Environ. Sci. Technol. 1999, 33, 1963-1969
Bioavailability of Polycyclic Aromatic Hydrocarbons in the North Sea T O R I L I . R Ø E U T V I K * ,† A N D S T A° L E J O H N S E N ‡ Norsk Hydro E&P Operations, Environmental Section, N-5020 Bergen, Norway, and Statoil R&D Centre, N-7005 Trondheim, Norway
Semipermeable membrane devices (SPMDs) and blue mussels (Mytilus edulis) were used to determine the bioavailable fraction of polycyclic aromatic hydrocarbons (PAHs) from oil field produced water in the North Sea. The SPMDs and mussels were deployed at 5, 10, and 50 m depth; 100 and 300 m downstream the discharge point; and at a reference site 16 km away. In both SPMDs and mussels, the concentration of PAHs increased significantly toward the discharge point, with the strongest contribution from the lower molecular weight compounds (naphthalene, phenanthrene, dibenzothiophene, and their C1-C3 alkyl homologues). The relative increase in PAH concentration from the reference site to the site at 100 m was higher for mussels than for the SPMDs. The SPMDs reflect the watersoluble fraction of the PAHs, which is probably the most important route of exposure for organisms at lower trophic levels and presumably also the fraction available for uptake by a respiratory route. Residues in the mussels represent both the water-soluble and particle-bound fraction and give information about bioavailability of the PAHs for organisms at higher trophic levels. The results of this study suggest that both techniques give important information about the bioavailability of PAHs to marine organisms.
Introduction Identifying potential environmental hazardous compounds in produced water has been regarded as an important task within the oil industry over the past decade. One of the most important criteria for a compound to fall in this category is its potential bioavailability. Knowledge of the bioavailable fraction of a compound is essential for environmental risk assessment and further effects studies. The purpose of this work was to determine the bioavailable fraction of polycyclic aromatic hydrocarbons (PAHs) from oil field produced water in the North Sea. The work is conducted as a part of a research program, which within the year 2000 will result in a new tool for environmental risk assessment of oil-related compounds to marine organisms. The research program includes development of a threedimensional dispersion model based on meteorological data of the North Sea (1), establishment of a database of detailed chemical composition of produced water from all fields in the Norwegian sector of the North Sea (2), laboratory studies of biological degradation (3), uptake/depuration kinetics (4, 5) and transport in marine food chains of selected compounds * Corresponding author telephone: (47) 55 99 64 98; fax: (47) 55 99 62 50; e-mail:
[email protected]. † Norsk Hydro E&P Operations. ‡ Statoil R&D Centre. 10.1021/es9804215 CCC: $18.00 Published on Web 04/29/1999
1999 American Chemical Society
in produced water at realistic environmental conditions (6), and finally effect studies of organisms at different trophic levels. The present study is designed to give information about bioavailability, and it is also expected to contribute to the evaluation of techniques that may be used in further environmental monitoring of the water column. Finally, there is a need for establishing background levels of organic compounds in the North Sea. Mussels and other bivalve molluscs have been widely used as indicator organisms due to their ability to accumulate trace levels of certain pollutants from the water column. In the Mussel Watch concept (7), pollutant levels and their changes with time in mussels taken from a given coastal marine zone provide a basis for coastal water quality. The use of mussels or other sentinel organisms for sampling hydrocarbons from the water phase may be limited by a number of factors, including variations in uptake with the amount of particulate matter in the water and the ability of the organisms to metabolize the compounds in question. Semipermeable membrane devices (SPMDs) seem to eliminate some of these problems. The lipid-containing SPMD was developed by Huckins et al. (8) for passive, in situ monitoring of aquatic contaminants and has been used in different environmental settings (9-23). SPMDs mimic phenomenologically the uptake via respiration