Environ. Sci. Technol. 1987, 21, 648-653

(2) Chrisp, C. E.; Fisher, G. L.; Lammert, J. E. Science (Washington, D.C.) 1978, 199, 73-74.

(3) Lao, R. C.; Thomas, R. S. In Polynuclear Aromatic Hy- drocarbons; Bjorseth, A.; Dennis, A. J., Eds.; Battelle: Columbus, OH, 1979; pp 829-839.

(4) Cope, V. W.; Kalkwarf, D. R., unpublished results. (5) Committee on Biological Effects of Atmospheric Pollutants

Particulate Polycyclic Organic Matter; National Academy of Sciences: Washington, DC, 1972.

(6) Pitts, J. N., Jr.; Lokensgard, D. M.; Ripley, P. S.; Van Cauwenberghe, K. A,; Van Vaeck, L.; Shaffer, S. D.; Thill, A. J.; Belser, W. L., Jr. Science (Washington, D.C.) 1980,

(7) Fishbein, L. In Chemical Mutagens; Hollaender, A., Ed.; Plenum: New York, 1976; Vol. 4, p 219.

(8) Cupitt, L. T. Fate of Toxic and Hazardous Materials in the Air Environment; Report to Environmental Sciences Research Laboratory: Research Triangle Park, NC, 1980;

(9) Jager, J. Fresenius' 2. Anal. Chem. 1971, 255, 281-284. (10) Pierce, R. C.; Katz, M. Environ. Sci. Technol. 1976, 10,

(11) Korfmacher, W. A.; Natusch, D. F. S.; Taylor, D. R.; Ma- mantov, G.; Wehry, E. L. Science (Washington, D.C.) 1980,

(12) Rappaport, S. M.; Wang, Y . Y.; Wei, E. T.; Watkins, B. F.; Sawyer, R.; Rappaport, H. Environ. Sci Technol. 1980,14,

(13) Epstein, S. S.; Mantel, N.; Stanley, T. W. Enuiron. Sci. Technol. 1968,2, 132-138.

(14) Korfmacher, W. A.; Wehry, E. L.; Mamantov, G.; Natusch, D. F. S. Enuiron. Sci. Technol. 1980, 14, 1094-1099.

(15) Pitts, J. N., Jr.; Van Cauwenberghe, K. A.; Grosjean, D.; Schmid, J. P.; Fitz, D. R.; Belser, W. L., Jr.; Knudsen, G. D.; Hynds, P. M. Science (Washington, D.C.) 1978, 202,

210, 1347-1349.

EPA-600/3-80-084,

45-51.

207, 763-765.

1505-1509.

515-519.

(16) Inscoe, M. N. Anal. Chem. 1964, 36(13), 2505-2506. (17) Zepp, R. G.; Schlotzhauser, P. F. In Polynuclear Aromatic

Hydrocarbons; Jones, P. W.; Leber, P., Eds.; Ann Arbor Science: Ann Arbor, MI, 1979; pp 141-158.

(18) Hughes, M. M.; Natusch, D. F. S.; Taylor, D. R.; Zeller, M. V. In Polynuclear Aromatic Hydrocarbons; Bjorseth, A.; Dennis, A. J., Eds.; Battelle: Columbus, OH, 1979; pp 1-8.

(19) Bowen, E. J. Adu. Photochem. 1963, I , 23. (20) Peters, J.; Seifert, B. Atmos. Environ. 1980, 14, 117-119. (21) Thomas, J. F.; Mukai, M.; Tebbens, B. p. Enuiron. Sci.

Technol. 1968, 2, 33-39. (22) Pitts, J. N., Jr.; Harger, W.; Lokensgard, D. M.; Fritz, D.

R.; Scorziell, G. M.; Mejia, V. Mutat. Res. 1982,104,35-41. (23) Katz, M.; Chan, C.; Tosine, H.; Sakuma, T. In Polynuclear

Aromatic Hydrocarbons; Jones, P. W.; Leber, P., Eds.; Ann Arbor Science: Ann Arbor, 1979; pp 171-189.

