearly-for hay in June, and again in September. It is the oldest experiment on grassland in the United Kingdom, and historical records show that the field had been under grass for at least two centuries when the experiment began in 1856. Continued manuring with different fertilizers and the effect of chalk dressings on soil pH have combined to affect both the botanical composition and the yield of the mixed population of grasses, clovers, and other species growing on the plots. The stored herbage samples had previously been air-dried, chopped into short lengths, and kept in sealed glass jars until subsampled. For this study only the hay samples (i.e., the June harvest) were analyzed. To look at the broad changes through time, samples taken in individual years were bulked in equal weights and finely ground to give samples representing intervals of approximately decades: 1891-1900,1934-1944, 1960-1970, and 1979-1988. It is important to note that these samples were unwashed prior to storage and analysis. Analytical Methods. Solvents were obtained from Burdick and Jackson, and redistilled in glass. Other essential chemicals, such as silica (70-230 mesh), sodium sulfate, and sulfuric acid were all obtained from Merck. (a) Soil Extraction. The extraction, cleanup, and analysis will be described in detail elsewhere (39). Briefly, 30-40-g samples of soil were weighed into glass-fiber thimbles, placed in a Soxhlet extractor, spiked with 2.5 ng of five 13C-labeled internal standards (2,3,7,8-TCDF, 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD, 1,2,3,4,7,8-HxCDF, OCDD), and extracted with 150 mL of dichloromethane (DCM)/acetone (1:l v/v) for 12 h. Correction for the water content was made by drying a 1-3-g subsample at 110 “C for 12 h. (b) Herbage Extraction. Herbage (70-80 g) was mixed with anhydrous sodium sulfate (1:3 w/w) in a blender. The mixture was then transferred to a 40 X 1200 mm column and extrated with 1 L of DCM/acetone. The eluate was spiked with the same internal standards as the soil, but also with the addition of 13C-labeled2,3,4,7,8-PeCDFand 1,2,3,4,6,7,8-HpCDD, Dry weights were determined as for the soil. A comparison of the extraction efficiency of different solvents has been made and verified the usefulness of the DCM/acetone mixture for environmental samples (39). ( c ) Cleanup. Pure grade tetradecane (50 pL, Fluka) was added to the extracts, which were then reduced by rotary evaporation, and the concentrate passed through three microcolumn cleanup stages: (I) a 38 X 200 mm
column, top packed with 40 mm of silica and 30 mm of potassium hydroxide silica and eluted with 200 mL of n-hexane; (11) ii double column (i.e., two 16 X 300 mm columns), the first of which was top packed with 25 mm of silica and 25 mm of sulfuric acid/silica, eluted with 50 mL of n-hexane, connected to a 40-mm basic alumina column (Fluka, activated at 130 "C), and eluted with 20 mL of hexane and 60 mL of DCM/n-hexane (1:l v/v); and (111) a Carbopac C column (40). Tetradecane was added at all the evaporation steps. (d) Analyses. To the final eluate were added 2.5 ng of 1,2,3,7,8-PeCDF and 1,2,3,4,6,7,8-HpCDFas recovery standards, and the samples were concentrated in 20 pL of tetradecane. The analyses were performed on a highresolution gas chromatograph with a 60-m Supelco SP2330 column, connected to a VG70-250s double-focusing mass spectrometer operating on a resolution of 1-100oO Da. The mass spectrometer was optimized and calibrated on 35 eV with polyfluorokerosene (PFK). Selective ion monitoring mode was used throughout on all the PCDD/Fs; M and M + 2 and M -t- 4 channels were monitored. A 3-rL aliquot of sample was injected into the instrument, which represented 5-6 g of soil. (e) Quality Assurance. Duplicate soil samples were prepared from the years 1846, 1944, and 1986, and one replicate injection was made of one of the 1986 samples. Four reagent blanks of the soil were made, and one was made for the herbage. All peaks were identified and 2,3,7,8-substituted isomers quantified and corrected for internal standard recoveries. "Nontoxic" isomers were identified and quantified against a mix containing all PCDD/F isomers. For peak detection a signal-to-noise ratio of 3 1 was set. The quality assurance procedure followed the principles of those laid down in the EPA guidelines method 8290 (41). Principal Components Analysis. Principal components are linear combinations of the original variables constructed orthogonal to each other in multidimensional space. Essentially, principal components analysis (PCA) can be viewed as a projection method where the intention is to preserve as much as possible of the variance of the data set, while projecting down onto as few principal axes as possible. Interpretation of object (sample) grouping and variable (measured feature) correlation is made on object score and variable loading plots, respectively. The multivariate analyses presented here were made with the SIMCA-3B (Soft Independent Modeling of Class Analogy) program. The principles and use of multivariate statistics are presented in ref 42. To remove the skewness, the PCDD/F data were first log-transformed and scaled to give equal standard deviations. Results and Discussion