of organic contaminants by aquatic organisms. Bioconcentration excludes the uptake of contaminants by ingestion of food and other organic matter (biomagnification), and SPMDs likewise avoid mimicry of food uptake of contaminant by the organisms. Although the compositions of the polymeric membranes (low density polyethylene) differ substantially from that of a gill membrane (bilayers of phospholipids and proteins), the diffusion of many lipophilic organic compounds through nonporous polymeric membranes is similar to diffusion through biomembranes (24). Huckins et al. (25) developed a mathematical model that makes it possible to calculate the average water concentration of selected contaminants during the sampling period. The steady-state flux of organic compounds into SPMDs is controlled by the sum of resistance to mass transfer in the membrane and the water boundary layer. The theory and model development has been described in detail (26). Biofouling of the SPMDs generally increases the resistance to mass transfer and decreases, but does not stop, the uptake flux. The decrease in uptake flux can be corrected for (26). Petty et al. (26) have performed laboratory experiments to validate the use of SPMDs as monitors of PAHs in the aquatic environment. They performed both flow-through and static tests at different concentration levels of PAHs, different temperatures, and different salinity. Their conclusions were that sampling rates appeared to be relatively independent of PAH water concentrations within the concentration range tested (1-100 ng/L), uptake rates increased with PAH octanol/water partition coefficients up to 200 000 and then declined as molecular size, or movement through the diffusion layer appeared to limit the SPMD uptake, and that there were only small differences in PAH uptake rate at temperatures tested (10, 18, and 26 °C) and at salinities tested (11‰ and 33‰). Recently, Huckins et al. (27) found that uptake rates of PAHs were controlled by both membrane permeability and diffusion through the aqueous boundary layer, depending on molecular size and the magnitude of the membrane-water partition coefficient. In the present study, mussels and SPMDs were deployed side-by-side in cages downstream from an oil platform in the central North Sea. The mussels and SPMDs were analyzed VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Map of oil and gas fields in the North Sea. for PAHs (EPA Priority Pollution List) and the alkyl homologues of naphthalene, phenanthrene, and dibenzothiophene.
Experimental Section Study Design: Field Experiment. Semipermeable membrane devices (SPMDs) and blue mussels (Mytilus edulis) were used in a side-by-side test for monitoring PAHs from produced water at sampling sites downstream from an oil production platform in the northern part of the North Sea in the period April-June 1995. A map of oil and gas fields in the North Sea is shown in Figure 1. The sampling equipment was placed at 5 and 10 m depth at sampling sites located 100 and 300 m from the platform in the direction of the prevailing current (Figure 2) and also at 50 m depth at the site at 300 m. A reference station was situated 16 km away (Figure 2). The SPMDs and mussels were collected after 4 and 8 weeks. Materials. The SPMDs (2.54 cm wide, 86 cm long) were purchased from Environmental Sampling Technologies, St. Joseph, MO. They were constructed from LDPE lay-flat tubing and contained 1 mL of 95% pure triolein. The SPMDs were transported from the United States to Norway in sealed steel containers and were placed in the glass vessels at the 1964
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laboratory or in the samplers at the laboratory on the supply ship only minutes before the samplers were submerged. Blue mussels (M. edulis) of uniform age and size (approximately 5 cm) were purchased from a local commercial grower at A° fjord, Central Norway, where the PAH level is presumably very low. The organisms, which were grown on ropes, were transported from the farm to the lab in a plastic container filled with seawater. They were also placed in the sampler at the laboratory on the supply ship. Methods. The field work was done as described in ref 28. The sampling device had two compartments: one to hold the SPMD frames and one to hold the mussel baskets (the screens on the SPMD frames were made of nylon). Each SPMD compartment included two SPMD frames, and each mussel compartment included two mussel baskets. The SPMD frames contained seven SPMDs on each side. Each mussel basket contained approximately 50 mussels. Moorings (consisting of an anchor, a wire, a subsurface buoy, and a guard buoy) were obtained from a contractor specializing in such equipment (Ocean Climate a.s.). At retrieval of the sampling devices, the SPMDs were placed in prelabeled Zip-Loc bags (3 in each), and the mussels were wrapped in aluminum foil (15 in each) and placed in