(24) Korfmacher, W. A.; Wehry, E. L.; Mamantov, G.; Nutusch, D. F. S. Environ. Sci. Technol. 1980, 14(9), 1094-1099.

(25) Daisey, J. M.; Lewondowski, G. G.; Zorz, M. Environ. Sci. Technol. 1982, 16(12), 857-861.

(26) Vollman, H.; Becker, H.; Corell, M.; Streeck, H.; Langbein, G. Justus Liebigs Ann. Chem. 1937, 531, 1.

(27) Fatiadi, A. J. Environ. Sci. Technol. 1967, 1(7), 570-572. (28) Fatiadi, A. J. J. Chromatogr. 1965, 20, 319-324. (29) Thekaekara, M. P. In Solar Energy Engineering; Sayigh,

(30) Okabe, H. Photochemistry of Small Molecules; Wiley-In-

(31) Inn, E. C. Y.; Tanaka, Y. Adv. Chem. Ser. 1959, No. 21,263. (32) Butler, J. D.; Crossley, P. Atmos. Enuiron. 1981,15,91-94.

A. A. M., Ed.; Academic: New York, 1977; pp 40-41.

terscience: New York, 1978; pp 237-249.

Received for review July 3,1986. Accepted March 9,1987. This work was supported by the U.S. Department of Energy under Contract DE-ACO6- 76RLO-1830 to Pacific Northwest Labora- tory.

Persistence of Benz[ a ]anthracene Degradation Products in an Enclosed Marine Ecosystem

Kenneth R. Hinga" and Michael E. Q. Pilson

Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882

Carbon-14-labeled benz[a]anthracene was introduced into an enclosed marine ecosystem that had planktonic primary production and a heterotrophic benthos. Benz- [alanthracene, labeled COz, and operationally defined fractions of labeled degradation products were followed in water and sediments for 202 days. The major fraction of intermediate degradation products was sufficiently water soluble so as not to be readily extractable with or- ganic solvents and at the end was still slowly decaying to COz. Both the parent benz[a]anthracene and degradation products found in the sediment appear to become pro- tected from further alteration after about 2 months and may persist indefinitely.

Introduction Polycyclic aromatic hydrocarbons (PAH) include car-

cinogenic and toxic compounds. Anthropogenic activities, especially combustion, have greatly increased the flux of PAH to the environment (I). Some pathways for PAH introduction to the marine environment have been quan- tified (2-7), and the presence of PAH in marine systems, especially those close to populated areas, is well docu- mented (i.e., ref 8 and 9).

PAH in the environment may have a direct toxicity, but the primary concern is that they may be acted upon by

mammalian enzymes to become carcinogenic compounds. In the environment, PAH may be photodegraded and biologically degraded, leading to intermediate products before complete remineralization. These intermediate products, if persistent, may present their own health or environmental hazard. For example, fungi produce initial degradation products from PAH that are very similar to the products responsible for carcinogenesis in mammals (10). Biological degradation pathways of smaller aromatics, which can be utilized as sole carbon sources by bacteria, have been determined, but only the initial steps in the degradation of larger PAH, which are only degraded through cometabolism, have been established (11 1. Sim- ilarly, the initial products of PAH photodegradation have been described (1 ,2) , but the behavior of the breakdown products in natural environments is not known.

Two previous experiments were conducted with radio- labeled PAH in enclosed marine ecosystems to measure the rate of disappearance of the parent and the reminer- alization rate to C 0 2 under near natural conditions (12, 13). The chemical fractionation used iu these experiments has also permitted a glimpse of the behavior of interme- diate products. In both these experiments we noted that intermediate degradation products of four-ringed ?AH, benz[a]anthracene and 7,12-dimethylbenz[a]anthracene, were found at the end of the 230- and 60-day experiments,

648 Environ. Sci. Technol., Vol. 21, No. 7, 1987 0013-936X/87/0921-0648$01.50/0 0 1987 American Chemical Society

respectively (12, 13). In a third experiment with benz- [alanthracene, described here, we document in greater detail the long-term persistence of degradation products in sediments and report for the first time water-soluble degradation fractions of moderate persistence.