Quality Assurance. Table I presents the data for the soils between 1846 and 1986, and Table I1 the bulked herbage samples. Some samples have been subjected to replicate analyses (1846, 1944, 1986). Table I shows generally excellent reproducibility for the data and indicates that the blanks are usually under the method detection limit. The detection limit of the method was 10-30 pg kg-'. Where some PCDD/F was detected in the blanks, the samples were clearly still significantly different. Recoveries of the 13C-labeled internal standards ranged between 50 and 150% for the tetra- and hexacompounds,and between 22 and 50% for [WIOCDD for the soils. Recoveries of [13C]-2,3,7,8-TCDFwere low (10-20%) in the two oldest herbage samples. All data reported have been quantified relative to internal standards. Environ. Scl. Technol., Vol. 25, No. 9, 1991
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Table 11. Toxic Isomers and CPCDD/Fs in the Park Grass Herbage for Bulked Groups of Years since 1891" isomer
1891-1900
time intervals 1934-1944 1960-1970
1979-1988
blank ND (6) 31 ND (8) ND 11 ND (6)
2,3,7,8-TCDF CTCDFs 2,3,7,8-TCDD CTCDDs 1,2,3,4,8-/ 1,2,3,7,8-PeCDF 2,3,4,7,8-PeCDF ZPeCDFs 1,2,3,7,8-PeCDD XPeCDDs 1,2,3,4,7,9-/ 1,2,3,4,7,8-H~CDF 1,2,3,6,7,8-H~CDF 1,2,3,7,8,9-H~CDF 2,3,4,6,7,8-H~CDF CHxCDFs 1,2,3,4,7,8-H~CDD 1,2,3,6,7,8-H~CDD 1,2,3,7,8,9-H~CDD CHxCDDs 1,2,3,4,6,7,8-HpCDF 1,2,3,4,6,7,9-HpCDF lV2,3,4,6,8,9-HpCDF 1,2,3,4,7,8,9-HpCDF 1,2,3,4,6,7,9-HpCDD 1,2,3,4,6,7,8-HpCDD OCDF OCDD
530 2400 20 570 350 210 3100 52 1200 190 140 ND (3) 60 1000 35 77 34 1400 380 33 54 32 470 430 330 760
460 2800 25 140 370 240 3700 68 1500 230 190 ND (3) 100 1500 35 95 67 1800 720 57 150 50 520 520 810 920
520 3900 46 1400 260 310 3900 120 3000 380 210 ND (3) 120 3800 120 630 350 7100 4000 120 4100 220 10000 11000 5000 39000
460 2500 32 860 180 200 2800 140 11000 320 160 23 150 3400 140 3000 1400 22000 1900 120 1700 140 7100 5900 2000 24000
CPCDD/F
12150
15140
96500
85360
11
ND ND 21 ND ND ND 21 ND ND ND 29 ND ND ND ND ND ND ND ND
(9) (5) (4) (5) (18) (14) (16) (8) (10) (10)
(16) (17) (17) (23) (21)
Values in picograms per kilogram of dry weight. ND, not detected. Values in parentheses indicate the samples were at or below the detection limits.
General Comments on the Trends and Concentrations. Two important points are apparent in the data from Tables I and 11. First, tetra- to octa-CDD/Fs were detected in all the samples, even the soils and herbage from the middle of the last century. Second, there is a clear trend of increasing concentrations of all the PCDD/Fs through time (Figure 1). We do not believe that losses through the cork lid during storage are significant. Any volatilized PCDD/Fs will be in the static air above the soil in the sealed jar and is presumably at equilibrium until the jar is uncorked. Cork has a high organic carbon content and therefore may adsorb vapor-phase PCDD/Fs on the inner surface. We do not believe that the more chlorinated isomers (hexa, hepta, and octa) will enter the vapor phase. Townsend et al. (43) had to resort to steam distillation to succesfully remove tetrachlorinated isomers from environmental samples and found it impossible to remove the hepta- and octacongeners in this way. Concentrations started to increase around the turn of the century and rose from 31 to 92 ng of CPCDD/Fs (kg of soil)-' between 1893 and 1986; CTCDD concentrations increased from 0.34 to 1.7 ng kg-l over the same time period. Concentrations of the individual isomers and the congener groups have increased steadily over the last century, by factors of between 1.7 and 5.1. It should be stressed that the Broadbalk control plot will only have received PCDD/F inputs via the atmosphere (i.e., not through the deliberate or accidental addition of amendments to the soil) and that the plot is not near to a local point source of PCDD/Fs. In fact, the site is semirural and should be indicative of broad regional trends in the PCDD/F content of soils. The contemporary soil CPCDD/F concentrations at Broadbalk are broadly representative of those reported by other workers ( I , 27,28, 3 3 , 4 4 , 4 5 ) . The typical range of concentrations in U.K. surface soils (0-5-cm depth) has been reported as ca. 45-2400 ng of CPCDD/F (kg of soil)-l, with a median of 1622 Environ. Sci. Technol., Vol. 25, No. 9, 1991
I'
204
1840
'
1
1860
'
I
1880
.