FIGURE 2. Schematic view of the design of the field experiment. prelabeled Zip-Loc bags. All samples were then frozen (-18 °C) until sample preparation in the laboratory. The sample preparation and analyses were performed as described in ref 28. The mussel samples were homogenized by maceration using a tissue homogenizer before subsampling and dry weight determination. Each sample was spiked with surrogate internal standards. After homogenization, the samples were extracted with dichloromethane (DCM) in Teflon jars containing solid sodium sulfate. The samples were centrifuged, and the extraction was repeated twice. The extracts were concentrated to 1 mL using a turbovap apparatus (30 °C). The extracts were then cleaned up on an alumina column, and the volume was reduced to approximately 1 mL using the turbovap apparatus. Surrogate internal standards were added to the SPMDs, which were subsequently extracted with cyclopentane in a Teflon tube. The samples were placed on a shaker table for 24 h. The extracts were reduced in volume using the turbovap apparatus. The extracts were cleaned up by gel permeation chromatography (GPC) on a Bio-Rad Bio-Bead SX-1 column. SPMD and mussel extracts were finally analyzed by GC-MS, using selected ion monitoring (SIM). All samples were prepared and analyzed by Battelle Ocean Sciences, Duxbury, MA. Quality Control. Field blank SPMDs and mussels were exposed to air similar to samples during the field deployment and retrieval phases to represent airborne contamination during preparation of the sample. These field blanks were processed and analyzed as deployed samples. PAHs were detected in the field blanks (Table 1), and the results from analysis of deployed SPMDs were background corrected. Samples from the field study were prepared and analyzed by Battelle Ocean Sciences. Analytical control samples of mussels were prepared and analyzed by the laboratory of Norsk Hydro Research Centre. The relative standard deviations of parallel samples (n ) 3) analyzed by one laboratory were up to 30% and highest for the high molecular weight compounds in samples from the reference site. Comparison of the results from the two laboratories showed that the profiles of the
TABLE 1. Concentration of PAHs in Blank Samples of SPMDs (ng/g of SPMD) and Mussels (ng/g wet weight) compound
concn in mussels (ng/g wet weight)
concn in SPMDs (ng/g SPMD)
naphthalene biphenyl acenaphthylene acenaphthene dibenzofuran fluorene anthracene phenanthrene dibenzothiophene fluoranthene pyrene benz[a]anthracene chrysene benzo[b]fluoranthene benzo[k]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene indeno[1,2,3-c,d]pyrene dibenz[a,h]anthracene benzo[g,h,i]perylene
0.4 NDa ND ND ND ND ND ND ND 0.2 ND ND ND ND ND ND ND ND ND ND ND
58 10 2 5 9 7 1 13 2 1 1 ND ND ND ND ND ND ND ND ND ND
a
ND, not determined.
PAHs were the same, but the total levels of the compounds varied by up to 58% (Table 2). The main reason for this divergence is the number of peaks included as alkylated homologues of the different compounds when standards were not available. Recoveries of the surrogate internal standard of SPMDs and mussels are given in Table 3.
Results and Discussion Normalization of Data. The results of the analysis for PAHs in SPMDs and mussels can be presented on a wet weight, dry weight, or a lipid weight basis. When comparing the results for the mussels and the SPMDs, there are some VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Results of Interlaboratory Comparison of Analysis of PAHs in a Mussel Sample (Mytilus edulis) (ng/g wet weight)a compound
concn in mussel sample concn in mussel sample (ng/g wet weight), lab 1 (ng/g wet weight), lab 2
naphthalene phenanthrene fluoranthene
1.08 ( 0.30 1.33 ( 0.23 0.63 ( 0.12
0.55 0.90 0.26
a Laboratory 1 is Battelle Ocean Sciences, and laboratory 2 is Norsk Hydro.
