Environ. Sci. Technol. 1997, 31, 2777-2781
Surface Microlayer Enrichment of Polycyclic Aromatic Hydrocarbons in Southern Chesapeake Bay† KEWEN LIU‡ AND REBECCA M. DICKHUT* Department of Physical Sciences, School of Marine Science, Virginia Institute of Marine Science, The College of William and Mary, Gloucester Point, Virginia 23062
Dissolved and particle-associated polycyclic aromatic hydrocarbons (PAHs) were measured in the surface microlayer (SM) of the York and Elizabeth River tributaries of Southern Chesapeake Bay throughout 1 year. Particle-associated PAH concentrations in the SM increased nonlinearly with total suspended particulate (TSP) levels, and a much more rapid rise in particulate PAHs with TSP was observed in the Elizabeth River vs the York River SM. The steep rise in particle-associated PAH concentrations along with observed nonequilibrium distributions of PAHs between the dissolved and particle phases in the Elizabeth River SM implies that atmospheric deposition of soot particles may be an important source of PAHs to this urban estuary. A significant relationship between dissolved PAHs and dissolved organic carbon (DOC) was also observed at the York River site, with the more hydrophilic PAHs exhibiting greater increases with DOC. PAHs were significantly enriched in the SM of both sites relative to the subsurface water concentrations. Particle-associated and dissolved phase enrichment factors were strongly influenced by wind speed at the Elizabeth River site; however, only particulate phase enrichment factors were dependent on wind speed at the York River site.
Introduction Numerous substances accumulate at the sea surface including lipids and amphophilic organic chemicals that exhibit a strong interfacial affinity. As a result of its organic nature, many pollutants concentrate in the surface microlayer (SM), especially hydrophobic organic contaminants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and other pollutants that adhere to particles and colloids or exhibit increased solubility with elevated dissolved organic matter. For example, total PAH concentrations in the SM were found to be 10-1000 times higher than in subsurface waters in coastal and estuarine systems of the east coast of the United States (1, 2). Enrichment factors of 103-104 for PCBs in the SM have also been reported from different water bodies (3). Although hydrocarbon concentrations of SM particulate matter have been observed to be more than an order of magnitude higher than those of subsurface water particles, average concentrations of hydrocarbons in the SM dissolved phase are often similar to those in subsurface waters (4). † Contribution No. 2070 from the Virginia Institute of Marine Science. * Corresponding author telephone: (804)684-7247; fax: (804)6847250; e-mail address:
[email protected]. ‡ Present address: Harbor Branch Environmental Lab, Harbor Branch Oceanographic Institution, 5600 US 1 North, Ft. Pierce, FL 34946.
S0013-936X(96)01080-2 CCC: $14.00
1997 American Chemical Society
The SM may affect gas transfer of semivolatile organic contaminants between the atmosphere and water through hydrodynamic effects and diffusive resistance (5). Air-water gas exchange may also be affected by biological uptake or particle sorption of contaminants during transport through the SM. The SM has been reported to have different microbiota and a considerably greater number of organisms such as early-life stage larvae of fish and invertebrates as compared to subsurface waters (6, 7). This SM biota may be exposed to higher levels of pollutants than organisms residing below the air-water interface. Therefore, from a pollution standpoint, the air-water interface is perhaps one of the most important but poorly characterized regions of the marine environment. In order to address the problems of chemical pollution, especially the atmospheric deposition of organic contaminants, it is necessary to quantitatively understand the structure of the SM as well as the major factors controlling SM characteristics and pollutant enrichment. The objectives of this study were to (1) examine the enrichment of PAHs in the SM relative to subsurface water, (2) compare PAH enrichment in the SM between an urban and a semiurban estuary in the Southern Chesapeake Bay region, and (3) determine the major factors controlling PAH enrichment in the SM.
