Environ. Sci. Technol. 1903, 17, 523-530
Long-chain Alkylbenzenes as Molecular Tracers of Domestic Wastes in the Marine Environmentt Robert P. Eganhouse,” Dara L. Blumfleld, and Isaac R. Kaplan Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, Los Angeles, California 90024
rn A suite of secondary C1+14-substitutedbenzenes known as linear alkylbenzenes (LABS) were found in southern California’s municipal wastes. These compounds are manufactured for the production of the linear alkylbenzenesulfonate (LAS) surfactants used in commercial detergent formulations. Thus, their appearance in wastes is believed to result from incomplete sulfonation of the linear alkylbenzenes and subsequent carryover in detergents and/or by desulfonation of LAS. LABS were evaluated as waste tracers in the marine environment by determining their concentration and composition in suspended particulate matter and sediments in the vicinity of a major wastewater outfall system. They appear to be preserved in sediments for time periods of 10-20 years. A complex assemblage of surfactant-related branched alkylbenzenes was also found in the waste-affected sediments. Sedimentary distributions of both the linear and branched alkylbenzenes are discussed in terms of the historical emission of wastes in Los Angeles and surfactant usage rates. The results demonstrate that LABS are potentially useful as molecular tracers of domestic wastes and, under appropriate conditions, as geochronological tools.
Introduction By current estimates, municipal wastewaters contribute approximately 3.0 X lo6 metric tons of petroleum to the coastal waters of the world each year ( I ) . This is believed to account for roughly 5% of the total global input. Whereas our knowledge of the magnitude of wastewater hydrocarbon emissions has been improving in recent years (2-4), the characterization, environmental behavior, and fate of these materials are still only poorly understood. One reason for this is that source identification of petroleum hydrocarbons introduced to marine sediments can be quite difficult, particularly after weathering has occurred. Petroleum is a highly complex mixture, and molecular scrambling of crude oils and various refined products occurs not only before and during waste treatment but also after discharge to ocean waters. The problem is exacerbated in regions where human activity is intense because of inputs from many sources. In this context, a passive molecular tracer acting as a source indicator in marine sediments would be highly useful. If conservative in behavior, such a compound or compounds might even be applied in a quantitative sense to estimate the accumulation rate of waste-derived petroleum in sediments. However, such a tracer would be required to meet certain minimum criteria: (1)that it be a true hydrocarbon and follow the general biogeochemical pathway(s) of petroleum-type hydrocarbons, (2) that it be source-specific, and (3) that it survive exposure in the marine environment to the extent that it can be found in detectable quantities &e., not be completely removed through physicochemical weathering or biodegradation). +Publication No. 2372. Institute of Geophysics and Planetary Physics, University of California at Los Angeles. *To whom correspondence should be addressed at Environmental Science Program, University of Massachusetts, Boston, MA 02125. 0013-936X183/0917-0523$01.50/0
Recently, we identified a group of secondary phenylalkanes also known as linear alkylbenzenes (LABS; C6H5-CnH2n+l, n = 1Q-14),which appear to manifest these qualities (5). The LABS occur as ubiquitous constituents in southern California’s municipal wastes, and although similar hydrocarbons have been noticed before in other waste effluents (6-8),their exact structures, origin(s), and possible uses were not further explored. The unique homologue and isomer distributions of the LABS indicate that they are related to the LAS (linear alkylbenzenesulfonate) surfactants (5,9, IO). Thus, their appearance in wastes is believed to stem from domestic and industrial detergent use. In 1979, Crisp et al. (11)first reported finding a suite of Clo_13-substitutedbenzenes in suspended particulate matter collected offshore from one of southern California’s major waste outfall systems. Subsequent examination of the hydrocarbons by GC/MS (gas chromatography/mass spectrometry) showed these alkylbenzenes to have the same structures and isomeric distribution as those found in the waste effluents (5). More recently, Ishiwatari et al. (12)identified LABS in the sediments of Tokyo Bay. We were thus encouraged to search for the presence of the linear alkylbenzenes in coastal sediments and further evaluate their use as molecular tracers of waste-derived petroleum. Because the southern California coastal zone, site of this study, receives hydrocarbons from a great many sources other than municipal waste discharges (23-I5), it is an appropriate location to test the applicability of potential hydrocarbon tracers. In undertaking this research, the main question we intended to address was UArethe longchain linear alkylbenzenes preserved in marine sediments and therefore viable tracers of waste-derived petroleum?” The results disclosed here give a clear affirmative answer.