TABLE 3. Recovery (%) of Surrogate Internal Standard in PAH Analysis of Blue Mussels (Mytilus edulis) and SPMDs naphthalene-d8 fluorene-d10 chrysene-d12
blue mussels (%)
SPMDs (%)
60-73 66-78 68-81
31-62 45-61 44-74
important aspects to be aware of when normalizing the data. A key assumption in the interpretation of lipid normalized data is that equilibrium has been reached between contaminant concentrations in the organism’s lipoidal tissues and the water (27). Also, partitioning is assumed to be independent of bulk lipid composition, and nonlipoidal tissue is assumed to have no significant role in contaminant accumulation (27). This means that it is not correct to compare lipid-normalized data from the mussels with the SPMDs where equilibrium has not been reached between the water and the mussel or the SPMDs. The uptake kinetics of the different PAHs in the SPMDs is known (26), but the uptake kinetics of PAHs in the mussels will depend on filtration activity and depuration time. The lipid content of aquatic organisms varies widely but generally lies in the 0.5-10% range, whereas an SPMD usually has >15% lipid, and the larger the lipid pool, the greater the retention time of non-metabolized residues (27). The SPMD (polyethylene) membrane itself also has considerable capacity to accumulate nonpolar organic compounds. Taking these aspects into consideration, the present results are presented on a wet weight basis for both mussels and SPMDs. Field Study. The results of the field study are presented as a function of sampling depth, exposure time, and distance from the discharge point. In Figure 3, the sum of PAHs in mussels and SPMDs at different depths is shown for 4 weeks
exposure time. The concentration of PAHs in mussel tissues is higher at 10 m depth than at 5 m depth at the sampling site 100 m from the platform, while the concentration is higher at 5 m than at 10 m at the sampling site 300 m downstream from the platform. This result indicates that the discharge plume is moving toward the surface from the discharge point at 30 m depth, which is in good agreement with previous dispersion modeling of the discharges (29). At the sampling site 300 m away from the platform, the concentration of PAHs in mussel tissues at 50 m depth is below the level at both 5 and 10 m depth at the reference site. This indicates that the level at the reference site is higher than the “background level” in the North Sea. For the SPMDs, there is no significant difference between values from 5 and 10 m depth at any of the sampling sites. The concentrations of PAHs in SPMDs at 50 m are, as for the mussels, lower than those at the reference station. In Figure 4, the total concentration of PAHs is shown as a function of exposure time at the sampling site 100 m from the platform and at 5 m depth. There is a significant difference between the amounts of PAHs in samples collected after 4 and 8 weeks exposure time for the mussels, which indicates that equilibrium has not been reached between the concentration of some PAHs in the mussels and the surrounding water. For the SPMDs, steady state seemed to be reached after 4 weeks of exposure. The variables depth and exposure time are of minor importance to the variation in the dataset as compared to the distance from the discharge point. As a typical example, a depth of 10 m and an exposure time of 8 weeks are chosen to show the trends in the dataset. In Figure 5, the distribution patterns of PAHs is shown for SPMDs at different distances from the platform. The same is shown for mussels in Figure 6. Considering the profiles of PAHs at the different sampling stations (Figures 5 and 6), the composition appears to be similar for SPMDs and mussels. At the reference station, the level of all components is lower for the mussels than for the SPMDs. The reason may be related to the active water filtration by the mussels. An increased amount of food close to the discharge point may lead to an increase in the filter feeding rate. If there is less food at the reference station as compared to the area close to the platform, the filtration rate and, consequently, the uptake of PAHs will be lower. The SPMDs concentrate PAHs over a certain period of time and will, as long as there is no significant biofouling on the membrane surface, reflect the average water-soluble con-
FIGURE 3. PAHS in blue mussels (Mytilus edulis) in ng/g wet weight and in SPMDs (ng/g SPMD) at 5, 10, and 50 m depth at the three sampling sites, 8 weeks exposure time. 1966
