Environ. Sci. Technol. 1997, 31, 1623-1629
Gaseous Exchange of Polycyclic Aromatic Hydrocarbons across the Air-Water Interface of Southern Chesapeake Bay† KURT E. GUSTAFSON* 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
The gas exchange fluxes of polycyclic aromatic hydrocarbons (PAHs) across the air-water interface of southern Chesapeake Bay were calculated using a modified twofilm exchange model. Sampling covered the period January 1994-June 1995 for five sites on the southern Chesapeake Bay ranging from rural to urban and highly industrialized. Simultaneous air and water samples were collected, and the atmospheric gas phase and surface water dissolved phase analyzed via GC/MS for 17 PAHs. The instantaneous gas flux was calculated for each compound using Henry’s law constants, diffusivities, and hydrological and meteorological parameters. The direction and magnitude of gas transfer were found to be controlled by water temperature and gas-phase concentrations. Fluxes were determined to vary in direction and magnitude both spatially and temporally across the air-water interface of southern Chesapeake Bay. The range of gas exchange is of the same order as atmospheric wet and dry depositional fluxes to southern Chesapeake Bay. The results of this study support the hypothesis that gas exchange is a major transport process affecting concentrations and exposure levels of PAHs in southern Chesapeake Bay.
Introduction Semivolatile organic compounds (SOCs), e.g., polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and organochlorine pesticides may cycle between air and water with periods of net upward flux during dry weather followed by periods of intense downward flux during rainfall (1, 2). Furthermore, it has been suggested that persistent, semivolatile, hydrophobic pollutants are transferred throughout the world via successive deposition and re-emissionsa “grasshopper” scenario (3). The physical-chemical properties of many trace organic contaminants indicate that SOCs will be long-lived in the environment, cycling between the atmosphere and water (1), thus increasing their effective residence times in the total environment. The original substances and their transformation products eventually will be deposited to the Earth’s surface and may impinge on communities or ecosystems hundreds or even thousands of kilometers removed from the original point of release (4). Thus, the importance of quantifying air-water exchange processes for SOCs is evident. In order to compile a legitimate mass balance and determine exposure levels for SOCs in an aquatic system, it † Contribution No. 2056 from the Virginia Institute of Marine Science. * Corresponding author present address: Institute for Coastal and Estuarine Research, Wetlands Research Laboratory, University of West FL, Pensacola, Florida 32514; e-mail:
[email protected]; phone: (904) 474-2052; fax: (904) 474-3496.
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1997 American Chemical Society
is necessary to consider all of the major air-water exchange processes (1). Air-water transfer processes for chemicals include volatilization and absorption of gases, dry deposition with particles, wet deposition by rain or snow, i.e., particle and gas “washout”, spray transfer, and bubble scavenging (5). In the Chesapeake Bay watershed, wet and dry depositional fluxes of selected SOCs and trace elements have been determined (6-8). Here we quantify the volatile-absorptive gaseous exchange fluxes of selected SOCs across the airwater interface at various sites in the southern Chesapeake Bay over the course of 1.5 yr.
Quantification of Gaseous Exchange Fluxes Gas Exchange Models. Quantification of the evaporation or absorption rate (volatile transport) of chemicals across the air-water interface relies primarily on the two-layer (film) model presented by Liss and Slater (9). The basic assumption of this model is that the two fluid phases are separated by a liquid film and a gaseous film, through which transport occurs largely due to molecular diffusion driven by the concentration (or fugacity) gradient of the chemical between the bulk reservoirs. This framework was extended by Mackay and Leinonen (10), wherein they presented calculations for the transport of low solubility compounds including selected saturated and aromatic hydrocarbons, pesticides, and PCBs expressed in terms of mass transfer coefficients instead of diffusion coefficients and boundary layer thicknesses. Diffusive transport across two boundary layers has been adopted for modeling air-water gas exchange of organic contaminants (2, 11-16). The volatile flux (Fvol) expression is
Fvol ) kol(Cd,w - Cg,a/Kaw)
(1)
where kol is the overall mass transfer coefficient or the inverse of the total resistance to mass transfer, Cd,w is the truly dissolved water concentration, Cg,a is the gas-phase concentration in the atmosphere, and Kaw is the air-water partition coefficient that defines the equilibrium distribution of the chemical in air and water. Kaw and kol are further defined by 1/kol ) RT/Hka + 1/kw and Kaw ) H/RT where kw and ka are the mass transfer coefficients across the stagnant water and air layers, respectively, H is Henry’s law constant, R is the ideal gas constant, and T is temperature (K) at the air-water interface. H values were calculated from subcooled liquid vapor pressures (17, 18) and subcooled liquid solubilities (19, 20; Supporting Information) at the temperatures of interest and corrected for salinity by the Setschenow equation (18, 20-24). For benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, and benzo[ghi]perylene, solubility and vapor pressure data are not available; therefore, flux calculations relied on temperature correlations (18) for H measured at several temperatures in freshwater (25). Mass transfer coefficients are related to the compoundspecific molecular diffusion coefficients and kinematic phase viscosities via the Schmidt numbers for a chemical in water (Scw) and air (Sca) and wind speed at a reference height of 10 m (U10) (26-27):
ka ) 0.001 + 0.0462(U*)(Sca)-0.67
(2)
kw ) 1.0(10)-6 + 34.1(10)-4(U*)(Scw)-0.5
(3)
kw ) 1.0(10)-6 + 144(10)-4(U*)2.2(Scw)-0.5
(4)
where eq 3 applies for U* > 0.3, eq 4 applies for U* < 0.3, and U* ) U10(6.1 + 0.63U10)0.5(10)-2. Surface skin temperatures have been determined to be only some tenths of a degree
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FIGURE 1. Sampling sites in the southern Chesapeake Bay where paired atmospheric and surface water samples were collected for the calculation of PAH gaseous exchange across the air-water interface. Celsius cooler than the underlying water due to energy losses from long-wave radiation and evaporation (28, 29). Therefore, in this study, surface water temperatures along with salinities were used for calculating parameters [i.e., diffusivity (see refs 30 and 31), density, and viscosity (see refs 32-35), and H] necessary to determine gaseous fluxes. Sampling Sites. Four main study sites were located throughout the southern Chesapeake Bay (Figure 1). The Wolftrap region (37°16.53′ N, 76°12.0′ W) is removed from local sources of contamination (land based) and is close to the Chesapeake Bay Atmospheric Deposition (CBAD) study site (37°26.15′ N, 76°15.25′ W) where SOC depositional inputs to the Bay (8) and air samples for paired Wolftrap water samples were collected. The CBAD study site is located approximately 100 m from the shore of the Chesapeake Bay in a high marsh area. The closest regional sources of contaminants to the site include shipping traffic on the main stem Bay and a coal/oil-fired power plant and refinery located approximately 30 km to the southwest. The Hampton Roads region study site (37°5.0′ N, 76°13.3′ W) was located near Grandview Beach; this site was considered urban, lying in the eastern most section of the city of Hampton and within 5 km of the cities of Newport News and Norfolk. The Elizabeth River site (36°52.0′ N, 76°19.6′ W) was located in an intensely industrialized waterway centrally located within the Hampton Roads Metropolitan area (population 1.5 million) and in close proximity (