Experimental Section Samping and Extraction. The procedures for sampling and analysis of wastewaters and suspended marine particulate matter have previously been described (2, 11, 13) and will not be further detailed here. However, the locations of the wastewater outfall system, sediment stations, and particle interceptor trap are shown in Figure 1. Marine sediments were collected by the Los Angeles County Sanitation Districts’ Ocean Monitoring Group on Apr 8-9,1981, using a specially designed gravity corer. The coring device was lowered to within ca. 60 cm of the bottom and allowed to free fall the remaining distance. This precaution was taken to prevent or at least minimize disruption of the sediment surface. Once recovered, the cores (in acetate liners) were rapidly cooled with liquid nitrogen and maintained frozen until such time as extrusion and sectioning (2-cm intervals; carbide cutoff wheel) could be carried out. After sectioning, each 2-cm interval was trimmed of the peripheral 2 cm of sediment to reduce the likelihood of contamination from the liner. The remaining inner portion of each sediment section (diameter = 10.5 cm) was then stored frozen in a clean glass jar until hydrocarbon analyses were performed. All sections of core 3C1 (located ca. 6 km to the northwest of the outfalls at
0 1983 American Chemical Society
Environ. Sci. Technol., Vol. 17,No. 9, 1983 523
PO'
40'
! W r e 1. Map showing the locations of the JWPCP outfall system, sediment stations. and collection she of the particle Interceptor trap.
a water depth of 62 m) were used in this study. For comparison, the upper (0-2 em) and bottom (48-50 cm) intervals of a deep-water sample, S P B l (13.5 km to the southwest; water depth = 869 m) were also analyzed. Although surface sediients in the proximity of the outfalls are frequently anaerobic due to the high rate of organic loading, those taken at 3C1 contained very small amounts of H,S (0.05 ppm) and appeared to be largely aerobic at the time of sampling (16). Successive ambient temperature extractions were performed by using 150 mL of a CH2C12/MeOHsolution (61) and 2.5-14.0 g of wet sediment. No UV fluorescence was observed in the extract after the fourth repetition. Following rotary evaporation of the combined extracts to about lC-15 mL, CHCI, was added to the concentrate, and the CHCI,/MeOH azeotrope was removed by rotary evaporation. The periodic addition of CHC?, (totalvolume ca. 400 mL) was continued until complete removal of MeOH was affected. The final concentrate was dried over Na2S04(anhydrous) and treated for elemental sulfur removal by using an activated copper column. The total hydrocarbons isolated by silica gel thin-layer chromatography (CH,CI, development) were analyzed for yield by microgravimetry. Throughout this procedure, special care was taken not to expose the samples to conditions that might lead to evaporative losses of the alkylbenzenes. A study of the behavior of n-alkanes (n-Cl0-n-C& spiked into a procedural blank sample showed that with our methodology n-alkane recovery efficiency maximized at n-C,,, remaining essentially constant with inereasing carbon chain length. This result is similar to that observed by Barrick et al. (17) and suggests that the overall recovery of the long-chain linear alkylbenzenes was nearly complete (83-100%). Gas Chromatography/Mass Spectrometry. The total hydrocarbon fractions were analyzed for their alkylbenzene and total DDT content by combined GC/MS (electron impact mode, 70 eV). Total DDT (i.e., o,p'-DDT, -DDD, -DDE, -DDMU and p,p'-DDT, -DDD, -DDE, -DDMU) was measured in this study because it provided additional information about the history of sedimentation near the outfall system. A high-resolution fused silica capillary column wall-coated with SE-54 (0.25 mm i.d. X 30 m; J & W Scientific) was used for the separations (Finnigan 9610 gas chromatograph) under splitless injection conditions (9,and mass spectral analysis of the 524
Environ. Sci. Technol., Vol. 17, NO. 9, 1983
