A record of polycyclic aromatic hydrocarbon (PAH) pollution obtained from accreting sediments of the Tamar Estuary, U.K.: Evidence for non-equilibrium behaviour of PAH

The Science of the Total Environment, 66 (1987) 73-94 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlands

73

A RECORD OF POLYCYCLIC AROMATIC HYDROCARBON (PAH) POLLUTION OBTAINED FROM ACCRETING S E D I M E N T S OF THE TAMAR ESTUARY, U.K.: EVIDENCE FOR NON-EQUILIBRIUM BEHAVIOUR OF PAH

J.W. READMAN and R.F.C. MANTOURA

The Institute for Marine Environmental Research, Prospect Place, The Hoe, Plymouth. Devon PLI 3DH (United Kingdom) M.M. RHEAD

Department of Environmental Sciences, Plymouth Polytechnic, Drake Circus, Plymouth, Devon PL4 8AA (United Kingdom) (Received November 9th, 1986; accepted J a n u a r y 8th, 1987)

ABSTRACT

Concentrations of polycyclic aromatic hydrocarbons (PAH) were quantified throughout a 21°Po-dated inter-tidal sediment core from the Tamar Estuary, U.K. in order to reconstruct the input history and investigate environmental reactivity of PAH in sediments. The profile recorded is similar to those reported in other aquatic sedimentary studies, with an approximately exponential increase in the concentrations of individual PAH from < 30 ng (g dry sediment) 1 prior to 1940 to between 100 and 1000 ng (g dry sediment)-1 in contemporary surface sediments. This corresponds to an increased input of total PAH from 0.23 to 21mg m 2 year-~. The PAH composition is dominated by parent compounds r a t h e r t h a n alkylated homologues and is characteristic of pyrogenic sources correlating with increased motor vehicle activity and road runoff into the Tamar. There is a remarkable compositional uniformity of PAH t h r o u g h o u t the polluted sediment core, indicating t h a t the biogeochemical transformation and exchange processes (sorption/leaching; microbial breakdown; photo-degradation; etc.) which are known to govern the fate of experimentally-added or petroleum-derived PAH, and which exhibit compound discrimination, appear not to affect PAH in the sediments. Using a linear free energy sediment-water exchange model to simulate the repartitioning and exchange of individual PAH between the surface-mixed layer of sediment and water, we demonstrate t h a t the current PAH concentrations in sediments are between 2 and 5 orders of magnitude greater t h a n those expected from equilibrium partitioning with observed water concentrations. This implies t h a t the PAH input to the sediments has been compositionally uniform and t h a t the PAH are chemically inert. Sorptive exchange with the aqueous phase and hence the potential bioavailability of PAH appear restricted by the existence of occluded and other micro-morphologically inert forms of particle-bound PAH.

INTRODUCTION

The use of geochronology to assess input histories of particle-associated pollutants has been successfully applied to trace metals (Goldberg et al., 1977)

0048-9697/87/$03.50

© 1987 Elsevier Science Publishers B.V.

74 and a variety of organic pollutants including pesticides, polychlorinated biphenyls, saturated hydrocarbons (Wade and Quinn, 1979; Venkatesan et al., 1980) and PAH (e.g. Muller et al., 1977; Hites et al., 1980; Wakeham et al., 1980a; Heit et al., 1981 and Ohta et al., 1983). Our previous studies of the estuarine chemistry of PAH (Readman et al., 1982, 1984a) indicate that PAH in the water column were present as either transient soluble/colloidal forms or as highly particle-associated forms. These 'species' probably related to petrogenic and pyrogenic sources respectively and are characterised by very different chemical and physical reactivities. The pyrogenic source dominates the distribution of PAH in the Tamar Estuary water column/suspended particulates and surface sediments. This form of PAH pollution consists of chemically inert constituent PAH which do not, for example, partition themselves between dissolved and particulate phases as observed in sorption experiments (with standard PAH) or as predicted by linear free energy theory of partition (Readman et al., 1984a). In this paper we examine the vertical distribution of PAH in a dated sediment core from the Tamar Estuary, Devon, U.K., to assess the input history of PAH and to examine the extent to which the detailed compositional record of PAH is consistent with the thermodynamically anomalous behaviour of PAH previously reported for the water column. MATERIALSAND METHODS Study area and sample collection The geographical location of the sample site is shown in Fig. 1. The Tamar catchment drains relatively pristine moorland. In contrast, urban Plymouth is situated adjacent to the lower estuary. The aquatic and surface sedimentary distributions of PAH throughout the Tamar Estuary have been described by Readman et al. (1982, 1984b). Clifton and Hamilton (1979) have previously obtained uniformly deposited inter-tidal sediment cores from the St. John's Lake area. Inter-tidal cores were obtained (October 1980) using hand held perspex (Decon 90 soaked, rinsed in distilled water) cylinders (6.6cm i.d. × 60cm). Once inserted, the tops of the cylinders were sealed using aluminium foil-lined plastic end-caps, and the surrounding mud excavated to allow withdrawal with minimum disturbance. Four replicate cores (stored vertically) were immediately returned to the laboratory. Methods X-ray morphological studies of the cores were performed upon return. The core showing least evidence of disturbance by benthic animals was transferred into a glove-box filled with aged nitrogen and the cylinder cut longitudinally on two opposing sides allowing removal of one-half of the liner for visual

75

o

PlymoUth

N

c~

Scale: o .........