Experimental Section

Enclosed Ecosystems. The Marine Ecosystems Re- search Laboratory (MERL) a t the University of Rhode Island maintains 14 large microcosms and is licensed for radiotracer work in four of these. Each microcosm is a fiberglass tank, 1.8 m in diameter and 5.5 m in height, containing 13 m3 of seawater and a 35-cm layer of sedi- ment. Unmodified sediments and water were taken from adjacent lower Narragansett Bay, a fairly typical New England coastal ecosystem, to initiate this experiment. The tank was operated in batch mode with water input or drain only to compensate for periods of excess evapo- ration or precipitation and was mixed 2 h out of every 6 h with a vertical plunger. The tanks are outdoors, exposed to ambient sunlight, and maintained a t temperatures within 2 "C of the adjacent bay.

The fundamental assumption for this type of experiment is that the ecosystem enclosed in the tank has the same components and processes that occur on a chemical scale in the natural coastal system. A number of tank-bay comparisons documenting the similarity between the ex- perimental and the reference natural ecosystem are available (14-26). In spite of some differences in physical properties between tanks and bay, all measured chemical and most biological parameters (e.g., large predators are excluded) fall within the range of measured values for lower Narragansett Bay throughout the year. MERL microcosms have now been operated for 10 years, up to 2 years on one loading of sediment, and continue to appear to contain ecosystems that are reasonable representatives of Narragansett Bay. Most processes that may act on PAH and their derivatives in a coastal system one therefore expects to be present in the MERL tanks and quantita- tively similar to those in lower Narragansett Bay. Ex- trapolating the results from MERL/bay to other envi- ronments requires the same judgements as deciding the appropriateness of applying results obtained in any en- vironment to another. I t should be noted that on time scales less than about 15-30 days there can be considerable variations between individual tanks and the bay and be- tween different parts of the bay, in such parameters as the initiation, composition, and duration of blooms, as may be expected from the variability in natural systems. These experiments also do not directly address processes such as large-scale horizontal transports of particle-bound pollutants that may affect their distribution in coastal or ocean systems.

Spike and Operation. On February 23,1982,700 pCi of [12-14C]benz[a]anthracene (Amersham Corp., 49 mCi/mmol) was introduced into a MERL tank within an oil-water mixture LO give a starting activity in water of 1.19 X lo5 dpm L-l. Prior to use the benz[a]anthracene was charged to a silica gel chromatography column, washed with hexane, and eluted with 4: 1 hexane-methylene chloride. No detectable labeled impurities were observed after thin-layer chromatographic analysis of the purified compound, nor were any other PAH found by GC-MS chromatography of the cleaned compound. Preparation of the carrier (15 L of seawater plus about 1 g of No. 2 fuel oil) used for introduction to the tank was previously de- scribed (12). The experiment was run for 202 days. Be- tween days 168 and 173 the water in the tank was partially

drained and replaced with fresh bay water 3 times. This procedure reduced the concentrations of water-soluble labeled fractions in the tank to about 10% of their former levels.

The benz[a]anthracene added to the tank, 3.5 mg, gave an initial concentration of 270 ng L-l, which represented 1% of the seawater saturation solubility a t the salinity (about 30%) and the beginning temperature of the ex- periment (27). Rates of microbial attack on PAH have been found to be elevated in sediments subject to chronic hydrocarbon and PAH contamination (28-30), so the spike was kept to a small size to minimize the possibility of stimulating a high rate of microbial degradation. The concentration of labeled benz[a]anthracene achieved in the experimental sediments was lower than that of normally occurring benz[a]anthracene in the area where the sedi- ments used in this experiment were collected (31). The small size of the spike relative to the ambient background materials as found in these experimental conditions did not facilitate identification of unknown compounds. Identification of specific degradation products was not an objective of these experiments.

Sample Collection and Processing. All water column samples were taken during a mixing cycle to ensure that the water column was homogeneous during sampling. Cores were taken with a 2.5 cm diameter stainless steel interface retaining corer (32) and then frozen until ex- traction. Figure 1 provides a summary of the detailed procedures listed below and identifies the fractions dis- cussed later.