Experimental Methods Study Area. Two estuaries adjacent to Southern Chesapeake Bay were sampled bimonthly during this 1-year investigation: the York and Elizabeth Rivers (8-10). The semiurban York River site was located ∼5 km northwest of a coal/oilfired power plant and oil refinery and 1 km east of a major vehicular river crossing. In contrast, the Elizabeth River is an industrialized urban estuary located centrally within the Hampton Roads metropolitan area with a population of about 1.5 million. The sampling site at this location was in the main stem of the river in close proximity (80% while those for PAHs passed through the stainless
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steel tank, GFF, and filter head have been previously demonstrated to be slightly lower (77 ( 16%) (11). Retention of dissolved organic carbon (DOC) and associated PAHs by XAD-2 may occur; however, no significant DOC sorption by XAD-2 resin was observed when DOC ranged from 5 to 7 mg/L (8). Similarly, we found no significant difference in a high DOC concentration (20.6 ( 0.95 mg/L) SM sample before passing over the XAD-2 resin as compared with the sample DOC level measured after it passed over the XAD-2 resin (20.1 ( 1.01 mg/L). Thus, sampling of DOC-associated PAHs by the XAD-2 resin was considered negligible. Sample Extraction and Analysis. A surrogate standard mixture containing five deuterated PAHs was added to all samples and blanks prior to extraction. The sieved particle fraction was extracted with 100 mL of methanol/acetone (50/ 50 v/v) for 1 h using an ultrasonic bath (Mettler, ME4.6) to loosen the particle matrix. The mixture was subsequently transferred to a separatory funnel and extracted three times with 100 mL of hexane. GFFs were sonicated for 1 h in 100 mL of methanol followed by two 1-h sonications each in 100 mL of hexane. The solvent extracts were combined in a sepfunnel, and the PAHs were back-extracted into the hexane by adding 100 mL of purified water. XAD-2 resin samples were sequentially Soxhlet extracted for 24 h each with acetone followed by hexane. The acetone fractions were then backextracted as described above, and the resulting hexane fractions for each sample were combined, concentrated to ∼5 mL using rotary evaporation, dried over anhydrous Na2SO4, and subjected to solid-liquid chromatography cleanup on silica (Bio-Sil A 100-200 mesh) (11). Lab blanks were held throughout sample storage and extracted along with the field samples. Prior to analysis, the extracts were spiked with an internal standard mixture consisting of additional deuterated PAHs and reduced to ∼100 µL under a stream of purified N2. Subsequently, each sample and lab blank was analyzed for 18 PAHs using gas chromatography/mass spectrometry (11). Individual PAHs were quantified relative to the closest eluting surrogate. Average recoveries for deuterated surrogate PAHs were 54 ( 14%, 96 ( 14%, 105 ( 13, and 90 ( 10% for naphthalene-d8, anthracene-d10, benz[a]anthracene-d12, benzo[a]pyrene-d12, respectively, indicating that the analytical procedures are precise and have an uncertainty of 0.5). Nonetheless, particulate PAH concentrations in the Elizabeth River SM, normalized to TSP dry weight, were about 1 order of magnitude higher than those in the York River SM, possibly due to a greater contribution of atmospheric deposition of soot-like aerosols to the SM particle-associated PAH concentration in the urban Elizabeth River estuary. Dissolved PAH concentrations in the York River SM were found to be correlated with DOC concentrations (Figure 2) but not in the Elizabeth River. The increase in dissolved PAH concentrations in the York River SM was related to compound hydrophobicity, with the relatively hydrophilic PAHs (e.g., fluorene) exhibiting a greater increase with DOC compared
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FIGURE 2. Relationship between dissolved PAH concentration and DOC in the York River SM. to the more hydrophobic compounds (e.g., indeno[1,2,3-cd]pyrene). This is in contrast to expectations based on solubility enhancement by DOC (12). Therefore, the results may indicate that kinetic factors such as desorption or gas exchange rates, which are faster for the more hydrophilic PAHs, are important in determining SM dissolved phase enrichment of PAHs.