i

2,

3,

Kilometres

Fig. 1. Description of the lower Tamar Estuary and sampling site. Urban regions in the area are indicated by open stipples. Dockyards located along the estuarine foreshore are indicated by hatched lines. The 1 km axial estuarine segment used in modelling sediment/water exchange of PAH is bordered by bold broken lines. Both boundary lines are marked with their respective distances from the weir at the top of estuary (29 and 30 km). i n s p e c t i o n of the sediment. T h e c o l u m n was m a r k e d for depth, and Eh determined t h r o u g h o u t (Pt/Ag/AgC1 electrode). Samples from the c e n t r a l p o r t i o n s (out of c o n t a c t w i t h the core liner surfaces) of selected individual sections were t r a n s f e r r e d into glass (solvent-rinsed) jars with a l u m i n i u m foil-lined screwcaps. E a c h sample was t h e n h o m o g e n i s e d and sub-samples t r a n s f e r r e d to prelabelled beakers. T h e glass jars were t h e n sealed and frozen a w a i t i n g P A H analysis. Weighed p o r t i o n s of the sub-samples were freeze dried to d e t e r m i n e w a t e r c o n t e n t . Aliquots of the dried samples were t h e n lightly c r u s h e d in a plastic ball mill for X-ray fluorescence (Si, A1, Zn, Cu), atomic a b s o r p t i o n (Pb) and ~l°Po analyses. A d d i t i o n a l sub-samples were finely g r o u n d in an a g a t e ball mill for c a r b o n and n i t r o g e n d e t e r m i n a t i o n w i t h p o r t i o n s being r e m o v e d for low t e m p e r a t u r e a s h i n g ( I n t e r n a t i o n a l P l a s m a C o r p o r a t i o n ) to d i f f e r e n t i a t e b e t w e e n o r g a n i c and i n o r g a n i c carbon. C a r b o n and n i t r o g e n in b o t h sets of samples were d e t e r m i n e d using a Carlo E r b a E l e m e n t a l A n a l y s e r Model 1106.

76

Elemental Analysis Silicon, aluminium, zinc and copper were determined using a Phillips 1220 X-ray fluorescence spectrometer. Dried, ground sediment samples were pressed into tablets (2 cm diameter). Silicon and aluminium were analysed using a Cr target X-ray tube and a PE analysing crystal. A LiF (200) analysing crystal with a gold target X-ray tube were used for zinc, and a tungsten target X-ray tube for copper. Following acid (concentrated HC1/HNO3) digestion of sediments (Van Loon et al., 1973), lead was determined by atomic absorption spectrophotometry (Instrumentation Laboratory Incorporated AA/AE spectrophotometer).

Lead-210 activities Lead-210 activities were obtained by measurement of its daughter product 21°po. The method used was that of Clifton and Hamilton (1979). In brief, freeze dried lightly ground samples (2 g) were spiked with 2°Spo as an internal standard. The sediments were then dried, treated with 5 cm 3 concentrated HNO3, warmed, concentrated HC1 added to expel the HNO 3 and then leached with 6 MHC1 (1 h, 90°C). The volume was then adjusted to 20 cm 3 and centrifuged. The 21°po was plated onto silver planchets according to the procedures of Flynn (1968). The 21°Po activity was determined by alpha spectrometry using a 200 mm 3 silicon surface barrier detector and yields corrected using 2°8po internal standard activity. Samples were counted for 103 min. Recovery of 2°Spo was typically > 60%.

P A H analyses All solvents were of Fison's or Rathburns HPLC grade. Glassware was acid-cleaned and pre-rinsed with solvent prior to use.

Extraction and clean-up Details of the extraction and clean-up procedures have been described previously (Readman et al., 1982). In summary, wet sediments were ground with anhydrous sodium sulphate and subjected to Soxhlet extraction into dichloromethane. Extracts were cleaned-up by passage through micro-columns of alumina (Neutral, Brockman grade 1, 7% deactivated) with hexane eluent. Recoveries of "spiked" standards ranged from 72 to 100% for individual compounds. Analyses of triplicate sediment sub-samples indicated PAH reproducibility to be + 6% of the mean concentrations. Re-extracts typically yielded less than 5% of the original extracts. Detection limits for the technique were typically < 3 ng (g dry sediment)-1 for individual compounds.

77

Reverse-phase high performance liquid chromatography (HPLC) Aliquots (20-30 mm 3) of sample extracts dissolved in acetonitrile were initially chromatographed on a Perkin Elmer 10gm HC-ODS (250 z 26mm i.d.) octadecylsilane (ODS) (PAH-specific) column as described by Readman et al. (1984b) after the method of Ogan et al. (1979). A solvent programme of 40% acetonitrile/60% distilled water for 15min prior to injection, followed by a gradient increase of 3% acetonitrile min -1 to 99% acetonitri!e/l% distilled water (which was maintained isocratically until completion of the chromatogram) was delivered at a flow rate of 0.5 cm 3min 1 using a Perkin Elmer Series 2 high-performance liquid chromatograph. Detection was by monitoring UV absorbance at 254 and 280nm (Perkin Elmer LC75 and Pye LCUV variable wavelength UV detectors in series). Peak identification was by co-injection of authentic standards, ratioing absorbance at 254 and 280 nm and by comparing stop-flow UV scans of standard peaks with corresponding exctract peaks (Readman et al., 1981). Capillary gas chromatography (as described below) was also used to confirm PAH composition. Calibration of the HPLC system was achieved using known concentrations of high purity PAH in acetonitrile. Using the above system, perylene was found to co-elute with benzo[b] fluoranthene, and although selective quantification was possible at 408nm, other HPLC columns were evaluated. A 5 #m HC-ODS "test" column (supplied by Perkin Elmer) used under the same elution conditions but with a flow rate of lcm~min ~ resolved the two compounds, although dibenz[ah]anthracene and benzo[ghi]perylene were then found to co-elute. A typical chromatogram is shown in Fig. 2. Sediment extracts were analysed using both the 5 and 10 #m HC-ODS systems. The large peak "X" in the chromatogram (Fig. 2) (with a retention time of ~ 22 min) corresponds to a peak reported previously (Readman et al., 1982, 1984b) which was shown by gas chromatography/mass spectrometry to be comprised mainly of phthalates and phenyl esters.

Capillary gas chromatography (GC) Selected extracts were also investigated by high resolution capillary GC analysis to confirm the HPLC results and to gain further information for source fingerprinting. Prior to GC, extracts from the alumina clean-up stage were further purified by normal-phase HPLC fractionation similar to that described by Killops and Readman (1985). HPLC was performed on a Partisil 5 gm PAC column (25 cm × 5 mm i.d.; cyanoamino (1:2) bonded phase column, commercially available from Whatman) connected in series with a silica pre-column. A linear solvent gradient was employed (100% hexane to 70:30 hexane/dichloromethane in 10 min at 3 cm 3min 1 flow rate) using a Spectra Physics SP8700 solvent delivery system with a Rheodyne 7125 valve injector and a Pye Unicam PU 4020 UV detector (monitoring at 254 nm). Aromatic hydrocarbons containing three to six aromatic rings were collected.