I
1900
'
I
1920
'
I
1940
'
I
1
1960
1980
Figure 1. CPCDD/F trends in the Broadbalk plough layer depth soil samples between 1846 and 1986 (see text).
ca. 330 (1,28,45). Given that CPCDD/Fs will enter soils from the atmosphere and are strongly retained in the surface layers of soils (46,47),these figures are in broad agreement with our result from plough layer depth samples. The broad trend of an increase in the soils is confirmed by the bulked herbage samples from Park Grass (Table 11). The bulked samples from 1960-1970 and 1979-1988 were 7-8 times higher than the sample from 1891-1900. The PCDD/F burden in these samples is thought to represent material deposited directly onto the foliage from the atmosphere rather than uptake of PCDD/Fs from the soil and translocation into the aboveground herbage. In fact, the herbage samples give better resolution to the variations in atmospheric deposition than the soils, because they are harvested annually. The samples analyzed in this study were bulked to reduce the problems of seasonal or
Table 111. Calculated Net Change in the PCDD/F Content of the Broadbalk Soils between 1893 and 1986”
isomer
predicted change, flux rate, pg/kg ratio ng m-2 1986 1893 1986:1893 year-’
2,3,7,8-TCDF CTCDFs 2,3,7,8-TCDD ETCDD 1,2,3,4,8-/1,2,3,7&PeCDF 2,3,4,7,8-PeCDF CPeCDFs 1,2,3,7,8-PeCDD ZPeCDDs 1,2,3,4,7,9-/1,2,3,4,7,8HxCDF 1,2,3,6,7,8-HxCDF 1,2,3,7,8,9-H~CDF 2,3,4,6,7&HxCDF EHxCDFs 1,2,3,4,7,8-H~CDD 1,2,3,6,7&HxCDD 1,2,3,7,8,9-H~CDD CHxCDDs 1,2,3,4,6,7,8-HpCDF 1,2,3,4,7,8,9-HpCDF EHpCDFs 1,2,3,4,6,7,8-HpCDD CHpCDDs OCDF OCDD
1030 10120 94 1830 1080 920 11410 270 3720 1370
480 4730 12 710 550 390 4390 60 965 760
2.1 2.1 7.8 2.6 2.0 2.4 2.6 4.6 3.9 1.8
1.7 17 0.25 3.5 1.6 1.7 22 0.64 8.5 1.8
840 ND 660 9500 350 610 470 8520 4250 420 6230 5650 11340 5200 23410
405 ND 400 4220 90 34 70 1740 1630 150 2150 370 1000 2540 6910
2.1
1.3
1.7 2.3 3.8 18.1 7.1 4.9 2.6 2.8 2.9 15.3 11.3 2.0 3.4
0.81 16 0.79 1.8 1.2 21 8.1 0.83 13 16 32 8.2 51
EPCDD/F
91400 29300
3.12
192
ND, not detected.
yearly “noise”, and to give a better overall picture of changes in air loading and deposition through time. The first soil sample (from 1846) is of interest because it appears not to fit the trends seen in the sample from the late 1800s. The CPCDD/F concentrations in 1846, 1856, and 1893 were 61, 31, and 31 ng kg-’, respectively (Table I). It is suspected that this early sample, taken at a time when the plough was still drawn by oxen, was just taken to a depth of about 0-12 cm and is therefore not directly comparablewith the later samples. Because inputs of PCDD/Fs would be via the atmosphere to the surface we suspect that the “anomalous” reading for the 1846 sample was due to this surface enrichment effect. Consequently, in Figure 1 a “depth-corrected” value for the 1846 sample of 29 ng-l has been used to plot the line of best fit. In an earlier publication reporting trends in the PAH content of these soils we also observed elevated concentrations in this early sample (23),but attributed this
to differences in the processing and storage procedure used on the early samples. We now suspect that this was not the correct explanation. In this context it is also interesting to note that there was little difference between the CPCDD/F content of the Broadbalk soil in 1856 and 1893. This implies that atmospheric inputs of PCDD/Fs were relatively stable in the latter part of the 18OOs, with the increased inputs beginning at the turn of the century. Net Rates of Increase in the Soil PCDD/F Burden. Data from Table I have been used to calculate annual rates of increase in soil PCDD/Fs at Broadbalk. The plough layer at Broadbalk contains 2870 t of soil ha-’ so the changes in soil PCDD/F concentration with time can readily be converted to ng of PCDD/F m-2 year-’. The data are presented in Table I11 and have been calculated from a linear regression of the concentration data nominally over the period 1893-1986. It should be noted, however, that deposition fluxes to the soil are likely to have altered during this time. Fluxes for the CPCDD/Fs averaged 192 ng m-2 year-’; 2,3,7,8-TCDDand CTCDD increased in the soil at average rates of 0.25 and 3.5 ng m-2 year-’, respectively. For each of the congener classes (tetra through to octa) there has been a greater net increase of dioxins that of the equivalent furans; in particular, they seem to have increased at a greater rate since 1944. This is discussed further below. Losses of PCDD/Fs can potentially occur from the soil system through microbial breakdown, photooxidation, volatilization, crop offtake, and leaching (1,43,48,49). The rates of increase in soil plough layer PCDD/Fs at Broadbalk are therefore not necessarily equivalent to deposition fluxes to the soil. Very little other information is available in the literature on deposition fluxes for PCDD/Fs, although some comparable data have been published from studies with lake sediment cores by Hites and co-workers for Lake Erie (8), three Swiss Lakes (15),and Lake Siskiwit, which will only have received inputs of PCDD/Fs via atmospheric deposition (8). All these studies obtained congener-specific data. Other isomer-specific data are presented by Broman et al. (50)for the Baltic Proper at locations near to Stockholm. These data are summarized in Table IV and compared to the Broadbalk data. It should also be stressed, of course, that deposition inputs to lake sediments and soils are not directly comparableclearly some of the data presented in Table IV have been obtained from lakes where direct discharges to the lake water, catchment runoff, and sediment focusing may greatly enhance the atmospheric deposition flux. The rates of PCDD/F input (Le., atmospheric deposition) clearly exceed rates of output (microbial breakdown, photooxidation, volatization, crop offtake, leaching) at Broadbalk.