eluting peaks was achieved with a Finnigan 4000 quadrupole mass spectrometer aided by an INCOS 2300 data system. The peak finding, spectral matching, integration, and quantitation functions were automatically performed by using the Finnigan target compound analysis routine in conjunction with the INCOS computer. After mass spectral verification, individual alkylbenzenes and chlorinated hydrocarbons were quantitated by comparing the integrated areas of the mass fragmentographic (base) peah with the peak area of 1-pbenyldodecane at m/z 91 (internal reference standard). Corrections for relative response a t the Corresponding mass/charge ratios were made possible by analyzing appropriate alkylbenzene or DDTJ1phenyldodecane standards under the same instrumental conditions (GC/MS) as those used for the sediment extracts. The external standard used in the quantitation of the alkylbenzene isomers was obtained as a mixture from Monsanto Chemical Co., the exact composition having been determined by repetitive gas chromatographic analysis with 1-phenyldodecane as the internal standard. The DDT isomers were high-purity (>99%) reference materials provided by the U.S. ~ ~ v i ~protection ~ ~ ~ Agency. Two distinct types of alkylbenzenes were encountered in sediment samples. The linear alkylbenzenes (LABS) consisted of all the secondary C,,4-substituted benzenes. 1-Phenylalkane isomers were not detected. Hereafter, a LAB whose phenyl group is substituted near the terminal carbon of the alkyl chain (e.g., 2-phenyldecane) will be referred to as an external isomer. Consequently, an internal isomer represents one whose phenyl group is attached to a carbon located a t the interior of the chain (e.g., 5-phenyldecane). The second variety of alkylbenzene observed in sediment samples had branched side chains of the type produced by reaction of tetrapropylene with benzene (TAB cf. ref 9 and 18). These compounds are synthesized by industry as precursors for the preparation of the branched ABS (alkylbenzenesulfonate) surfactants commonly used in the United States prior to ca. 1965. The alkylation reaction results in a complex assortment of various TABS (more than 8oOOO possible isomers) ranging in molecular weight from 204 to 288 (i.e., C+15-benzenes; cf. ref 19). However, the major components (>50% by mass) contain 1 2 carbons in the side chain. GC/MS analysis of a TAB standard mixture (kindly provided by Monsanto Chemical Co.) showed that the 12 largest peaks were identical with those present in the sediments. Figure 2a shows mass fragmentograms (m/r 105) of the sediment hydrocarbons (3C1; 14-16 cm) and the TAB standard mixture along with representative mass spectra corresponding to one of the major peaks wmmon to both. The elution and distribution patterns of the TAB peaks are virtually identical, and the mass spectra match quite well. We were able to quantitate the 12 major TAB peaks for which spectral matching between the standard and sediments bad been verified by repetitive GC analysis with 1-phenyldodecane as the internal standard. Relative response factors were then determined by GC/MS analysis of the TAB/1-phenyldodecane standard. In sediment samples containing significant quantities of both alkylbenzene types (linear and branched), one problem that sometimes arose (three cases) was the partial or complete coelution of LAB and TAB isomers. This situation is illustrated in Figure 2b. The chromatographic interference can readily be overcome in GC/MS analysis (but not GC) by taking advantage of the distinct mass spectral qualities of the respective compound types. For
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TIME I) Flgure 2. (a) Mass fragmentograms at m / Z 105 for 3C1 core sediments (14-16 cm) and TAB standard mixture with mass spectra for one of the major components common to both (indicated by asterisk); (b) mass fragmentograms at mlz 91 and 105 for the 8-10cm section of core 3C1 depicting the coelution of LAB and TAB isomers (LAB isomers identified as shaded peaks).
example, all of the LABS (except 2-phenylalkyl isomers) have base peaks at m/z 91 corresponding to formation of the C7H7+ion, with relatively small m/z 105 idns. Only the 2-phenyl isomers have a base peak at m/z 105. In contrast, due to branching at the benzylic carbon (19,201 the major TAB peaks produce predominant fragments at m/z 105 or 119. Currently we are investigating methods for the complete preparative chromatographic separation of LAB and TAB isomers to facilitate direct GC analysig.