78

UV Absorbance ( 254 nm. )

6 7

1 I 2

0

t 10

11 910

= 20 Time

~ 30

] 40

( min )

Fig. 2. HPLC chromatogram showing PAH separation of a sediment (1 2cm depth interval) extract using the 5 ttm HC-ODS Perkin Elmer column. Peak numbers correspond to (1) phenanthrene, (2) anthracene, (3) fluoranthene, (4) pyrene, (5) benz[a]anthracene, (6) chrysene, (7) benzo[e]pyrene, (8) benzo[b]fluoranthene, (9) perylene, (10) benzo[k]fluoranthene, (11) benzo[a]pyrene, (12) coincident dibenz[ah]anthracene and benzo[ghi]perylene, (13) indeno[1,2,3-cd]pyrene. X is an unresolved mixture (see text). Analytical conditions are described in the text. A r o m a t i c f r a c t i o n s were a n a l y s e d using a Carlo E r b a M e g a 5260 GC with a Grob split/splitless i n j e c t o r and a flame i o n i z a t i o n d e t e c t o r (FID). A 25 m × 0.2 mm i.d. silylated P y r e x glass (Grob et al., 1979) 0.2 ttm W C O T OV-73 c o l u m n was used w i t h h e l i u m c a r r i e r gas (2.2 kg cm-2). After an initial period (1 min) at 60°C, the c o l u m n was t e m p e r a t u r e p r o g r a m m e d from 60 to 300°C at 6°C min 1 and was m a i n t a i n e d i s o t h e r m a l l y at 300°C for 10 min. P e a k identification was a c h i e v e d by c o m p u t e r i s e d c a p i l l a r y gas chromatog r a p h y - - mass s p e c t r o m e t r y (CGCMS) using a F i n n i g a n 3200 q u a d r u p o l e mass s p e c t r o m e t e r c o n t r o l l e d by a n I N C O S 2015 (Nova 4) computer. The mass r a n g e m/z 50-450 was s c a n n e d at 1-s intervals. W h e r e mass spectra failed to different i a t e b e t w e e n isomers, identification has been based upon observed elution orders on SE52 s t a t i o n a r y phase ( W a k e h a m et al., 1980a).

Statistical analysis of results To assess l i n e a r r e l a t i o n s h i p s b e t w e e n all pairs of variables, P e a r s o n p r o d u c t - m o m e n t c o r r e l a t i o n coefficients (r) were calculated. RESULTS AND DISCUSSION Visual i n s p e c t i o n of the s e d i m e n t core r e v e a l e d an oxic pale b r o w n surface l a y e r (0-1 cm) a b o v e a d a r k grey a n o x i c core. This was confirmed by E h measu r e m e n t s w h i c h d e c r e a s e d e x p o n e n t i a l l y with depth. W a t e r c o n t e n t of the sediment t h r o u g h o u t the core exhibited o n l y m i n o r f l u c t u a t i o n s (57.6 + 3.6%)

79 i n d i c a t i n g n e g l i g i b l e c o m p a c t i o n of t h e core. O r g a n i c c a r b o n d e c r e a s e d f r o m 1.3-1.5% in t h e s u r f a c e ( < 5 cm depth) s e d i m e n t s to < 1% at a d e p t h of 52 cm in a c c o r d w i t h d i a g e n e t i c trends. T h e r a t i o S/A1 was u n i f o r m (2.9 _+ 0.2) t h r o u g h out t h e core. Polonium-210 a c t i v i t y was s h o w n to d e c r e a s e a p p r o x i m a t e l y e x p o n e n t i a l l y w i t h d e p t h (Fig. 3). B a c k g r o u n d a c t i v i t y w a s e s t i m a t e d to be 0.27 pCi g 1. D a t e s w e r e e s t i m a t e d f r o m 21°po a c t i v i t y in the i n d i v i d u a l s e c t i o n s r e l a t i v e to t h a t r e c o r d e d at t h e surface. D a t e s for the core s e c t i o n s n o t a n a l y s e d for 21°po w e r e e s t i m a t e d f r o m v a l u e s d e r i v e d by i n t e r p o l a t i o n of the 21°Po:depth profile. An a v e r a g e s e d i m e n t a t i o n r a t e of 0.8 cm y e a r - 1 or 5.0 k g d r y s e d i m e n t m 2 y e a r 1 was c a l c u l a t e d . T h e c o r r e s p o n d i n g flux of 21°po c a n be c a l c u l a t e d as 0.24 pCi cm -2 y e a r 1 ( u n s u p p o r t e d s u r f a c e 21°Po a c t i v i t y = 0.47 pCi (g d r y sedim e n t ) 1; w e t s e d i m e n t d e n s i t y = 1.502 g c m - 3 ; s e d i m e n t w a t e r c o n t e n t = 58% by weight). T h i s a g r e e s w i t h a v a l u e of 0.28 pCi cm 2 y e a r 1 p r e v i o u s l y r e p o r t e d for t h e l o w e r T a m a r E s t u a r y by Clifton a n d H a m i l t o n (1979). T h e d e p t h profile of 21°po s h o w n in Fig. 3 identifies a d i s t u r b a n c e at a d e p t h of a p p r o x i m a t e l y 13 cm w i t h a l o w e r t h a n " e x p e c t e d " 21°po activity. A n o m a l o u s d i s t r i b u t i o n s of c a r b o n a n d n i t r o g e n a n d r e d u c e d c o n c e n t r a t i o n s of P A H a n d h e a v y m e t a l s in the 10-15 cm s e c t i o n of t h e core coincide w i t h this d i s t u r b a n c e . T h e S:/A1 ratio, h o w e v e r , r e m a i n e d u n c h a n g e d r u l i n g o u t i n c l u s i o n of s e d i m e n t f r o m a different g e o l o g i c a l p r o v i n c e . T h e d i s t u r b a n c e could be e x p l a i n e d by t h e i n c o r p o r a t i o n of " o l d e r " s e d i m e n t c o n t a i n i n g c h a r a c t e r i s t i c a l l y l o w e r levels of 2'°po - bkg

(a]

Zinc

( pCi.( g dry sediment ).1) 0

0.2

0.4

+

0 t-

Lead

( pg.( g dry sediment )-1) 100

140

180

.