Table IV. Comparison of the Net Change at Broadbalk with Deposition Measurements to Lake Sediments location Broadbalk Swiss lakes Baldegg Zurich Lugano Great Lakes Erie I Erie I1 Siskiwit Lake Baltic Proper Stockholm archipelago remote N site
EPCDD/F flux, ng m-2 year-’
comments
ref
192
averaged over the last century
a
2100 2900 2900
contamporary fluxes (1980s)
16
contemporary fluxes (1980s) possibly enhanced by sediment focusing
8
contemporary flux (1980s) to a remote site only receiving atmospheric inputs
8
measurements of settling particulate matter
50
20000 28000 230 9100 820
“This study. Environ. Sci. Technol., Vol. 25, No. 9, 1991
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Table V. Percentage of the CPCDD/F Burden in Each Congener Group Calculated for the Soil and Herbage Samples TCDF
PeCDF
HxCDF
HpCDF
OCDF
CPCDF
1891-1900 1934-1944 1960-1970 1979-1988
20 18 4 3
25 24 4 3
8 10 4 4
4 6 9 4
3 5 5 2
Herbage 60 63 26 16
1846 1856 1893 1914 1944 1956 1966 1980 1986
14 18 10 13 16 14 9 12 10
17 17
13
8 5 6 6 8 9 6 7 6
5 3 4 3 12 6 4 5
Soil 56 56 44 48 63 56 38 47 43
11
15 14 15 9 13 12
13 13 11 13 12 10 10 10
5
PCDD/F Offtakes in Herbage. Yield data are available for the Park Grass plot and were used to calculate the amount of PCDD/Fs removed with the herbage for the time intervals analyzed in Table 11. Annual average offtakes can be calculated from crop yield data (not shown), and these again highlight the substantial difference between the early (1891-1900, 1934-1944) and later (1960-1970, 1979-1988) samples. The EPCDD/F loads removed for each time interval were 1.7,1.1,14.2, and 10.8 ng m-2 year-’, respectively;offtakes of 2,3,7,8-TCDDvaried for each time interval between 2.8, 1.9,6.8, and 4.0 pg m-2 year-’, respectively. Given that there are 287 kg of soil m-2 at Broadbalk, and that the CPCDD/F plough layer burdens were therefore 8.9, 17.2, 25.5, and 26.4 pg m-2 in 1893, 1944,1966, and 1986 respectively, these offtakes represent only ca. 0.02, 0.006, 0.06 and 0.04% of the soil burden in each time interval. Temporal Trends Related to Possible Sources. There are numerous potential sources of PCDD/Fs to the environment, and these were conveniently subdivided in the Introduction into sources arising from chemical manufacturing and industrial processes, or from various combustion activities. These have been discussed in detail in refs 1and 2. It is pertinent here to consider two questions: (a) What were the source(s) of PCDD/Fs in the oldest samples? (b) Which of the range of possible sources can be invoked to explain the temporal trends of PCDD/Fs observed in the soils and herbage from Rothamsted? Clearly the United Kingdom has a long history of industrial activity, dating back to the smelting of metals in pre-Roman times and the widespread combustion of timber, coal, and coke started many centuries ago. These combustion processes may account for the low levels of PCDD/Fs observed in the samples from the middle of the last century. It seems probable that the increasing levels observed in the soils in the latter part of the 1800s are also due to combustion activities, perhaps most notably the domestic consumption of coal. The bombing of London and its environs in the 1940s may also have resulted in high and unusual releases from fires. The postwar years have been a diversification in sources of release. In addition, the widespread use of chloroaromatic chemicals in industry and agriculture will have superimposed onto these combustion sources. Indeed, Czuczwa and Hites ( 7 , 8 ) concluded that the post-1940 increase in inputs of PCDD/Fs to sediments taken from the Great Lakes basin was probably due to the combustion of chlorinated organic products present in various wastes. The direct dumping of chemical wastes (for example, from PCP production) is an alternative, but unlikely interpretation of their results. The authors stated that “... the agreement of the congener 1624
Environ. Sci. Technoi., Voi. 25, No. 9, 1991
TCDD
PeCDD
HxCDD
HpCDD
OCDD
CPCDD
5
10 10 3
11
7 7 22 15
6 6 40 28
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and isomer profiles of PCDD/F in the sediments with those in combustion effluents and in air particulates and the coincidence of the production and concentration profiles are persuasive pieces of evidence that combustion is the major source. Direct dumping and coal or natural combustion may be real, but minor sources”. We can obtain further clues about the relative importance of these many sources by studying congener- and isomer-specific changes. Compound-Specific Observations and PCA. It has been suggested that PCDD/Fs can undergo photooxidative degradation in the atmosphere (8) and may show congener-group separation between the gas and aerosol phases (43, 49). Similarly in soils PCDD/Fs are susceptible to slow biodegradation (48),volatilization (51),and probably also to abiotic breakdown. These processes will all act to modify the congener/isomer profile of environmental samples (8,9). It is therefore important to exercise some caution before using the “pattern matching” approach to try and relate environmental mixtures to particular source categories. Indeed, for these samples, pattern matching has been most useful in identifying groups of similar samples, which can perhaps be in turn related to possible sources. It is interesting, in the first instance, to simply broadly compare the percentage of the CPCDD/F burden in each congener group for selected samples. Table V shows this information for the soils and herbage samples. In the soils there is a fairly equal division between PCDDs and PCDFs; dioxins account for between 37 and 62% of the sample (mean 50%). In general, the congener profile in soils contains a high proportion of OCDD (generally>20% of the total) and typically >lo% of the total as the TCDF, PeCDF, HxCDF, and HpCDD congener groups. This is in general agreement with congener profiles from air particulates and surface sediments from the Great Lakes, and broadly characteristic of US. combustion sources (7, 8). The tetra- and penta-CDDs are minor constituents, comprising 70% PCDDs)
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1944b VORLD WAR II SOILS 1944a
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Flgure 2. PCA object plot of the herbage and soil samples. After the flrst component is removed (containing 61 % of the variation due to the degree of contamination), the figure shows the variation in the second and third components. The xdirection accounted for 19% and the y direction for 5 % of the total variation. The position of the dates corresponds to the positions of each sample point on the plot. The boxes show arbitrary groupings of the samples. The axes and origin correspond to those shown in Figure 3.