Water samples (1-2 L, unfiltered) were extracted 3 times with chloroform (20 mL/L), the activity in an aliquot of each extract was counted, and the samples were combined. Beginning on day 20 the water samples were acidified to pH 2 after the initial extractions and extracted twice more with chloroform, and the additional aliquots were counted.

Samples of particulate material were obtained by fil- tering 0.5-2 L of water through a 47 mm diameter glass- fiber filter (Whatman GF/A). The filters were placed in 10 mL of scintillation fluid and counted. Duplicate sam- ples were extracted in a single-phase chloroform-metha- nol-water solution for 8 h, then the solution was converted to two phases by the addition of water, and aliquots of both the chloroform and methanol-water fractions were coun- ted.

Total COz was collected by transferring 500-mL water samples to a bottle fitted with two suspended wells, each containing a piece of filter paper and 0.2 mL of phen- ethylamine, and then acidified to a pH of <2.0. The sample was stirred for 2-4 h, then the filters were replaced with a fresh filter paper with phenethylamine, and the sample was extracted for an additional 2-4 h. Both sets of filter papers were counted. About 1% of the total 14C02 recovered was typically found in the second extraction. The concentrations of labeled COz present in the tank were converted to labeled COz produced up to the sampling date by correcting for loss to the atmosphere and primary production, which has been empirically determined for the MERL tanks (33).

Total non-C02 activity in water samples was measured by c.ombining 10 mL of water, previously acidified and stirred to remove COz, with 10 mL of scintillation fluid in a scintillation vial. The total activity in the water after chloroform extraction and the activity remaining in water after acidification and chloroform extraction were mea- sured by this technique.

Half the cores were processed by extruding the sediment and slicing in 0.5- or 1.0-cm intervals to 6 cm of depth. The

Environ. Sci. Technol., Vol. 21, No. 7, 1987 849

add c o l l e c t CO on C02 e I 3 H a c i , I p h e n e t l y l a i i n e k

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f i l t e r on p a r t i c l e s A g l a s s f i b e w

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TOTAL ON P A R T I C L E S

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BAND 1

O R I G I N

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METHANOL FRACTION

Flgure 1. Procedures. A summary of procedures used to extract and fractionate labeled compounds. Solid circles indicate where liquid scintillation counting occurred.

top 6 cm of the other cores was extracted as single samples. The results of previous experiments (12,13) and the sec- tioned cores from this experiment indicate that very little of the label penetrated to 6 cm of depth in the sediment. Sediment was extracted by refluxing with 4 mL of toluene and 4 mL of KOH in methanol for each 1 cm3 of sediment. Aliquots of both the methanol and toluene fractions were counted.

Aliquots of the chloroform extracts of water and sus- pended particles and of the toluene fraction of the sedi- ment extract were reduced in volume, applied to silica gel thin-layer chromatography plates, and run in a toluene- hexane, 3:7, solvent system. Each plate was sprayed with a fluor, placed in contact with a sheet of X-ray film, and exposed for 3-14 days. The autoradiographs were used to identify the locations of individual bands, which were then scraped off the plates and counted.

All counts of 14C were corrected for background, de- termined for each type of sample, and for counting effi- ciency for each sample (33).

Results Water Column, Solvent-Extractable Compounds.

The compounds extracted from the water at seawater pH were separated into three fractions by thin-layer chro- matography. These were the parent benz[a]anthracene, a band on the thin-layer chromatography plates at R, 0.12 (band l), and the material remaining at the origin of the thin-layer chromatography plate (origin). Benz[a]- anthracene concentrations decreased rapidly so that by day 30 less than 1% of the benz[a]anthracene added to the system remained in the water column while band 1 and the origin initially increased and then decreased (Figure 2). Discussion of the observed rates of benz[a]anthracene removal and transformation may be found elsewhere (33).

Band 1 was not further separated in a variety of solvent systems on silica gel thin-layer chromatography and is likely to be a single compound. An attempt to identify the compound by GC-MS (courtesy of U.S. EPA, Narragan- sett Laboratory) revealed that the spectrum of benz[a]- anthracene-7,12-dione contained 3-4 times the expected counts in the base plus two peak. This is consistent with the 14C-labeled 7,12-dione coeluting with a greater normal background concentration of unlabeled compound.