Results and Discussion Linear Alkylbenzenes in Municipal Wastes. Figure 3 presents the structures of the linear alkylbenzenes along with mass fragmentograms ( m / z 91) representative of those found in waste samples and one commercial detergent. In most of the effluent samples arid the detergents we analzyed, the complete set of Cl~14-substituted benzene isomers (excluding the 1-phenylalkases) were observed. This is consistent with a process originating with the Friedel-Crafts alkylation of benzene wit4 C10-14 linear olefins (9,18,21,22). Generally, LAB h d LAS mixtures produced by industry and those found in detergents (cf. Figure 3) are dominated by the Cll-12 species (10,23,24). With the exception of the Hyp-7mi and Hyp-5mi effluents (City of Los Angeles, Bureau of Sanitation), the same pattern is observed for LABSin waste samples. The reason for the greater relative abundance of the higher molecular weight homologues in the Hyp-5mi and Hyp-7mi effluents is presently not known. As postulated in an earlier communication (5), the LABS are believed to occur in wastes either as uneulfonated hydrocarbon residues associated with detergents or as the products of LAS desulfonation. To examine the former possibility, we conducted a survey in which we determined the LAB contents of the major U.S.laundry detergents. Our intention was to compare the amounts of unsulfonated
Time Flgure 3. Mass fragmentograms at m l z 9 1 depicting the long-chain linear aikyibenzene assemblages found in wastewater effluents and a commercial detergent. Note: JWPCP &os Angeles County Sanitation Districts); Hyp-Bmi, Hyp-7mi (Cky of L a Angeles, Bureau of Sanitation); OCSD (Orange County Sanitation District); CSD (City of San Diego).
Table I. Concentractions (rg/g) of Total Linear Alkylbenzenes in Various Commercial Laundry Detergents Sampled from the Los Angeles Areaa product type detergent 1
grandar
detergent 2 detergent 3 detergent 4 detergent 5 detergent 6 detergent 7 detergent 8 detergent 9 detergent 10
granular granular granular granular granular liquid granular granular liquid
total LAB concn 20.6, 43.2, 47.1,b 192, 204 77.1 30.8 5040 21.7 52.4 556 97.0 61.2 1440
a Analytical details t o be published separately. value based on five replicate analyses,
Mean
residual LABS that could conceivably enter waste streams via domestic detergent use with data on the output of LABS from the Los Angeles County wastewater treatment system. Table I gives the results of the detergent survey. The products tested not only include the major selling brands but also represent a cross section of detergent types. With the exception of detergent 4, the major granular detergents contain LABS at concentrations ranging from 20 to 200 wg g-l. By comparison, the LAB contents of the liquid concentrates appear to be substantially higher. It is also clear that there is probably considerable sample-to-sample variability for any given brand on the basis of our analysis of the five samples of detergent 1. Informed sources in Enviton. Sci. Technol., Vol. 17, No. 9, 1983 525
Table 11. Concentrations (fig/L) of Total Linear Alkylbenzenes (C,,.,,-substituted) in the Los Angeles County (JWPCP) Final Effluent, 197ga
date sampled
a
total LAB concn
Jan 1979 107.7 Feb 1979 242.0 Mar 1979 98.0 Apr 1979 140.4 May1979 78.4 June1979 302.5 Analytical details t o be
date sampled
total LAB concn
July 1979 113.6 Aug 1979 142.4 Sept 1979 155.4 Oct 1979 87.2 g e c 1979 173.0 X +S 149.2 rt 68.8 published separately.