(Table V). More specifically, the proportions of tetra- and penta-CDFs have declined, while hepta- and octa-CDDs have increased. It should also be noted that these two “types” of congener profile in the herbage are both clearly different from that in the soil. Of course, if atmospheric deposition inputs have changed through time, one would expect the herbage to respond more rapidly to mirror this change than the soils. Hence, the changes in composition observed in the herbage are likely to provide a closer reflection of the changing PCDD/F air chemistry. More detailed information about similarities/differences between samples emerges from the PCA plots prepared from the congener- and isomer-specific data from Tables I and 11. The PCA calculations highlight three components of the total variability, which account for 85% of the total variability in the data set. The first principal component (PC1) alone accounts for 61%, due to differences in PCDD/F concentrations between samples. Figures 2 and 3 are the object (sample) and variable loading plots, respectively, prepared from the soils and herbage data. Figure 2, a plot of PC2 (19% of total variability) against PC3 (5% of the total variation), is interesting since it shows clear differences between groups of samples. It highlights three different groups of soils-pre-1944 (“early”) soils, 1944 on its own, and post-1944 (“recent”) soils-and the two “types” of vegetation pattern, as mentioned above. Replicate analyses are each plotted separately to emphasize the good reproducibility on samples. The 1944 soil is unusual in having a relatively high proportion of OCDF and TCDD, and a low proportion of HpCDDs and OCDDs (Table V; see Figures 2 and 3). Figure 3 plots the eigenvalues for PC2 and PC3 for each of the PCDD/F variables. Variable 1 (2,3,7,8-TCDF;see Table I), for example, gives a high positive value for PC3 and a negative value for PC2; it will be an important component of the samples grouped in the correspondingposition in Figure 3. Other PCA data (not shown) indicate that the PCDD/F composition of modern air most closely matches the old herbage samples, and that the 1944 soil (which is dominated by a high PCDF content) is the one most akin to the mixture of contemporary combustion sources.
13
Figure 3. PCA variable plot corresponding to the variable plot in Figure 2. The numbers (1-25) correspond to the PCDD/F variables in Table I, variable 26 is time. Penta- to octa-CDDs are separated on the x axis, with the hexa- to octa-CDDs separated on the y axis.
Interestingly, the two old herbage samples perhaps most closely resemble contemporary municipal incinerator emissions with, for example, a characteristic isomer pattern for the hepta-CDFs. Hagenmaier and Brunner (52) suggested this as a marker for PCDD/Fs produced from chlorophenoltechnical products. This may imply that the inputs of combustion-derived PCDD/Fs dominated the deposition input around the late 1800s and early part of the twentieth century, with the principal source(s) being the burning of fossil fuels and wood (53). In contrast, following the unusual combustion-derivedinputs from the war years, the later soils, and more particularly the herbage, may be strongly influenced by the postwar production and use of chlorinated organic chemicals in the United Kingdom, notably pentachlorophenol and 2,4,5-T (1). PCDD/F releases from the combustion of leaded fuel may also be important, since its use increased dramatically in the postwar years, peaking in ca. 1973, and declined gradually more recently, following reductions in the amounts of lead additives permissible in U.K. fuel (25). I t is also possible that higher PCP consumptionprincipally as a wood preservative-and its subsequent release following volatilization and combustion could give rise to secondary formation of PCDD/Fs in the atmosphere by photocoupling (54) or by an enzymatically mediated formation (55, 56). Hagenmaier and Brunner (52) have reported specific isomers present in PCP formulations. Our study also finds these markers. Indeed, there is some evidence from the congener/isomer pattern that the 1960-1970 and 1979-1988 herbage samples are influenced by chloroaromatic inputs. A number of isomers increased in concentration by over 1 order of magnitude, namely, 1,2,4,6,8-/1,2,4,7,8-PeCDD, 1,3,4,6,7,8-HxCDF, 1,2,4,6,8,9-HxCDF, and 1,2,3,6,7,9-/1,2,3,6,8,9-HxCDD. These increases are also reflected to a lesser extent in the post-1940 soil samples (Table 111). Reference was made earlier to the degradation of PCDD/Fs in soils. The data provide some evidence to suggest that the turnover of PCDD/Fs may be of the order of years in the Broadbalk soils. Given that we have argued that the 1944 sample has an unusual PCDD/F pattern, it is interesting to note that by 1956 the pattern is clearly different, suggesting either that the new deposition inputs are strongly superimposedover the 1944 pattern, that some Environ. Sci. Technoi., Vol. 25, No. 9, 1991
