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Validating the Use of Performance Reference Compounds in Passive Samplers to Assess Porewater Concentrations in Sediment Beds Jennifer N. Apell* and Philip M. Gschwend R. M. Parsons Laboratory, Department of Civil and Environmental Engineering Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: Hydrophobic organic compounds (HOCs) like polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) tend to accumulate in sediment beds when they are released into aquatic environments. Due to this buildup of HOCs in the sediment, the highest water concentrations are often in the pore water. Passive samplers can be used in the field (i.e., in situ) to measure freely dissolved porewater concentrations if target contaminants diffusing through the sediment and into the sampler exhibit the same diffusive retardation factors as performance reference compounds (PRCs) that are diffusing out of the sampler and into the sediment. To test this assumption, polyethylene (PE) passive samplers were placed in an organic- and black- carbon-rich sediment bed in the laboratory with samplers removed every 30 days for 4 months. The concentrations of target contaminants in the PE at each time point, corrected using measures of the losses of PRCs, were in good agreement with separately measured equilibrium concentrations in a well-mixed system. Concentrations in the PE passive samplers, normalized by their polyethylene-water partition coefficients, were also in good agreement with directly measured porewater concentrations. Finally, PE-deduced porewater concentrations were compared with the traditional equilibrium partitioning models and showed that considering sorption to only organic carbon substantially overestimated porewater concentrations. However, predictions improved greatly if sorption to black carbon was also considered.
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INTRODUCTION Hydrophobic organic compounds (HOCs) often persist in aquatic environments because they become associated with sediments. When the HOC contamination ceases, the built up concentration in the sediments can then become a continuing source of contamination to the overlying water as well as to the aquatic organisms. Historically, sediment concentrations have been used to decide which sediment locations are the greatest source of HOCs to the overlying water and therefore pose the highest risk to organisms. In this method, porewater concentrations are estimated using partitioning calculations that assume the product of the organic carbon content of the sediment, f OC, and the HOC’s organic-carbon normalized partition coefficient, KOC, yield the compound’s sorption coefficient, Kd. The Kd should then allow an investigator to use the sediment’s concentration to calculate dissolved porewater concentrations (EqP).1,2 However, sorption can vary greatly between sediments, especially if the HOCs of concern have significant affinities for black carbon.3−5 Passive sampling techniques have been developed to estimate freely dissolved water concentrations without using estimates of sediment sorption behavior. The passive sampler polymer accumulates HOCs in proportion to their freely dissolved concentrations and each HOC’s polymer−water partition coefficient. However, the approach to equilibrium can be quite slow when the sampler is used passively (i.e., without © 2014 American Chemical Society
mixing) and uptake slows substantially as the passive sampler accumulates target HOCs, thereby requiring impractically long deployment times for in situ use (i.e., in the field) to reach equilibrium. Performance reference compounds (PRCs), impregnated in the polymer before deployment, have been used to extrapolate measured in situ passive sampler concentrations to corresponding equilibrium concentrations. Several methods have been used for this extrapolation with the most common being those based on mass transfer models.6,7 The major assumption of these models, however, is that the PRCs, moving out of the sampler into the surroundings, and the target HOCs, moving from the surroundings into the sampler, exhibit the same mass transfer rate. If the transport is being controlled by diffusion through the polymer, this assumption is likely valid. However, in the case of HOCs, transport will likely be controlled by diffusion through the sediment bed. Therefore, PRCs and target contaminants must exhibit the same retarded diffusion through the sediment bed. If PRCs and target contaminants do not exhibit the same retarded diffusion rates through the sediments, for example due to differential sorption versus Received: Revised: Accepted: Published: 10301
June 3, 2014 July 31, 2014 August 5, 2014 August 5, 2014 dx.doi.org/10.1021/es502694g | Environ. Sci. Technol. 2014, 48, 10301−10307
Environmental Science & Technology
Article
consisting of PCB congeners 39, 55, 104, 150, and 188 (Ultra Scientific). PE-Sediment Equilibration. Sediment was placed into a 250 mL round-bottom flask with deionized water (≈100 g wet sediment containing about 3 g organic carbon, 150 g deionized water). A 20 mg piece (≈ 4 cm × 2 cm) of PRC-impregnated PE was also added. Phase ratios were determined so that the PCB uptake in the PE was