/

( pg.( g dry sediment )-~) 20

+

~ *,

÷+//+

40

+

./*-

60

~÷

i t2o lO

lO

Depth (cm)

,

2o! 30"

40!-

r

5o! s .

+

+/ .~' +/

/

/* / ,' ,I I I.,'210 rI:)~.h v

/+

/

Y +

I /

]

// /+

,//

+

/

t

*

÷

I

+

I'

,~ I

0

I \

I Zn

'r

Cu

\

':

1

.i i

j

J_~

I

2

4

I

Total PAH ( /Jg.(g dry sediment )-1)

]

0

Depth (cm)

'~

i ÷

PAH

1

Pb

, j

,

,

4~0

,

/

80

Copper ( pg.( g dry sediment ) ~)

Fig. 3. Depth profiles of unsupported ~l°Po, "total" PAH (the sum of those compounds quantified), zinc, copper and lead. The vertical bar on each cross represents the depth interval analysed. The horizontal bar approximates analytical precision (in the case of 2~°poit represents counting error, taken as _+x/~unts). (a) 21°Povalues are corrected for background (bkg) activity, lpCi = 0.037Bq.

80 21°po activity, PAH, heavy metals, carbon and nitrogen. This may have arisen for example, by lateral "slumping" or shearing of sediments during storm events. Zinc, copper and lead show exponential increases in concentration from a depth of approximately 30 cm to the surface (Fig. 3). Distributions generally co-vary [correlation coefficients (r) > 0.9]. Increases in concentrations of the metals from 30cm depth to the surface are: 115 to 200; 25 to 90 and 25 to 70#g (g dry sediment) -1 for zinc, copper and lead respectively. Increases in concentrations of lead are recorded below a depth of approximately 35 cm (Fig. 3). This may be associated with the end of the mining activities in the Tamar catchment at the turn of the century (Clifton and Hamilton, 1979). Concentrations o f " t o t a l " PAH (i.e. the sum of the compounds quantified) in the surface sediments [ ~ 4000 ng (g dry sediment)- 1] are more than two orders of magnitude higher than those recorded below 30 cm depth [< 20ng (g dry sediment) 1]. The increase in PAH is approximately exponential and co-varies with the heavy metals [zinc (r = 0.96), copper (r = 0.97) and lead (r = 0.92)], probably indicating a related or common anthropogenic origin for both classes' of pollutants.

Composition of the PAH A generally uniform composition of PAH (e.g. see Figs 2 and 4) was observed throughout the upper core (above 25 cm depth). Dominance of parent PAH over individual alkyl-substituted homologues is most apparent (Fig. 4) and precludes undegraded oil as a major contributor of PAH (Speers and Whitehead, 1969; Youngblood and Blumer, 1975; Jones et al., 1986). However, the unresolved complex mixture (UCM) apparent in the GC-FID trace of the sediment extract (Fig. 4) probably indicates the presence of highly degraded petroleum residues in the sediments (Jones et al., 1983, 1986). Parent PAH assemblages remarkably similar to that generally observed throughout this core have been reported in sediments from the Severn Estuary, U.K. (Thompson and Eglinton, 1978), the Humber Estuary, U.K. (Jones et al., 1986), Monchaltorfer Aa and Lake Greifensee, Switzerland (Giger and Schaffner, 1978) and Lakes Lucerne and Zurich, Switzerland and Lake Washington, U.S.A. (Wakeham et al., 1980a). Wakeham et al. (1980a) identify this sedimentary composition as ubiquitous, citing literature from the marine environment (Giger and Blumer, 1974; Youngblood and Blumer, 1975; Hites et al., 1977; Laflamme and Hites, 1978); freshwater sediments (Grimmer and Bohnke, 1975; Muller et al., 1977; Giger and Schaffner, 1978); and in soils (Youngblood and Blumer, 1975; Laflamme and Hites, 1978). Giger and Schaffner (1978) and Wakeham et al. (1980a) attribute "street dust" as the major source. Giger and Schaffner (1978) also suggest a similarity of airborne particulates and Wakeham et al. (1980a) emphasise the importance of asphalt particles in street run-off and exclude exhaust emissions and tyre wear as major contributors. Other primary sources suggested include combustion of fossil fuels (Hites et al.,

81

4157 6

10

Sediment Extract

Flame Ionisation Detector Response

•j•m, 4

.

,-?,,

UCM

i

Asphalt Extract

6 7

~o

2

12 11

F .... 60

~ 100

~ 150

-7 200

Temperature

r 250

T 300--~

oC

Fig. 4. Capillary GC-FID chromatograms of PAH separated from extracts of sediment (1 2 cm depth interval) and an asphalt sample. Peak numbers correspond to (1) phenanthrene, (2) anthracene, (3) methyl-phenanthrenes, (4) fluoranthene, (5) pyrene, (6) benz[a]anthracene, (7) chrysene/triphenylene, (8) benzo-fluoranthenes, (9) benzo[e]pyrene, (10) benzo[a]pyrene, (11) perylene, (12) indeno[1,2,3-cd]pyrene, (13) benzo[ghi]perylene. An unresolved complex mixture is indicated by "UCM", the baseline beneath which is shown by a dotted line. Analytical conditions are described in the text. 1977, 1980; L a f l a m m e a n d Hites, 1978; G s c h w e n d a n d Hites, 1981; H e i t et al., 1981; T a n a n d Heit, 1981), c o m b u s t i o n p r i m a r i l y of coal ( G r i m m e r a n d B o h n k e , 1975; M u l l e r et al., 1977), a n d " n a t u r a l " fires ( B l u m e r a n d Y o u n g b l o o d , 1975; Y o u n g b l o o d a n d Blumer, 1975). G s c h w e n d a n d H i t e s (1981) c a l c u l a t e d ind i v i d u a l P A H r a t i o s to i n v e s t i g a t e sources. T h o s e a p p l i c a b l e to this s t u d y a r e phenanthrene/anthracene, fluoranthene/pyrene and benzo[e]pyrene/ b e n z o [ a ] p y r e n e . P l o t s of t h e s e r e l a t i o n s h i p s (Fig. 5) a r e a p p r o x i m a t e l y linear,