FIGURE 3. Relationship between particulate PAH concentration and TSP for selected PAHs in the York (O) and Elizabeth (0) River SMs. Particulate PAH concentrations increased nonlinearly with TSP in the SM (Figure 3), with slopes of the lines for plots of log particulate PAH concentration vs log TSP for the York River ranging from 1.16 to 1.39 (r 2 ) 0.75-0.89) and from 2.66 to 3.16 (r 2 ) 0.90-0.94) for the Elizabeth River. At both sites the regression slopes did not vary significantly (p > 0.3) among PAHs, indicating similar sources for the various particle-associated PAHs at each site. The fact that the slopes of the lines in Figure 3 are >1 also suggests that the particulate PAHs are not uniformly distributed on the TSP accumulating at the air-water interface. This is particularly evident for the Elizabeth River. If PAHs were equally distributed on the various pools of particulate matter entering the SM (on a w/w basis), then an increase in mass of any one fraction of the TSP would result in a corresponding increase in PAH loading to the SM. The greater rise in PAH concentration in the SM per unit of TSP added indicates that particulate matter enriched in PAHs accumulates in the SM. Moreover, the steeper rise in particleassociated PAH concentrations with TSP in the SM of the Elizabeth River as compared with the York River is reflective of the urban aerosol pool, which tends to have a greater abundance of coarse (1-10 µm) atmospheric aerosols with high deposition velocities as compared to rural areas (13), which have been observed to contain high levels of PAHs particularly in urban areas (14, 15). In addition, the C/N ratio of the TSP in the Elizabeth River SM increased markedly with TSP, whereas in the York River SM this ratio remained relatively constant (9, 10). This increase in C/N of the particulate matter accumulating at the air-water interface of the urban Elizabeth River estuary is consistent with an increase in allochthonous, refractory organic matter, such as soot. Loading of this type of material to the SM would significantly increase the particle-associated PAH concentration in the SM with comparatively little change in TSP, as observed for the Elizabeth River. Assuming atmospheric deposition is the primary source of particulate PAHs to the SM, then higher PAH concentrations would be expected on small (0.1). Similarly, at both the Elizabeth and York River sites, the PAH composition of the SM particulate matter was not significantly different (p values >0.1) than that for aerosol particles in the area (Table 1). PAH ratios on the SM and aerosol particles indicate that street dust is most likely an important source of these particle-associated contaminants to the atmosphere and airwater interface of these urban/semiurban estuaries (Table 1). The only significant (p < 0.1) PAH compositional difference between the York and Elizabeth River SMs was for the b[e]p/ b[a]p ratio in the dissolved phase (Table 1). Higher b[e]p/ b[a]p in the Elizabeth River as compared with the York River SM may be due to higher crude oil-related PAH sources at this site or to preferential loss of b[a]p via photodegradation. The half-life of b[a]p in the SM is on the order of minutes (9) unless it is sorbed to a particle matrix such as fly ash where it is resistant to photolysis (21). Photodegradation of PAHs in the SM depends on their residence time at the air-water interface as well as the abundance and nature of DOC and POC, which have been observed to both promote and retard PAH photolysis (9). In Situ Distribution Coefficients. Organic carbonnormalized distribution coefficients (Koc) for the PAHs were found to be substantially higher than the compound octanolwater partition coefficients Kow values (9). This may be because of differences in the nature of the organic matter associated with SM particles as compared to 1-octanol (22) or because only a fraction of the total particulate PAH concentration is available to come to equilibrium with the surrounding media (23). In the latter case, a fraction of the PAHs may be constrained in intraparticle spaces (24-27) where they are unavailable for equilibrium partitioning on the time scale represented by the particle residence time in the SM. As a result, measured Koc values would be higher than expected, as observed here, and show evidence of nonequilibrium partitioning between the dissolved and particle phases. Theoretically, at equilibrium, log Koc is linearly related with log Kow for a specific class of organic chemicals, with a slope of 1 and an intercept dependent upon the characteristics of sorbate and sorbent (22, 28). Some studies have shown a discrepancy between field observations and lab data in that field partitioning coefficients do not depend as strongly on Kow as predicted. For example, for PCBs, slopes ranging from 0 to 0.6 have been observed as opposed to laboratory-derived values, which range from 0.72 to 1.0 (29-31). In the Elizabeth River, the average slope of the log Koc-log Kow regressions for the PAHs was 0.64 ( 0.12, whereas in the York River SM the
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average regression slope was 0.78 ( 0.12, with some of the summer samples nearer equilibrium. Overall, York River SM samples had significantly higher log Koc-log Kow (p < 0.03) regression slopes than the Elizabeth River SM samples; however, the average slopes for both sites were significantly lower than those measured in corresponding subsurface water samples (p values