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modification of the 1944 pattern has taken place, or that both processes are occurring. However, since we have shown earlier that net annual changes in the soils are less than 1% of the total soil plough layer burden (Tables I and 111),it seems likely that the degradation of PCDD/Fs in the soil plays a key role in changing the pattern. Little field- based data have been reported for these compounds in the literature, although a study by Di Domenico et al. (57) suggested that the half-life of TCDD in soils from Seveso was >10 years. This estimate is supported by information reported by Young (58)for 2,3,7,8-TCDD in soils a t the Eglin Air Force Base, FL. Wild et al. (59) reported soil half-lives for a range of similarly recalcitrant PAHs in sludge-amended soils to be in the range 9 (benzo[ghi]perylene and coronene) years. These figures would seem to be of the same order of magnitude as those implied by this study. General Comments If the production, use, and subsequent combustion of chloroaromatics has dominated PCDD/F emissions in recent decades, it would explain why the most recent herbage samples clearly have a different PCDD/F composition from the early ones. It would also imply that municipal and hazardous waste incinerators-combustion sources that have actually increased in recent decades-are not as important as a source of PCDD/Fs into the environment as the production and use of chloroaromatic chemicals, a t least to the semirural Rothamsted site. I t is interesting to note that the 1979-1988 herbage sample contains lower concentrations of PCDD/Fs than the one from 1960-1970 (Table 11). This fits with the changing production and use figures for the chloroaromatics,but not with the trend for increasing municipal incineration, which has become increasingly popular as a waste disposal option in the United Kingdom in recent years. Czuczwa and Hites (8) also found some evidence that atmospheric inputs of PCDD/F to Siskiwit Lake and Lake Erie peaked in the early-/mid-l97Os and declined somewhat subsequently. They attributed this to reduced particulate emissions from combustion sources as a result of the US.Clean Air Act introduced in 1970. Phasing out/banning of 2,4,5-T in the United States (contaminated with 2,3,7,8-TCDD) has resulted in a decrease in human adipose tissue concentrations through the 1970s and 1980s (60). There has been considerable discussion about the historical release of PCDD/Fs. It has been suggested that traces of PCDD/Fs have been released into the environment from combustion sources throughout history (61). Our data provide some support for this assertion, since we measure PCDD/Fs in soils sampled prior to the industrial revolution. However, we clearly show that anthropogenic sources far outweigh any possible contribution from natural combustion activities, and that there is an imbalance between contemporary rates of PCDD/F release into the environment and their destruction. The U.K. Interdepartmental Working Group on PCDD/Fs report (1)reviewed existing data and suggested that "The release of dioxins probably reached a peak in post war years and is now declining. However, because dioxins are so persistent, there is a legacy of contamination from activities in the past. This legacy means that actions which we take now will only gradually show up as reduced levels of environmental contamination". Again, this study supports their assertion, but needs further verification. We are therefore currently looking with greater temporal resolution at herbage samples taken over recent decades to see if the downturn in PCDD/F releases suggested by 1626
Envlron. Scl. Technol., Vol. 25, No, 9, 1991