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weeks, the uptake in the SPMDs is expected to be linear and can be described by
CSPMD )
FIGURE 4. Concentration of PAHs in mussels (ng/g wet weight) and SPMDs (ng/g SPMD) at the sampling site 100 m from the platform, 5 m depth, at 4 and 8 weeks exposure time. centration of PAHs. Visual observations of the SPMDs at the sampling sites indicated that there was some biofouling on the membrane surface during the experiment and that the growth increased with nearness to the platform. The concentrations of PAHs in the SPMDs were used to estimate the concentration of PAHs in the water. For the PAHs for which equilibrium has not been reached after 4
CWRSt VSPMD
(1)
where CSPMD is the concentration in SPMD, Cw is the concentration in water, Rs is the SPMD sampling rate in L/day, t is time, and VSPMD is the SPMD volume (lipid and polyethylene). Both lipid and polyethylene have a density of 0.9 g/mL. The sampling rates for the different PAHs at the actual temperature are shown in Table 4 (from ref 31). The estimation of PAH concentrations in water from the SPMD results is based on a linear uptake model (eq 1) even though there may be a risk that some of the compounds have reached equilibrium with the surrounding water. The resulting PAH concentration in water (Figure 7) reflects the gradient toward the platform. The levels appear to be in good agreement with those from measurements by Riksheim and Johnsen in 1994 (30) based on liquid/liquid extraction of water samples. In that study, it was not possible to detect the low levels of the high molecular weight PAHs. The present results show that the gradient toward the platform is steepest for the low molecular
FIGURE 5. Profiles of selected PAHs in SPMDs from the samples taken at 10 m depth and 8 weeks exposure time as a function of distance from the platform (log concentration ng/g of SPMD).
FIGURE 6. Profiles of selected PAHs in mussels from the samples taken at 10 m depth and 8 weeks exposure time as a function of distance from the platform (log concentration ng/g wet weight). VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Concentration of PAHs (ng/L) in water as a function of distance from the platform calculated from the SPMDs (10 m depth and 8 weeks exposure time).
TABLE 4. Literature Values of Sampling Rate from Flow-Through Diluter Exposures at 10 °C, 10 ng/L Exposure Concentration and 21 Days Exposure Timea compound
sampling rate (RS) L/day (10 °C)
naphthaleneb acenaphthylene acenaphthene fluorene anthracene phenanthrenec fluoranthene pyrene benz[a]anthracene chrysene benzo[b]fluoranthene benzo[k]fluoranthene benzo[a]pyrene indeno[1,2,3-c,d]pyrene dibenz[a,h]anthracene benzo[g,h,i]perylene
1.3 2.3 2.7 3.0 3.0 3.9 4.3 5.1 3.6 4.0 3.2 3.4 3.5 3.3 2.3 1.9
aFrom ref 31 with permission from authors. b Value of naphthalene also used for C1-C3 naphthalenes. c Value of phenanthrene also used for C1-C3 phenanthrenes.
weight PAHs, which is in good agreement with the composition of PAHs in produced water discharges. For the high molecular weight fraction (chrysene and higher), the concentration level seems to be quite similar at the three sampling stations. Due to greater biofouling near the platform, the uptake rates of compounds with high Kow may have been affected more than for the compounds with low Kow (27). The field experiment provides valuable information about the bioavailability of produced water compounds. The uptake/depuration kinetics studies (4, 5) and the food chain transfer studies (6) indicate that the water-soluble fraction of PAHs is the most important route of exposure for marine organisms at the lower trophic levels. This is the fraction represented by the SPMDs. The next step will be to perform effect studies in both laboratory and field scale using marine organisms at different trophic levels including fish. The present study gives an indication of the background levels of PAHs in the North Sea, but more effort should be made in this area to establish a basis for further environmental monitoring. Both the use of mussels and SPMDs should be 1968
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considered as monitoring tools since these techniques will probably provide reproducible, comparable data from one year to the next and because they have been shown in the present study to be suitable for this purpose.