the detergent industry (25)state that the per capita usage of detergents is approximately 266 g/ (capita week). Applying the concentration range of the major granular detergents that we found (20-200 pg of LAB/g of detergent) to this usage rate, one calculates that approximately 5.3-53 mg of LAB/(capita week) should enter waste streams as a result of detergent usage, assuming no losses. Table I1 lists the data for the LAB content of the JWPCP final effluent. The average concentration in 1979 was approximately 150 pg of LAB/L of effluent. When this value is combined with the daily effluent outflow of 1.39 X lo9 L dag1 and the population served by this plant (3.65 million (2)), an average weekly per capita mass emission rate of 400 mg of LAB/(capita week) is obtained. Thus, even assuming that no residual LABS are lost during or after detergent use, our calculations indicate that, at most, 13% of the LABS found in the JWPCP wastes can be accounted for by introduction of unsulfonated residues. These results suggest that other sources of LABS may contribute to the wastewater LAB burden. Because LAS production is the only end use for LABs, the two most obvious mechanisms for generating LABs that one might consider are microbial and chemical desulfonation of LAS. As for microbial desulfonation, laboratory and field studies of LAS metabolism in aerobic systems have shown repeatedly that microorganisms favor breakdown of the hydrocarbon side chain (via w-oxidation and progressive @-cleavage,for example) over removal of the sulfonate group during primary biodegradation (18,26). This leads to the formation of various sulfophenylcarboxylicacids as transient intermediates (18,26-29). In certain cases where only pure cultures have been studied (cf. ref 21), LAS desulfonation has, in fact, been observed during early stages of biodegradation. However, the stable intermediates were either hydroxylated or in one case (cf. ref 30) nonhydroxylated benzenecarboxylic acids, not hydrocarbons. We know of no published data demonstrating the microbially mediated reductive desulfonation of LAS without concomitant or prior o-oxidation and cleavage of the alkyl side chain. Thus, in view of the large body of evidence showing oxidative metabolism of LAS, an origin for the LABs by reductive microbial desulfonation seems highly unlikely. Ishiwatari et al. (12) reached similar conclusions in their study of Tokyo Bay sediments. The only remaining possibility is chemical desulfonation of LAS, which can be occur because of the reversibility of the sulfonation/desulfonation reaction. Under conditions of high temperature (100-175 "C) and an acidic medium, formation of the free hydrocarbonsfrom the corresponding sulfonates is promoted. At the present time we have no data suggesting either that such conditions are attained in washers or industrial settings or that this reaction does pccur during or after detergent use. Thus, the question of LAB generation via chemical desulfonation remains open. 526
Table 111. Concentrations of LAB, Total Hydrocarbqhs, and ZDDT in JWPCP Effluent Particulates and Surface Sediments (0-2 c m ) at Stations 3C1 and SPBl
Environ. Scl. Technol., Vol. 17, No. 9, 1983
JWPCP effluent particulates sediments 3C1(0-2 cm) SPBl (0-2 cm) concentration ratios JWPCPl3Cl JWPCPjSPBl
total hydrocarbons, XDDT, ZLAB, mg/dry wg/dry a/dryg g g 1342 59.9 31.2 21.4 1.1
62.7 1220
7.1 2.4
11.0 2.6
8.4 25.0
2.8 12.0
Behavior of Linear Alkylbenzenes during Sedimentation. The quantitative and qualitative data we have collected thus far suggest that the linear alkylbenzenes discharged with wastes undergo substantial alteration during sedimentation. Because the estimated sedimentation rate of refractory effluent particulate matter is far greater (8-630 times; cf ref 31) than that of natural particulates on the San Pedro Shelf, the input of waste-derived organics to surface sediments should dominate inputs from natural sources. This expectation is fully supported by a number of organic geochemical studies that have been performed in the area of the San Pedro Shelf and Basin (11,14,32,33). Assuming that the uppermost sediments represent mostly recently deposited particles, a simple comparison of the concentrations for any waste-related parameter on effluent particulates and surface sediments should reveal fractionations that occur during the sedimentation process. Table I11 presents the concentrations of total hydrocarbons, CDDT and CLAB for JWPCP effluent particulates and surface sediments (0-2 cm) at stations 3C1 and SPB1. Effluent/surface sediment concentration ratios for the three parameters are also provided. For both the 3C1 and SPBl sediments the ratios rank in the order CDDT < total hydrocarbons < CLAB, with CLAB values being 7-100 times greater. This suggests that the LABs are fractionated (i.e., lost) relative to either the total hydrocarbons or CDDT during passage through the water column. In addition, the concentration ratios are always greater for the SPBl sediment than those measured for station 3C1. This effect may be related to the difference in the settling times of the waste particulates deposited at each station. Such an interpretation is consistent with the fact that the San Pedro Basin (SPB1) sediments are located farther away from the outfall system and clearly away from the trajectory of the prevailing (northwesterly trending) subsurface currents (34). Inspection of the C12-LAB isomer distribution for these samples (Figure 4a) sheds some light on one of the possible causes for this apparent fractionation. A progressive relative enhancement of the internal isomers (e.g., 6phenyldodecane) at the expense of the external isomers (e.g., 2-phenyldodecane) is seen on going from waste effluent to suspended ocean particulates to the surface sediments. Moreover, the pattern is developed to a greater extent for the SPBl surface sediments than those collected at 3C1. This sequence is completely analogous to that observed for LAS surfactants during primary biodegradation (18, 24, 35). However, it raises an interesting question concerning the mechanism of LAS metabolism. Swisher (35) suggested that the greater rates of biodegradation for external LAS isomers (relative to internal
