Evaluation of physical/chemical mechanisms controlling PCB release from river sediments

Experiments were conducted to assess the mass transfer release of polychlorinated biphenyls (PCBs) from Grasse River (Massena, NY) sediments under quiescent conditions and these results were compared with fluxes inferred from field measurements. Water-sediment batch equilibration tests provided data on PCB porewater concentrations for various river sediments and sediment mixtures containing different amounts of PCBs. Individual PCB congener measurements performed during the batch equilibration tests were used to estimate PCB-homolog partitioning between water and sediment, and this information was used to assess the PCB-homolog porewater concentrations for the various river sediments and sediment mixtures used in this study. A modified Freundlich-type relationship represented the partitioning of PCB-homologs between water and the organic carbon contained in the different river sediments and sediment mixtures.; Flow-through column studies were employed to evaluate the release flux of PCBs under quiescent conditions for the various river sediments and sediment mixtures and to estimate overall mass transfer coefficients for release of PCBs from the different sediments and sediment mixtures tested. The observed total PCB release fluxes ranged from about 1 to 15 mg/m{dollar}sp2{dollar}yr.; The aqueous phase PCB-homolog distributions were similar in relative proportions for the batch water leach tests, the column flux tests, and measurements in the Grasse River. The aqueous-phase PCBs comprised mainly dichloro-, trichloro- and tetrachloro-PCB homologs. The information on release of PCBs from river sediments obtained in the laboratory work was compared with estimated values of release of PCBs from sediments in the Grasse River. The estimated release of PCBs in the Grasse River was based on several sets of field measurements of congener-PCBs in the water-column. This information was available for different river flow conditions comprising low, medium, and high flow conditions. Mass transfer coefficients for release of PCBs in the Grasse River were estimated using correlations based on turbulent flow theory. These correlations predicted mass transfer coefficients of about 2 cm/d, as was observed in the river for low flow conditions ({dollar}<{dollar}10 m{dollar}sp3{dollar}/s), but which underestimated the mass transfer of PCBs for river flows higher than 10 m{dollar}sp3{dollar}/s. This difference was attributable to other factors such as sediment mixing/resuspension, groundwater discharge, and bioturbation. The load of PCBs contributed to the river from contaminated sediments is judged to be dominated by the large amount of sediments with low-level PCB concentrations (5-20 mg/kg total PCBs), rather than from a few hot spot locations with higher-level PCB concentrations.; A complementary laboratory study was performed to assess the release of hydrophobic organic compounds (HOCs), specifically naphthalene, phenanthrene, and pyrene, from organic phases where the formation of an HOC-depleted zone within a viscous organic phase may become the limiting step for the dissolution of HOCs into water. A mathematical model for this case was proposed and experimental data from long-term experiments were used to test this model. These results confirmed that diffusional resistance in the nonaqueous phase controlled mass transfer for viscous organic materials, i.e., transmission oil, petrolatum, and paraffin, wherein naphthalene diffusivities, for example, in the organic phase were less than about 10{dollar}sp{lcub}-8{rcub}{dollar} cm{dollar}sp2{dollar}/sec. The transition from aqueous-phase dissolution control to nonaqueous-phase dissolution control appears predictable, and this provides a framework to assess long-term release of HOCs from viscous, nonaqueous phases liquids and semisolids.