dissolved phases of surface sea water including minerals (e.g., iron oxides) and live and dead cells. These partitioning processes include adsorption where the hydrocarbon attaches to the two-dimensional surface of a solid or other interface and absorption where the chemical partitions into the interior of a cell or detrital particle. Understanding the distribution of petroleum hydrocarbons between the dissolved phase and the variety of aquatic particles is important for determining the fate of hydrocarbons in the sea and the bioavailability of these chemicals to marine biota.
For nonpolar organic chemicals, including most of the components of petroleum, the science of partitioning is based on early studies of pesticide retention by soils, which formed the basis for later work with sediments. This field has been extensively reviewed and synthesized (e.g., Karickhoff, 1984; Schwartzenbach et al., 1993; Chiou et al., 1998; Stangroom et al., 2000). For these compounds, associations with solids result primarily from nonspecific interactions with the solids, often driven by hydrophobic exclusion from the dissolved-phase. The simplest model of dissolved-solid distributions is the equilibrium isotherm. The concentration of a solid-associated chemical is related to the dissolved phase concentration by an equilibrium constant K = Cp/Cd, where Cp is the solid-associated concentration of a chemical (mass of chemical per unit mass of solid) and Cd is the thermodynamically dissolved concentration. (In practice, “dissolved” is an operational definition corresponding to the water fraction that can pass through a 0.2 or 0.45 μm membrane filter and includes solids such as minerals, bacteria cells, and colloids; in these cases, the true equilibrium partition coefficient, K, is approximated by a distribution coefficient, Kp.) For nonpolar compounds, the magnitude of K is inversely proportional to the compound’s aqueous solubility and is directly proportional to the octanol-water partition coefficient (Kow). Furthermore, K varies with the nature of the solid phase, especially the fractional organic carbon content and grain size.
Since the NRC (1985) report, there have been great advances in analytical measurements of the different fractions and in predicting the dissolved-solid distributions and bioavailability of hydrocarbons, especially PAH. Initially, partitioning of PAH was modeled as an equilibrium process, based on laboratory observations that the distribution of hydrocarbons apparently reached constant conditions after a few hours. Subsequent work established that desorption may be much slower that adsorption, especially after the sorbed chemical has been allowed to age within the solid (Karickhoff, 1984; Wu and Gschwend, 1988; Ball and Roberts, 1991; Huang et al., 1998; Kan et al., 1998, 2000). Slow desorption has important implications for the fate and transport of hydrocarbons, both in surface waters and in the subsurface environment (Mackay et al., 1986). For example, concentrations of PAH attached to particles from land-based sources may be supersaturated with respect to the corresponding dissolved phase in coastal waters, resulting in a desorption gradient driven by diffusion. The extent to which this disequilibrium will persist depends on the relative rate of desorption compared to the residence time of the particle in the coastal waters As another example, the amount of hydrocarbon released from contaminated sediments that are resuspended into the water column as a result of storms, tides, or dredging depends directly on the desorption rate. Also, the surface to volume ratios (S:V) of laboratory studies should be considered when evaluating field conditions, and compared with the S:V of oils occupying different environments. During the past 15 years, kinetic adsorption-desorption algorithms that describe rate processes have begun to find their way into newer hydrocarbon fate and transport models. Most commonly used “off-the-shelf” modeling packages continue to employ equilibrium partitioning.
Another significant improvement in the description of dissolved-solid partitioning is the recognition that highly sorbing phases within aquatic particle populations can greatly reduce hydrocarbon bioavailability and reactivity in the marine environment. The presence of soot particles in coastal marine sediments significantly alters the partitioning of PAH between sediments and porewater (McGroddy and Farrington, 1995; Gustaffson et al., 1997a; Naes et al., 1998). Whether this alteration is due to highly energetic adsorption sites on the soot particle surfaces or to correspondingly slow desorption kinetics from these particles (or, more likely, both) is not yet clear. This strong binding within the sediments likely decreases the availability of PAH to benthic organisms (Maruya et al., 1996; Naes et al., 1998; Lamoureux and Brownawell, 1999; Krauss et al., 2000). The composition of sedimentary organic matter also affects the efficiency with which benthic organisms extract PAH from sediments (Landrum et al., 1997; Standley, 1997; Weston and Mayer, 1998; Baumard et al., 1999).
Organisms are exposed to petroleum hydrocarbons in the marine environment. They are not exposed to the total amount of hydrocarbons in the water and sediment, however, because some portions of the chemical occur in forms not accessible to the organisms. The processes controlling bioavailability have been reviewed by. Partitioning strongly affects the mechanisms and magnitude of exposure of aquatic organisms to hydrocarbons. Dissolved hydrocarbons can diffuse across gill and cell membrane surfaces, and those associated with particles can be ingested during feeding. If oil droplets are present in the water column, marine filter feeders are exposed to PAH by direct uptake of the oil (Menon and Menon, 1999). Unlike other nonpolar compounds such as polychlorinated biphenols (PCBs) and certain pesticides, PAH sometimes bioaccumulate in the food chain depending on the metabolic rate of the organism.
Single-cell organisms, such as phytoplankton, are exposed to hydrocarbons primarily through partitioning of dis