Acknowledgments We gratefully acknowledge Carol Peven at Battelle Ocean Sciences; Hilde Riksheim and Bodil Thorvaldsen at the Analytical laboratory of Statoil R&D Centre; Anita Seljelund, Erling Odden, Gerd Aasen, Siw T. Pettersen, Jorunn Gundersen, and Terje Karstang at Norsk Hydro Research Centre, Section of Analytical Chemistry, for experimental assistance. We also acknowledge James N. Huckins and Jimmie D. Petty at USGS, Midwest Science Center, and Eiliv Steinnes, Norwegian University of Science and Technology, for interesting discussions and good advice.
Literature Cited (1) Reed M.; Johnsen S.; Melbye, A.; Rye, H. In Produced water 2-Environmental Issues and Mitigation Technologies; Reed, M., Johnsen, S., Eds.; Environmental Science Research Vol. 52; Plenum Press: New York, 1996; pp 317-330. (2) Røe, T. I. Chemical characterisation of produced water from four offshore oil production fields in the North Sea. In preparation. (3) Brakstad, O. G.; Olsen, A. J.; Nordtug, T.; Frost, T. K.; Aunaas, T.; Johnsen, S. SPE Pap. 1996, No. 35910. (4) Frost, T. K.; Olsen, A. J.; Nordtug, T.; Røe, T. I. Uptake and depuration kinetics of naphthalene, phenanthrene and p-cresol in the copepod Calanus finmarchicus. In preparation. (5) Frost, T. K.; Olsen, A. J.; Nordtug, T.; Røe, T. I. Uptake and depuration kinetics of naphthalene, phenanthrene and p-cresol in marine planktonic organisms. In preparation. (6) Røe, T. I.; Olsen, A. J.; Nordtug, T.; Frost, T. K.; Johnsen, S. Transport and accumulation of naphthalene, phenanthrene and p-cresol in two marine food chains. In preparation. (7) Goldberg, E. D.; Bowen, V. T.; Farrington, J. W.; Harvey, G.; Martin, J. H.; Parker, P. L.; Risebrough, R. W.; Robertson, W.; Schneider, E.; Gamble, E. Environ. Conserv. 1978, 5, 101-125 (8) Huckins, J. N.; Tubergen, M. W.; Manuweera, G. K. Chemosphere 1990, 20, 533-552. (9) Lebo, J. A.; Zajicek, J. L.; Huckins, J. N.; Petty, J. D.; Peterman, P. H. Chemosphere 1992, 25, 697-718. (10) Prest, H. F.; Jarman, W. M.; Burns, S. A.; Weismu ¨ ller, T.; Martin, M.; Huckins, J. N. Chemosphere 1992, 25, 1811-1823. (11) Lebo, J. A.; Gale, R. W.; Petty, J. D.; Tillitt, D. E.; Huckins, J. N.; Meadows, J. C.; Orazio, C. E.; Echols, K. R.; Schroeder, D. J. Environ. Sci. Technol. 1995, 29, 2886-2892.
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J. S., Eds.; ASTM STP 1219; American Society for Testing and Materials: Philadephia, 1995; pp 239-260. Lieb, W. R.; Stein, W. D. Nature 1969, 224, 240-243. Huckins, J. N.; Manuweera, G. K.; Petty, J. D.; Mackay, D.; Lebo, J. A. Environ. Sci. Technol. 1993, 27, 2489-2496. Petty, J. D.; Huckins, J. N.; Orazio, C. E.; Lebo, J. A.; Clark, R. C.; Gibson, V. L. A Report to the American Petroleum Institute; National Biological Service, Midwest Science Center: 1994. Huckins, J. N. Personal communication, 1996. Battelle Ocean Sciences. Standard Operating Procedures for the North Sea Bioaccumulation Project, Report to Statoil. 1994. The North Sea Produced Water Project: Calculation of concentration fields. Report to the Norwegian Oil Industry Association, Sintef: 1998; Report STF66 F98-124. Riksheim, H.; Johnsen, S. SPE Pap. 1994, No. 27138, 471-484.17. Huckins, J. N.; Petty, J. D. Unpublished results, 1998.
Received for review April 27, 1998. Revised manuscript received November 16, 1998. Accepted March 22, 1999. ES9804215
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