82 Fluoranthene

Phenanthrene

Benzo (e) pyrene

0.8] ,,+

0"31

0.2

O.1

,,"

,'+'"

0.8

+

"'

0.6

* ÷~,. +/'

0.4

+"

~tt'"

0.2

0 0.05 0.10 0.15 Anthracene

,,~,"

0

+,"

0.6 0.40.2"

,'+'"

0.4 0:8 if2

Pyrene

Concentration Units :

+

0

,/

,+,' +

""+ ,~

t+"

0.2

0.4

8enzo (a) pyrene

~ug ( g dry sediment )-1

Fig. 5. Cross-correlation between selected PAH throughout the core. Linear regressions of the points are indicated by the broken lines and result in slopes of 1.94, 0.7 and 1.37 for phenanthrene/ anthracene, fluoranthene/pyrene and benzo[e]pyrene/benzo[a]pyrene respectively.

confirming a uniform composition of the paired PAH throughout the core. Regression analyses give values of phenanthrene/anthracene = 1.9, fluoranthene/pyrene = 0.7 and b e n z o [ e ] p y r e n e / b e n z o [ a ] p y r e n e = 1.4. The phenanthrene/anthracene ratio of 1.9 compares best with values of ~ 3 quoted by Gschwend and Hites (1981) for Charles River and Narragansett Bay sediments and fir wood or coal fires. The fluoranthene/pyrene ratio of 0.7 is the same as the value cited for Zurich air (Giger and Schaffner, 1978). Zurich street dust (Giger and Schaffner, 1978) exhibits the same ratio of benzo[e]pyrene/ benzo[a]pyrene as that reported for the Tamar sediment core (1.4). Wakeham et al. (1980a) indicate that abrased asphalt provides an important contribution of PAH to street dust. A GC-FID chromatogram (Fig. 4) of an extract of weathered asphalt (sampled from the surface of a car park) supports their suggestion, although subsequent analyses of road asphalts (unpublished data) indicate considerable variability in PAH content; further research is necessary to evaluate contributions from this source. PAH in "used" crankcase oils can also be dominated by unsubstituted PAH (Colmsjo et al., 1984) representing another potentially important contributor to street dust. From the compositional data it can be summarised that the majority of unsubstituted PAH in the Tamar sediments are derived from fossil fuel combustion and/or street runoff. The distribution of perylene relative to other PAH (Fig. 6) is anomalous in that perylene is notably present at significant levels [5-10ng (g dry sediment)- 1] in the lower portion of the core. This is consistent with previous work suggesting a biogenic origin of the compound (e.g. Wakeham et al., 1980b; Tan and Heit, 1981).

83 Total

#g.( g dry

1980 I 1970,

1960 Date

of

0

1

i

'-'

~

Perylene

PAH sediment

2

) -1

3

4

'

I

+~÷~ +

t/

\

pg.(

/\;\

io

4-

~

/

\

002

t

j

i

/

//

1920 L -

4" !

:~ fluoranthene [] pyrene + benzo (a) pyrene • benzo (ghi) perylene

t95oF

1930i

-

1980

197C

1960

/

t

)-I

0,04

j..l::3

.o////oJ

Deposition

1940

g dry sediment

0

i

I

1910 ¥

I

0

~

I

Date 1950

of

Deposition

1940

4 1930

i

02

i

0.4

_J-

l

0.6

i

i

08

I_

.J 10

J1920

concentrafion Jug,( g dry ~ t }-1

PAH

Fig. 6. Profiles of "total" PAH (the sum of those compounds quantified) and selected individual PAH plotted against date of deposition.

The historical record of PAH The distribution of PAH with estimated date of deposition (Fig. 6) indicates low concentrations in sediments deposited prior to the 1940s [individual PAH concentrations typically < 30 ng (g dry sediment) 1]. Levels then increase to between 10 and 130ng (g dry sediment) 1 in the 1960s and exponentially increase to the surface of the core (1980) where individual PAH concentrations range from 100 to 1000 ng (g dry sediment) 1. Fluxes of "total" PAH (including all the compounds quantified) can be calculated to be ~ 2 1 m g m -~ year 1 in 1980, two orders of magnitude higher than in 1940 ( ~ 0.23 mg m-2 year-l). Most published data suggesting fossil fuel combustion as the primary source (including: Grimmer and Bohnke, 1975; Hites et al., 1980; Gschwend and Hites, 1981) describe a general distribution of negligible concentrations prior to the late 1800s, which then increase and reach a maximum in the region of 1960. After this period concentrations gradually decrease to the present day, an effect attributed to the change from coal as the primary fossil fuel to oil and gas which emit less PAH upon combustion. Heit et al. (1981), however, who also suggest fossil fuel combustion as the major source, describe distributions similar to that described for the Tamar core. Isolation of carbon particles using the method of Smith et al. (1975) followed by electron microscopic examination (Joel JSM35C scanning electron microscope) revealed a selection of combustion particles similar to those described by

84

Griffin and Goldberg (1981). No obvious difference was, however, observed in the quantitative content of these particulates in surface sediment when compared with sediment deposited prior to 1940. Since the 1950-1960 period, when the PAH concentrations rose exponentially, there have been no drastic changes in the local light industry nor any significant increases in domestic fuel burning (particularly of coal). It is also unlikely that the prevailing south westerly air stream in the region, which originates from the Atlantic Ocean, would contain sufficiently high concentrations of PAH to support the levels encountered. The most important contributory factor to PAH emission during this time period is probably the increase in petrol and diesel driven vehicles. In 1961 the Tamar Road Bridge was opened enabling unrestricted entry into Cornwall. This is the major route of vehicles into the South West Peninsula and has increased traffic in the area. A motor vehicle related source (probably linked with road runoff and including abrased asphalt, crank-case oils, etc.), is likely to be most compatible with the historic record of PAH described for the sampling area. Copper, zinc and lead are also likely to co-vary with a road-runoff source (Harrison and Johnson, 1985). Modelling the fate of P A H deposited into sediments Processes which can control the transport and degradation of PAH in sediments include: (a) partition of the compounds between aqueous (pore water) and particulate phases, (b) microbial degradation, (c) uptake, metabolism and depuration of PAH by the benthos, (d) photo-oxidation (at the surface), (e) chemical oxidation and (f) biosynthesis. These processes are compoundselective and will act to change the composition of the suite of PAH with time. Probably the most influential of these processes are partitioning of the PAH between the aqueous and particulate phases and microbial degradation. For this reason we have selected these processes as examples to model compositional changes in PAH within the Tamar Estuary sediment core. (a) P A H exchange and repartitioning between sediments and water PAHs in the surface mixed layer of sediments undergo cyclic resuspension and deposition by tidal currents and may thus exchange and repartition with PAH in the water column. The dynamics of this exchange process may be assessed for PAH using the model depicted in Fig. 7 (J.R.W. Harris, in preparation). This incorporates sediment (total mass B) and particulate material (total mass S) suspended in a water column of volume V. Equilibrium partitioning is assumed to occur rapidly between PAH in solution [X] and/or suspended particulates [XS],, which are then exchanged with sediment-bound PAH ([XS]~). It can be shown that in response to an input I (g day 1) of PAH, the change in PAH concentrations in sediments [XS]8 is described by Eqn (1): d[XS]s _ dt