850 Environ. Sci. Technol., Vol. 21, No. 7, 1987

25t I

a

P LL 0 rp

D A Y

Flgure 2. Water column, solvent-extractable compounds. Percent of the total label added to the tank, extracted from seawater with chlo- roform, and found as benz[a]anthracene (0), band 1 (V), and ma- terials remaining at the origin of thin-layer chromatography separation (+I.

Thin-layer chromatography of the origin in a second solvent system (acetone-hexane-acetic acid, 70:30:1) re- sulted in separation into three poorly resolved bands. (The large amount of extraneous organic material carried in the extract interfered with solvent advance on the TLC plate.)

Sediment Extracts. About 13% of the benz[a]- anthracene added to the system was found in the sedi- ments early ip the experiment (Figure 3a). Between days 37 and 68 the amount of labeled benq[a]anthracene de- creased to about 6% of the total added and remained constant for the duration of the experiment.

The same band 1 that was found in the water column early in the experiment was also found in the sediments. Band 1, origin materials, and compounds partitioning to methanol all increased initially and then remained con- stant, within analytical and sampling uncertainty, for the latter half of the experiment (Figure 3a,b).

Suspended Particles. The labeled compounds ex- tracted from suspended particles consisted of benz[a]-

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D A Y Figure 4. Water-soluble organic fractions and CO,. Percent of the total label added to the tank found as not readily chloroform-extractabie compounds or CO,: (a) total label remalnlng in water after chloroform extractions (O), label remaining in water after acidification and chlo- roform extraction (+), and label removed by chloroform extraction after acidification (by difference) (A); (b) label appearing as COP

the partitioning to the organic phase may not be equivalent for the two different extraction techniques (methanol being present in the particulate extraction). Nevertheless, the calculated fraction of origin compounds on particles steadily decreased from 35% to 15% by day 17. This is consistent with multiple compounds in the origin and a removal of the more strongly particle-associated com- pounds with time. Presumably the more particle-bound compounds are subject to a faster removal rate through particle scavenging and sedimentation.

Water-Soluble Organic Fractions and COz. The non-C02 label remaining in the water after the chloroform extractions reached a maximum of approximately 30% of the label added to the tank on about day 70 (Figure 4a). The presence of significant amounts of label remaining in the water after extraction was not expected so procedures to further fractionate and study these materials were not initiated at the start of the experiment. Starting on day 20 the water previously extracted under neutral conditions was acidified and reextracted. This procedure typically removed one-third of the label remaining after the initial extraction and represented 6-10% of the label added to the system (Figure 4a). Inspection of the amount of label in each sequential aliquot of the chloroform extractions (three a t seawater pH, two after acidification) indicates that the material extracted after acidification represents a distinct fraction and is not simply a tailing from incom- plete extractions. The label remaining in the water after

r

0 1 ' I ' 1 ' I " ' I I 0 40 80 120 160 200

DAY

Figure 3. Sediment extracts. Percent of the total label added to the tank extracted from sediments and found as (a) benz[a ]anthracene (0) and band 1 (V) and as (b) label partitioning to the methanol phase of sediment extract (0) and remaining at origin layer of thin-layer chromatography separation (A). Center points are the average of two or four replicate cores. The bars indicate the range of replicate cores. The Inner marks on the range bars, where outside the size of the center point, indicate the range that results just from the variability in the percentage found in the different fractlons in the replicate cores. Most of the total range is due to the high variability in total inventories in replicate cores. A much smaller range results from differences in the relative amounts in the four fractions In replicate cores. Water temperatures and months are shown in (c).

anthracene, the same band 1, and a set of compounds remaining a t the origin of the thin-layer plate. The con- centrations of benz[a]anthracene and band 1 on suspended particles were a constant fraction of the total extracted from water (discussed above) up to the last particulate sample taken on day 17. During this time, the total benz[a]anthracene concentration varied by a factor of 100 and that of band 1 by a factor of 20. With an average suspended load of particles in the MERL tanks of 3 mg L-l, partitioning coefficients are calculated to be 2.8 X lo5 mL g-' for benz[a]anthracene and 5.0 X lo4 mL g-' for band 1. These represent 46% and 12% of the total on particles, respectively, and for benz[a]anthracene this is similar to partitioning coefficients previously reported for marine particles (34).