the most recent herbage sample analyzed in this study can be substantiated and to study in more detail the relative importance of the different PCDD/F sources. Literature Cited (1) Department of the Environment. Dioxins in the Enuironment; Pollution Paper No. 27, HMSO: London, 1989. (2) National Swedish Environmental Protection Board. Dioxins: a program for research and action. Solna, Sweden, 1988. (3) World Health Organization. Polychlorinated dibenzopara-dioxins and dibenzofurans. Enuiron. Health Criter. 1989, No. 88. (4) Rappe, C. Enuiron. Sci. Technol. 1984, 18, 78A. ( 5 ) Rappe, C.; Andersson, R.; Bergqvist, P.-A.; Brohede, C.; Hansson, M.; Kjeller, L.-0.; Lindstrom, G.; Marklund, S.; Nygren, M.; Swanson, S. E.; Tysklind, M.; Wiberg, K. Waste Manage. Res. 1987, 5, 225. (6) Hutzinger, 0.; Blumich, M. J.; Berg, M. v. d.; Olie, K. Chemosphere 1985, 14, 581. (7) Czuczwa, J. M.; Hites, R. A. Enuiron. Sci. Technol. 1984, 18, 444. (8) Czuczwa, J. M.; Hites, R. A. Enuiron. Sci. Technol. 1986, 20, 195. (9) Eitzer, B. D.; Hites, R. A. Enuiron. Sci. Technol. 1989,23, 1396. (10) Bumb, R. R.; Crummett, W. B.; Cutie, S. S.; Gledhill, J. R.; Hummel, R. H.; Kagel, R. 0.;Lamparski, L. L.; Luoma, E. V.; Miller, D. L.; Nestrick, T. J.; Shadoff, L. A,; Stehl, R. H.; Woods, J. S. Science 1980, 210, 385. (11) Lamparski, L. L.; Nestrick, T. J.; Stenger, V. A. Chemosphere 1984, 13, 361. (12) Schecter, A.; Dekin, A.; Weerasinghe, N. C. A,; Arghestani, S.; Gross, M. L. Chemosphere 1988, 17, 627. (13) Ligon, W., V.; Dom, S. B.; May, R. J.;Allison, M. J. Environ. Sci. Technol. 1989, 23, 1286. (14) Tong, H. Y.; Gross, M. L.; Schecter, A.; Monson, S. J.; Dekin, A. Chemosphere 1990,20, 987. (15) Czuczwa, J. M.; McVeety, B. D.; Hites, R. A. Chemosphere 1985, 14, 623. (16) Czuczwa, J. M.; Niessen, F.; Hites, R. A. Chemosphere 1985, 14, 1175. (17) Hagenmaier, H.; Brunner, H.; Haag, R.; Berchtold, A. Chemosphere 1986, 15, 1421. (18) Friege, H.; Klos, H. Proceedings Dioxin '90 Meeting; Bayreuth, FRG, September 1990; Vol. I, pp 521-528. (19) Jones, K. C.; Symon, C. J.; Johnston, A. E. Sci. Total Enuiron. 1987, 61, 131. (20) Jones, K. C.; Symon, C. J.; Johnston, A. E. Sci. Total Enuiron. 1987, 67, 75. (21) Jones, K. C.; Stratford, J. A.; Tidridge, P.; Waterhouse, K. S.; Johnston, A. E. Enuiron. Pollut. 1989, 56, 337. (22) Jones, K. C.; Johnston, A. E. Enuiron. Pollut. 1989,57,199. (23) Jones, K. C.; Stratford, J. A.; Waterhouse, K. S.; Furlong, E. T.; Giger, W.; Hites, R. A.; Schaffner, C.; Johnston, A. E. Enuiron. Sci. Technol. 1989, 23, 95. (24) Jones, K. C.; Grimmer, G.; Jacob, J.; Johnston, A. Sci. Total Enuiron. 1989, 78, 117. (25) Jones, K. C.; Symon, C.; Taylor, P.; Walsh, J.; Johnston, A. E. Atmos. Enuiron. 1990,25A, 361. (26) Jones, K. C.; Johnston, A. E. Enuiron. Sci. Technol. 1991, 25, 1174. (27) Rappe, C.; Kjeller, L.-0. Chemosphere 1987, 16, 1775. (28) Her Majesty's Inspectorate of Pollution (HMIP). Determination of Polychlorinated Biphenyls, Polychlorinated Dibenzo-p-dioxins and Polychlorianted Dibenzofurans in UK Soils: Technical Report. Her Majesty's Sationery Office: London, 1989. (29) Creaser, C. S.; Fernandes, A. R.; Harrad, S. J.; Cox, E. A. Chemosphere 1990,21, 931. (30) Jones, K. C.; Stratford, J. A,; Waterhouse, K. S.; Vogt, N. B. Enuiron. Sci. Technol. 1989, 23, 540. (31) Jones, K. C. Chemosphere 1989, 18, 1665. (32) Sacchi, G. A.; Vigano, P.; Fortunati, G.; Cocucci, S. M. Experientia 1986, 42, 586.
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(33) Eduljee, G. H. Chem. Br. 1988, 24, 1223. (34) Rappe, C.; Nygren, M.; Lindstrom, G.; Buser, H. R.; Blaser, 0.;Wuthrich, C. Enuiron. Sci. Technol. 1987, 21, 964. (35) Travis, C. C.; Hattemer-Frey, H. A. Chemosphere 1987,16, 2331. (36) Jones, K. C.; Bennett, B. G. Sci. Total Environ. 1989, 78, 99. (37) Svensson, B. G.; Nilsson, A.; Hansson, M.; Rappe, C; Akesson, B.; Skerfving, S. N . Engl. J . Med. 1990, 324, 8. (38) Johnston, A. E.; Garner, J. V. Report of the Rothamsted Experimental Station for 1968; Part 2, pp 12-25. (39) Kjeller, L.-0.; Johnson, P.; Rappe, C., in preparation. (40) Nygren, M.; Hansson, M.; Sjostrom, M.; Rappe, C.; Kahn,
P. C.; Gochfeld, M.; Velez, H.; Ghent-Guenther,T.; Wilson, W. P. Chemosphere 1988, 17, 1663. (41) Tondeur, X.; Beckert, W. F.; Billets, S.; Mitchum, R. K.
Chemosphere 1989, 18, 119. (42) Wold, S.; Albano, C.; Dunn, W. J.; Edlund, U.; Esbensen,
K.; Geladi, P.; Hellberg, S.; Johansson, E.; Lindberg, W.; Sjostrom, M. Multivariate Data Analysis in Chemistry; B.. Kowalski. Ed.: Proceedings of the NATO Advanced Study on Chemometrics; Maihematics and Statistics in Chemistry. Cosenza, Italy; D. Reidel: Dordrecht, Holland,
1984; pp 1--79. (43) Townsend, D. I.; Lamparski, L. L.; Nestrick, T. J. Chemosphere 11987,16, 1753. (44) Nestrick, T. J.; Lamparski, L. L.; Frawley, N. N.; Hummel, R. A,; Kocher, C. W.; Mahle, N. H.; McCoy, J. W.; Miller,
D. L.; Peters, T. L.; Pillepich, J. L.; Smith, W. E.; Tobey, S. W. Chemosphere 1986, 15, 1453. (45) Creaser, C. S.; Fernandes, A. R.; Al-Haddad, A.; Harrad, S. J.; Homer, R. B.; Skett, P. W.; Cox, E. A. Chemosphere 1989, 18, 161.