(K2S + ~){KII - [KI(p + a) + v]} [XS] 8 B{KI(K2S + p + a) + v}

(1)

85

--•-t water

net flow

V

~

input

EX3

--~

--

K1 * S

~

EXS]w--

-p-¢~

EXS]s

t

particle i n p u t -

mobile

B

sediments

deep consolidated

nr ~ - -

sediments

burial

t

cm

Fig. 7. A two-box model for the reversible sorption, resuspension and sedimentation of PAH in an estuarine segment. The notation is summarised and explained in Table 1. W h e r e / ( 2 is the exchange rate of particles ( d a y - l ) between the sediment and suspended load, a is the net s e d i m e n t a t i o n rate (g d a y - l ) , p and v are the t h r o u g h p u t s of particles (g d a y - 1) and w a t e r (m 3 day ') respectively, and K1 is the p a r t i c l e - w a t e r p a r t i t i o n coefficient (m ~g - l ) of the PAH. /(l was derived from the linear free energy equation ( K a r i c k h o f f et al., 1979) and expanded to include the effects of estuarine salinity s(%o) on PAH p a r t i t i o n i n g (Harris et al.,

1984). logK, or/(1

=

log Kow + logfo¢ - 6.21 + 1.65 × l O - 2 K s s

= 6.2 × 10-TKowfoceXp (3.8 × 10 2 K , s)

(2)

Where Kow is the o c t a n o l - w a t e r p a r t i t i o n coefficient, foc is the fractional organic carbon c o n t e n t of particles and K, is the Setschenow salting-out c o n s t a n t (May, 1980) re-expressed here in units of (%0) ~. The steady state solution to Eqn (1), i.e. d [ X S L / d t = 0) yields [XS],

=

[XS]*

=

K , I / ( K 2 p + v + ~)

(3)

with an a p p a r e n t turn-over time ~(day) for PAH in the mixed layer of sediments: r

=

BKI/KI(p

+ ~) + v

(4)

Defining an exchange time 0 = B / K 2 S for the mixed layer of sediments, and i n t e g r a t i n g Eqn (1) with respect to time yields a relation of the form: [XS] t =

[ X S ] * + ([XS] ° + [XS]*)e -~'

(5)

Where w = (1 + aO/B)/(z + O) and [ X S ] ° is the initial c o n c e n t r a t i o n of PAH in surface mixed sediments. Thus the dynamics of PAH exchange between sediment and water m a y be characterized by either an exchange half-life ().~) or the 90% equilibration time (290)

86

As0 -

0.693 -;Ag0 W

2.303

(6)

W

Since our sediment station is intertidal, one would expect the sediment resuspension to occur with the ebb and flood currents so t h a t K2 ~ 4 day 1. The model was parametizecl (Table 1) to simulate the PAH exchange process in a 1 km axial segment of the Tamar Estuary centred at St John's Lake sampling site (Fig. 1). Using the observed sediment concentrations and averaged water concentrations derived from our previous estuarine PAH survey (Readman et al., 1982) (see Table 1), the model was applied to estimate the exchange dynamics and equilibrium repartitioning of benzo[~]pyrene (Bap) and phenanthrene (Phen). The results of equilibrium modelling which are shown in Fig. 8 and Table 1 are striking in their conclusions: (i) The estimated steady state sediment concentrations [XS]* of benzo[a]pyrene and phenanthrene (Bap = 1.2 ng g-l, Phen = 0.03 ng g-l) required TABLE 1 E n v i r o n m e n t a l a n d p h y s i c o - c h e m i c a l p a r a m e t e r s u s e d in t h e s e d i m e n t - w a t e r e x c h a n g e m o d e l for p h e n a n t h r e n e a n d b e n z o [ a ] p y r e n e a t St J o h n ' s Lake, T a m a r E s t u a r y Notation

E x p l a n a t i o n (units)

Value

Ref. a

A v e r a g e for 29-30 k m s e g m e n t A B V v p

S s foe

gow

K, [X] + [XS]w [XS]o

[xs]*, T1/2

S e d i m e n t a r e a (m 2) M a s s of mobile s e d i m e n t (g) T i d a l l y a v e r a g e w a t e r v o l u m e (m 3) V o l u m e t h r o u g h p u t of p a r t i c u l a t e s ( g d a y -1) M a s s t h r o u g h p u t of p a r t i c u l a t e s (g d a y - 1) N e t s e d i m e n t a t i o n r a t e (g d a y -1) T o t a l m a s s o f s u s p e n d e d solids (g) A n n u a l a v e r a g e s a l i n i t y (%0) Fractional organic carbon

O c t a n o l - w a t e r p a r t i t i o n coefficient S e t s c h e n o w c o n s t a n t (%0 i) T o t a l w a t e r c o n c e n t r a t i o n ( g m 9) I n i t i a l s u r f a c e sed. conc. ( g g - 1 ) Calc. s t e a d y - s t a t e surf. conc. ( g g - 1 ) Calc. s e d . - w a t e r e x c h a n g e h a l f life (days)

9.80 7.10 1.12 2.31

× × × ×

105 109 107 106

1 1 1 1

8.71 × 107

1

7.28 × 106 4.22 x l0 s 33 0.02

2 3 3 2

Phenanthrene

Benzo[a]pyrene

3.63 × 104 0.275 8.10 × 10 -6 2.74 × 10 -7 Model 2.96 × 10 TM 4.1

1.10 × 0.333 5.47 × 4.50 × output 1.22 × 27.6

106 10 -6 10 7

4 5 6 2

10 -9

a 1 -Bale et al., 1985; 2 - - t h i s work; 3 - - H a r r i s et al., 1984; 4 - - K a r i c k h o f f et al., 1979; 5 - - d e r i v e d from M a y , 1980 a n d C h i o u et al., 1982 w i t h c o n v e r s i o n to %0-1; 6 - R e a d m a n et al., 1982.