I t is not completely valid to compare the TLC origin materials extracted from particles to those extracted from the water. Compounds remaining a t the origin of the thin-layer plates are relatively polar and may not have a high efficiency of extraction into organic solvents. Thus,

Environ. Sci. Technol., Vol. 21, No. 7, 1987 651

both sets of extractions reached a maximum of about 20% of the label added to the system on about day 60 (Figure 4a).

Labeled C02 was produced continuously during the experiment and accounted for 44% of the label added to the tank by day 163 (Figure 4b). The tank water was exchanged between days 163 and 170 with unlabeled sea- water, so that concentrations of labeled compounds in the water column were reduced by 90%. The rate of labeled C 0 2 production then dropped by an equivalent amount. Extrapolating the rate of COz production before the water exchange predicts that 55% of the label added to the tank would have been remineralized to C02 by day 202.

Discussion The labeled benz[a]anthracene added to the system was

lost rapidly from the water column. Volatilization of benz[a]anthracene is far too slow to be significant in systems configured like the MERL microcosms (35). La- beled benz[a]anthracene found in the sediments must have been scavenged from the water column. This is expected for a compound that strongly associates with particles (20). Some benz[a]anthracene was probably photodegraded to produce the solvent-extractable compounds found early in the experiment. On the basis of laboratory measure- ments, biological degradation of benz[a]anthracene in water either does not occur or is slower than the technique can measure, about 0.001% per day (30, 34, 36). The observed initial rate of degradation product appearance is also roughly consistent with predictions of the rate of benz[a]anthracene photodegradation (37), as is the ap- pearance of multiple compounds ( I , 2, 38). Finally, as discussed earlier, band 1 is likely to be a dione, which is one type of compound expected to be produced by pho- todegradation ( 1 , 2 , 38). The later disappearance of the band 1 and origin materials from the water may result from sedimentation, microbial degradation, or further photo- degradation. From this experiment it is not possible to quantify the relative importance of the three processes.

Interpretation of the behavior of sediment-bound breakdown products is hindered by the high variability, often a factor of 2, in the total inventory of label in rep- licate cores. This high variability is a common feature of this type of experiment (12, 13) and appears in spite of active bioturbation and the relatively long experimental period. Since the tank water is well mixed, the variability between cores is likely a result of bioturbation occurring at different rates at locations separated by as little as a few centimeters. The relative proportions of label in the fractions of sediment extracts were much more uniform between replicate cores than were total inventories. The processes altering the compounds within the sediments appear to be more spatially uniform than the initial in- corporation.

The decrease in benz[a]anthracene between days 38 and 68 is evidence for microbial degradation of benz[a]- anthracene in sediments. Assuming a linear rate between these days, the rate of benz[a]anthracene degradation was 1.7% per day, which is similar to rates reported for short-term bottle experiments (29, 39,40) . Degradation products in the sediments may result both from scavenging water column produced products and by production within sediments.

A remarkablu feature of the data is that neither benz- [alanthracene nor the degradation products appear to further degrade after a couple of months in the sediments. Given the high variability in inventories between replicate cores, some decrease in these fractions cannot be ruled out. Since the origin and methanol fractions may consist of

652 Environ. Sci. Technol., Vol. 21, No. 7, 1987

multiple compounds, there may also be some unobserved changes within these fractions. However, the four sedi- ment fractions were in nearly the same portions in each core taken on or after day 67. The simplest interpretation is that there is no, or an extremely slow, alteration of the sediment-bound fractions after this time. While a halt in degradation is consistent with the observation that PAH may be found deep in old sections of cores, the mechanism of protection provides an interesting puzzle.