marine sediments. The results of our analyses for the long-chain alkylbenzenes in the 3C1 sediment core are presented in Figure 5a. Two varieties of alkylbenzenes were found in these sediments (cf. Experimental Section) corresponding to (1)the linear alkylbenzenes (LABS) and (2) the tetrapropylene-based alkylbenzenes (TAB) having branched side chains. Whereas the former are currently used in the manufacture of LAS surfactants, the TABS are hydrocarbon precursors employed in the production of the branched ABS surfactants no longer used in the United States (cf. ref 9). The LAB profile (Figure 5a) shows a subsurface maximum at the 2-4-cm interval followed by a continuous decline in concentrations with depth until the 16-18-cm section is reached. TAB concentrations maximize at 12-14 cm, decreasing with approach to both the surface and the 22-24-cm interval. The trace amounts of alkylbenzenes found at sediment depths below 24 cm are believed to be due to contamination from the upper layers via bioturbation, diffusion through pore waters, or the coring/sectioning procedure. No alkylbenzenes of either type were found in the procedural blank. Parts b and c of Figure 5 present data on the total hydrocarbon and CDDT content of these sediments, which are of use in interpreting the alkylbenzene profiles. It is generally recognized that the concentration of any geochemical component at some depth in a sediment column is the net result of i n p u t and post-depositional alteration. Therefore, we will attempt to rationalize the alkylbenzene profiles at station 3C1 by considering the following: (1)known historical facts surrounding the operation of the JWPCP outfall system, (2) data on historical surfactant usage, and (3) inferences from the organic geochemical record as a whole. The input of alkylbenzenes (LAB and TAB) to these sediments should depend on both the historical discharge of wastes via the outfall system and variations in surfactant (Le., detergent) usage. Figure 6a presents data on the emission rate of suspended solids from the JWPCP outfall system from 1946 to the present. From 1946 until 1971 emissions increased steadily. Thereafter, stabilization in the urban population and improvements in source control and waste treatment resulted in a decline that continues to the present time. One would expect that oil and grease (and thus total hydrocarbon) emissions would follow the same general trend because these two organic fractions comprise roughly 22% and 7% of the total solids, respectively (2). Unfortunately, a historical correlation cannot be confirmed for the entire period of discharge because monitoring data for oil and grease concentrations were collected only after 1971 (cf. Figure 6a), and total
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Phenyl Position of PhenyldodecaneIsomers Figure 4. Isomeric distributions of phenyldodecanes: (a) sewage effluent, suspended marine particulates, and surface sedlments taken in the vicinity of a major waste outfall system: (b) sectlons from sediment core 3C1.
isomers) might be due to fixation of the sulfonate group on the oxidative enzyme a certain distance away from the enzyme site where w-oxidation of the alkyl chain was initiated. Because of this geometric constraint, isomers with longer chains (i.e., external isomers) could undergo w-oxidation more readily because of their ability to span the distance. As the LABS contain no hydrophilic &e., sulfonate) group, this mechanism of sulfonate fixation cannot explain the progressive depletion of external LAB isomers with exposure to the environment that we observe. Either a separate mechanism producing the same effect (e.g., ring fixation) is operative for the LABS or sulfonate fixation does not play a role in (isomer-selective) LAS biodegradation. Although we cannot yet offer an alternative mechanism for the apparent fractionation of the LABS that occurs during sedimentation, selective microbial metabolism is probably involved. Once deposited, the LABS undergo further depletion of the external isomers with depth in the sediments as demonstrated in Figure 4b. The changes, however, appear to be less pronounced. The susceptibility of the linear alkylbenzenes to removal would seem to limit their utility as waste tracers to the general environs of a sewage disposal system. Nevertheless, the differential activities of the LAB isomers may provide an indication of the extent to which waste hydrocarbons have been altered during early diagenesis. Long-chain Alkylbenzenes in Marine Sediments. As stated earlier, the principal aim of this study was to determine if the linear alkylbenzenes could accumulate in
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Environ. Sci. Technol., Vol. 17, No. 9, 1983 527
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