87 I

5

-

Observed

6

_

[ BAP]., .

.

-'h 7 - l o g (PAH g/g)

Observed

[ PHEN ]

8

[ BAP ] 9

10

[ PHEN ]

11

[BAP]

m

1000

[PHEN]

100 BAP

: PHEN ratio

10 iiiiiiiiiiiiiiiitiii!iiiii!i!!!!!!•ii•ii1iiiiiiiiiii{i!i!•- Obse v ~d

0

t00

[BAP ] : [PHEN]

200 Time

300

1

400

((Jays)

Fig. 8. Model results of PAH repartition dynamics (Eqn. (5)) of exchangeable benzo[a]pyrene (BAP), phenanthrene (PHEN) and their ratio, in the surface mixed layer of sediments with overflowing water of constant PAH concentrations (Table 1). Comparisons with observed surface (0-2 cm) sediment data are also shown. to be in equilibrium with observed water c o n c e n t r a t i o n s are 2 to 4 orders of m a g n i t u d e less t h a n the measured c o n c e n t r a t i o n s in the surface sediments (Bap = 450 ng g- 1, P h e n = 274 ng g- 1) (Fig. 8). Phrased in a n o t h e r way, if P A H sediments were in true equilibrium, t h e n the o n l y way in w h i c h the observed sediment c o n c e n t r a t i o n s could be m a i n t a i n e d is for the water c o l u m n conc e n t r a t i o n s of Bap and P h e n to be 3.4 and 2 1 6 0 # g d m 3 respectively, w h i c h correspond to 62 times and 266000 times respectively their observed water c o n c e n t r a t i o n s (Table 1). Clearly this is i n c o n s i s t e n t with k n o w n water concentrations and leads us to the c o n c l u s i o n that P A H in the Tamar Estuary sediments appear to be irreversibly bound and u n a v a i l a b l e for desorptive depuration to the water column. (ii) The sediment/water e x c h a n g e half-lives for Bap and P h e n (27.6 and 4.1 days respectively) are rapid and significantly shorter t h a n the estimated particle residence time ( = B / ~ ~,, 2.7 years) in the surface mixed layer. This implies that any equilibrium repartitioning of P A H with the water c o l u m n could h a v e been achieved prior to burial. Thus, the chemical characteristics of P A H in deep sediments s h o u l d reflect the extent to w h i c h equilibrium repartitioning

88 may have occurred. For example, equilibrium repartition would have increased the Bap/Phen ratio from 1.6 to ultimately 40.8 (Table 1; Fig. 8), yet throughout the polluted portion of the core, Bap/Phen ratios remained remarkably constant (1.38 + 0.51), implying that the PAH input to sediments of the Tamar Estuary has been compositionally uniform, is chemically inert, and has never been at equilibrium with the overlying water. Qualitatively similar conclusions about the Bap/Phen ratios can be reached when considering post-depositional leaching of PAH by flowing pore waters associated with inter-tidal pumping and hydraulic drainage. Following periods of high rain fall, the pore water salinity at 20 cm depth of sediment near St John's Lake was found to decrease by ~ 10%o (Bryan and Uysal, 1978) representing a net volume replacement of ~ 30% in 3 months. Even shorter pore water replacement times (3 weeks, top 30 cm) have recently been reported (Agosta, 1985) for the top 30 cm of mesotidal sandy sediment in S. Carolina. Assuming fast equilibration (24 h, Karickhoff et al., 1979) relative to pore water replacement time, it is possible to model the selective leaching effects of flowing pore water on particle-associated PAH by combining linear free energy equation (2) for K 1 with a simple step-wise leaching equation used in liquid/ liquid extraction theory (Irving and Williams, 1961) to yield the concentration of PAH resulting from n changes of pore water [XS]n to be:

[XS]n = [XS]0 \ K , P + 17)~ Where P and V are the weight (g) and volume (m 3) of sediment and pore water in 1 m 3 of sediment. Thus assuming n = 4 year -1, we calculate leaching half lives of Bap =- 76.8 years and Phen = 6.7 years, resulting in an increase in the Bap/Phen ratio from 1.95 to 75.5 over a 40 year period. Again we conclude that the constancy in the observed Bap/Phen ratio (1.38 + 0.51) indicates the PAH to be remarkably unavailable to natural leaching processes.

(b) Microbial degradation It is now well established that microbial degradation of PAH occurs primarily in the aerobic zone (Bauer and Capone, 1985; and refs. therein) with highest rates occurring with low molecular weight homologues (Lee et al., 1978; Gardner et al., 1979; Readman et al., 1982). Consequently, any degradation should result in selective losses of, say, anthracene relative to benzo[a]pyrene, and so affect the ratio of residual PAH. Using Gardner et al.'s (1979) data for the degradation of anthracene, fluoranthene, benz[a]anthracene and benzo[a]pyrene in coastal sediments and applying it to our Tamar PAH (see Table 2) we calculate that up to 80% of anthracene and 40% benzo[a]pyrene could, theoretically, be degraded during the approximately 2 year particle/PAH passage through the aerobic layer. At the same time, the anthracene/ benzo[a]pyrene ratio would be expected to decrease from a surface sediment value of 0.32 to 0.11 in the anaerobic sediments. This is not evident in the anaerobic (2-8cm depth) region of the Tamar sediment core where ratios

2.18 1.67 1.17 0.67

Anthracene Fluoranthene Benz[a]anthracene Benzo[a]pyrene

0.87 1.14 1.63 2.72

Half-life (years)