(46) Di Domenico, A.; Silano, V.; Viviano, G.; Zapponi, G. Ecotoxicol. Enuiron. Saf. 1980, 4, 327.
(47) Nielsen, P. G.; Lokke, H. Ecotoxicol. Enuiron. Saf. 1987, 14, 147. (48) Arthur, M. F.; Frea, J. I. J. Environ. Qual. 1989, 18, 1. (49) Nakano, T.; Tsuji, M.; Okuno, T. Atmos. Environ. 1990, 24A, 1361. (50) Broman, S.; Naf, C.; Zebuhr, Y.; Lexh, K. Chemosphere 1989, 19, 445. (51) Miller, G. C.; Herbert,V. R.; Miille, M. J.; Mitzel, R.; Zepp, R. G. Chemosphere 1989, 18, 1265. (52) Hagenmaier, H.; Brunner, H. Chemosphere 1987,16,1759. (53) Nestrick, T. J.;Lamparski, L. L. Anal. Chem. 1982,54,2292. (54) Lamparski, L. L.; Stehl, R. H.; Johnson, R. L. Environ. Sci. Technol. 1980, 14, 196. (55) Svensson, A.; Kjeller, L.-0.; Rappe, C. Environ. Sci. Technol. 1989, 23, 900. (56) Oberg, L. G.; Glas, B.; Swanson, S. E.; Rappe, C.; Paul, K. G. Arch. Enuiron. Contam. Toxicol. 1990, 19, 930. (57) Di Domenico, A,; Silano, V.; Viviano, G.; Zapponi, G. Ecotoxicol. Enuiron. Saf. 1980,4, 339. (58) Young, A. L. In Human and Environmental Risks of Chlorinated Dioxin and Related Compounds;Tucker, R. E., Young, A. L., Gray, A. P., Eds.; Plenum: New York, 1983; pp 173-190. (59) Wild, S. R.; Berrow, M. L.; Jones, K. C. Enuiron. Pollut. 1991, 72, 141. (60) Stanley,J. S.; Ayling, R. E.; Cramer, P. H.; Thornburg, K. R.; Remmers, J. C.; Breen, J. J.; Schwemberger,J.; Kang, H. K.; Watanabe, K. Chemosphere 1990, 20, 895. (61) Crummett, W. B.; Townsend, D. I. Chemosphere 1984,13, 778.
Received for review January 8,1991. Revised manuscript received April 9,1991. Accepted May 13,1991. We are grateful to the U.K. Agricultural and Food Research Council for financial support.
“Unmixing” of 137Cs,Pb, Zn, and Cd Records in Lake Sediments Erik R. Christensen* and Richard J. Klein Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee,
The inverse Berger-Heath model to unmix sedimentary records of particle-associated tracers, originally used for l80records in deep-sea sediments, is extended to include radioactive materials, environmental pollutants, compaction of sediments, and error analysis. The method is applied to the fallout tracer 137Cs,and to Pb, Zn, and Cd in sediment cores from Lake Michigan. The reconstructed 137Csinput records from northern Lake Michigan are in good agreement with 137Csfallout data. The unmixed influx records of Pb, Zn, and Cd show excellent agreement with input records reconstructed previously by a frequency domain method, thus supporting the validity of the approach. Assuming that tracer concentrations as well as sedimentation and mixing parameters are known with sufficient accuracy, and that the forward model is correct, the ultimate limitation of this or any other reconstruction method lies in the finite depths of the physical sampling intervals. Introduction
Determination of historical fluxes of pollutants to lake or near-shore ocean sediments is of value for environmental 0013-936X/91/0925-1627$02.50/0
Milwaukee, Wisconsin 5320 1
modeling and management. For example, historical records of sulfur have been used to document anthropogenic sources of acid precipitation (1). Other examples include the linking of polycyclic aromatic hydrocarbons in the , demonenvironment to fossil fuel combustion ( 2 , 3 ) the stration of environmental lead reduction in response to federally mandated curbs on lead in gasoline ( 4 ) )and the correlation of reduced concentrations of polychlorinated biphenyls (PCBs) in lake sediments with the 1977 termination of U.S. domestic PCB production (5). Accurate source functions, as revealed from sedimentary records, can also be used in source apportionment including the assessment of the environmental effects of changes in industrial activity. In the case where no mixing takes place, the historical influx of particle-associated pollutants is directly reflected in the sedimentary record, except for any modification that may occur caused by chemical processes, radioactive decay, or biodegradation. Assuming that the latter processes are either insignificant or take place at a known rate, the influx is directly reflected in the sedimentary record. Mixing, often caused by the activities of bottom-dwelling organisms, can drastically change the input record (6, 7).
0 1991 American Chemical Society
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