Temperatures were increasing during the experiment (Figure 3c) so a seasonal cooling did not slow the rate of degradation of compounds. The degradation of PAH and many other compounds has been found to be slowed if not altogether stopped in anaerobic conditions (1 1). Sediments in the MERL tanks are anaerobic below a few millimeters. However, a t least smaller aromatics are subject to anae- robic degradation including ring cleavage (41), and an upper portion of the sediments in the MERL tanks remain oxidized. It may be necessary to invoke other mechanisms to account for the permanence of PAH in natural sedi- ments. A consequence of the halt in the breakdown of benz[a]antrhacene is that the results of short-term ex- periments can not alone accurately reflect microbial deg- radation of PAH in sediments.

The presence of moderately persistent and relatively abundant water-soluble benz[a]anthracene products was not expected. So far as we known there is no previous report of such compounds found in other PAH degradation experiments. [Techniques were used by Atlas et al. (39) that should have quantified water-soluble products in short-term biodegradation experiments, but this fraction was not reported.] This fraction may have been formed by the degradation of the solvent-extractable fraction or by degradation within sediments and release to the water column. In addition, the total label accounted for in the system relative to that added dropped to about 80% shortly after the start of the experiment and then increased to near 100% by the end. This indicates that there may have been an initial sorption to the tank walls, which could have been another contributor to the water-soluble frac- tion. During the later third of the experiment, the rate of decrease of the water-soluble fraction was about equal to the rate of labeled C 0 2 production. Draining the tank to reduce the concentration of water-soluble compounds in the water reduced the labeled COz production by es- sentially an equivalent amount. Since there were no dis- cernible changes in sediment inventory during the later half of the experiment and no solvent-extractable fraction remained, it is clear that during this period labeled C02 production must have been supported primarily by the remineralization of the water-soluble compounds. As- suming the remainder of the water-soluble fraction would be subject to remineralization at the same rate as found during the last third of the experiment, this fraction would be exhausted and labeled C 0 2 production would cease by about day 260. The distinct changes in the rate of labeled COz production occurring twice in the experiment (Figure 4b) may represent the appearance and later exhaustion of various compounds.

Conclusions (1) The rate-limiting step for the remineralization of

PAH in the environment is not the initial degradation step. Both photodegradation in the water and microbial decay in sediments occur faster than remineralization. This results in the accumulation of intermediate products.

(2) After about 2 months, some unknown mechanism acts to stop, or greatly slow, the degradation of both the parent and intermediate products in the sediments.

Therefore, some portion of the parent and its degradation products may be preserved indefinitely in sediments.

(3) A group of persistent intermediate products are produced that are sufficiently water soluble so as not to be readily extractable by organic solvents. These water- soluble products require months to remineralize.

Registry No. Cot, 124-38-9; benz[a]anthracene, 56-55-3.

Literature Cited (1) National Academy of Science Particulate Polycyclic Or-

ganic Matter; National Academy of Sciences: Washington, DC, 1972.

(2) Neff, J. M. Polycyclic Aromatic Hydrocarbons in the Aquatic Environment; Applied Science: Barking, England, 1979.

(3) Barrick, R. C. Environ. Sci. Technol. 1982, 16, 682-692. (4) Gschwend, P. M.; Hites, R. A. Geochim. Cosmochim. Acta

(5) Hoffman, E. J.; Mills, G. L.; Latimer, J. S.; Quinnn, J. G.

(6) Hamilton, S. E.; Bates, T. S.; Cline, J. D. Environ. Sci.

(7) Lopez-Avila, V.; Hites, R. A. Enuiron. Sci. Technol. 1980,

(8) Lafamme, R. E.; Hites, R. A. Geochim. Cosmochim. Acta

(9) Farrington, J. W.; Goldberg, E. D.; Risebrough, R. W.; Martin, J. H.; Bowen, V. T. Environ. Sci. Technol. 1983,

(10) Cerniglia, C. E. Adv. Appl. Microbiol. 1984, 30, 31-71. (11) Atlas, R. M. Microbiol. Rev. 1981,45, 180-209. (12) Hinga, K. R.; Pilson, M. E. Q.; Lee, R. F.; Farrington, J.

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Received for review June 16, 1986. Revised manuscript received January 27, 1987. Accepted March 15, 1987.

Environ. Sci. Technol., Vol. 21, No. 7, 1987 853