80 71 57 40

% PAH aerobically degraded b

0.32 1.72 0.64

0-1 cm

0.26 _+ 0.08 1.70 + 0.49 0.79 + 0.13

Observed 2 ~ cm

PAH/benzo[a]pyrene ratios

0.11 0.83 0.46

Theoretical c 2 ~ cm

a G a r d n e r et al.'s (1979) d a t a r e c a l c u l a t e d from u n i t s of % w e e k 1 a t 20°C to r a t e c o n s t a n t (day 1) at 11.5°C ( a v e r a g e T a m a r s e d i m e n t t e m p e r a t u r e ) by a s s u m i n g first o r d e r k i n e t i c s a n d b a c t e r i a l Q~0 of a p p r o x i m a t e l y 2. b C a l c u l a t e d % d e g r a d a t i o n of P A H d u r i n g t h e a p p r o x i m a t e l y 2 y e a r P A H / p a r t i c l e r e s i d e n c e t i m e in t h e a e r o b i c layer. C a l c u l a t e d v a l u e a s s u m i n g a e r o b i c d e g r a d a t i o n as r e p o r t e d by G a r d n e r et al. (1979).

Degradation rate a c o n s t a n t (× 10 ~ d a y ~)

PAH

T h e o r e t i c a l i m p a c t of d e g r a d a t i o n o n s e d i m e n t e d P A H

TABLE 2

90 remain constant at 0.26 + 0.08. This implies that unsubstituted PAH in the Tamar sediments, unlike the spiked standard compounds (Gardner et al., 1979; Readman et al., 1982) are apparently not available for microbial degradation.

The thermodynamically anomalous behaviour of the P A H The generally uniform composition of unsubstituted anthropogenic PAH within the Tamar Estuary sediment core [confirmed by high correlation coefficients and linear regressions (Fig. 5) between individual compounds], together with the co-variation between PAH and heavy metals (Fig. 3), indicate that the PAH are "unavailable" for degradation. Examples of thermodynamic models constructed to investigate transport and degradation of PAH within the sediments, however, predict substantial compound-selective discrimination. Gschwend and Hites (1981) who also describe constancy of PAH composition within a dated core, comment on the absence of isomeric discrimination. Comparing their results with the degradation of benz[a]anthracene demonstrated in a Marine Ecosystems Research Laboratory (MERL) microcosm study (Hinga et al., 1980), they suggest that the oil-associated PAH used in the MERL study might be more available for microbial degradation than PAH associated with combustion particles. Support for this hypothesis has recently been obtained by Jones et al. (1986), who show that whilst oil-derived aromatic hydrocarbons can be rapidly biodegraded in sediments, combustion-derived aromatic hydrocarbons in the same sediments are relatively resistant to degradation. Similar anomalous behaviour of PAH has also been reported by Farrington et al. (1983) who suggest that petroleum-derived PAH are more available for uptake by mussels than are pyrogenic PAH. Varanasi et al. (1985) have also recently shown that sediment "dosed" with standard 14C-labelled benzo[a]pyrene is more bio-available than the benzo[a]pyrene actually present in polluted environmental sediments. The anomalous partition behaviour of PAH previously reported for the Tamar Estuary water column (Readman et al., 1984a) is therefore extended to the sediments and would appear to control the long-term fate of PAH following their deposition. Differences between thermodynamically-predicted and observed behaviour of PAH can be explained by considering the degree of binding of the pollutant molecule to the matrix with which it is associated. Physico-chemical interactions which could occur between PAH and particulate phases in the aquatic environment might include: dissolution within oil droplets; surface sorption onto particulates; sorption under convoluted surfaces or debris (e.g. intralamellar bound within clays or trapped under organic debris); occluded in bacteria or trapped within combustion particles. Particle binding would appear to be of fundamental importance when considering the environmental reactivities and bioavailability of PAH. This could be verified by developing sol-

91 vophyllic leaching protocols to resolve particle binding and polydispersity characteristics of PAH in natural waters. CONCLUSIONS (i) Concentrations of individual PAH in the surface sediments [100-1000 ng (g dry sediment) 1] are more than two orders of magnitude higher than those recorded below 30 cm depth [ < 20 ng (g dry sediment) 1]. The increase in PAH towards the surface is approximately exponential and co-varies with increases in zinc, lead and copper. (ii) Throughout the upper core (above 25 cm depth) the composition of PAH is uniform, with parent PAH dominating alkylated homologues. The PAH assemblage is typical of that associated with a combustion origin of the compounds. (iii) The distribution of "total" PAH (i.e. the sum of the compounds quantified) with date of deposition indicates low concentrations in sediments deposited prior to the 1940s [~ 30 ng (g dry sediment) 1, corresponding to a flux of less than 0.3mgm -~ year 1]. Levels then increase to ~ 500ng (g dry sediment) 1 in ~ 1960 and exponentially increase to the surface of the core (1980) where the "total" PAH concentration is ~ 4000 ng (g dry sediment) i (corresponding to a flux of ~ 21mgm 2 year l). In the area concerned this historical record (together with the "combustion"-derived composition) suggests a motor vehicle related/road runoff source. (iv) The compositional uniformity of unsubstituted PAH indicates that these PAH are chemically unavailable to post-depositional compound-specific transformation processes (desorption-exchange, biodegradation, etc.). This is contrary to the results predicted using thermodynamic modelling and degradation data, which suggest that there should have been substantial compound discrimination. (v) Occlusion of unsubstituted PAH within particulate matrices could provide a logical explanation for the anomalously low environmental reactivity and bioavailability of these compounds. ACKNOWLEDGEMENTS The authors wish to thank R. Clifton and Dr E. Hamilton for expert advice and assistance with the ~l°Po dating of the sediment core and the X-ray fluorescence analyses. We gratefully acknowledge the help of Dr K.R. Clarke for performing the computing and statistical work and Dr J.R.W. Harris for the sediment exchange modelling. We also thank Dr P. Donkin and Dr L. Brown for useful discussion. Dr Readman's research was funded by Devon Education Authority. Dr Mantoura's r61e in this research forms part of the Biogeochemistry Programme of IMER and was partly supported by the Department of the Environment (Contract DGR 480/48).

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