Using Passive Polyethylene Samplers to Evaluate Chemical Activities Controlling Fluxes and Bioaccumulation of Organic Contaminants in Bed Sediments
Evaluating the hazard posed by organic contaminants in sediment beds remains a difficult challenge. It requires quantifying the contaminants' presence in terms that reflect their “availability” to move to other locations (e.g., overlying waters), to various receptors (e.g., shellfish and fish), and to microorganisms that might facilitate degradation. The Department of Defense needs an efficient and accurate means to characterize such bed sediments at sites near training facilities. If deposits are confidently deemed to be hazardous and the spatial extent is accurately determined (both depth into the bed and lateral distribution), effective remedial actions for contaminated sediments can be planned and executed.
The objective of this project was to demonstrate the efficacy of using polyethylene (PE) strips to assess the hazard posed by sediment beds contaminated by hydrophobic organic compounds (HOCs) such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs).
The method involves insertion of PE strips, pre-loaded with internal standards, across the bed-water interface at sites of concern, leaving the PE to absorb HOCs, and then retrieving the samplers and measuring the HOCs accumulated in the PE by solvent extraction and gas chromatography-mass spectrometry (GC/MS). After using the internal standards to extrapolate observed HOC loads to their equilibrium levels, PE-water partition coefficients can be used to calculate contaminant pore water concentrations (or chemical activities) as a function of depth into the bed.
Independent measures of pore water concentrations showed good correspondence with the PE-inferred results. Moreover, PE-inferred pore water concentrations of PAHs at six intertidal field sites were consistent with levels needed to explain PAH tissue burdens in soft-shelled clams living at those locations. Given the relative ease of the PE passive sampling method, these results strongly support use of PE passive samplers to assess the risks posed by organic contaminants in sediments. The technology is currently being demonstrated and validated under ESTCP project ER-200915.
Passive PE samplers offer several advantages. The extraction and analysis of the PE after deployment has proven to be much easier than corresponding sediment or organic tissue measures. Notably, there is little need to use normal phase liquid chromatography steps to clean up the extracts before gas chromatography. The PE samplers also can be loaded with internal standards or performance reference compounds (PRC) so adjustments are made for their use at every site in varying conditions (e.g., temperature, biofouling). Moreover, these PRC allow case-specific passive sampler calibration so that other target contaminants of interest can be quantified from the same sediment. Deployment times of 1 month or more, however, may be necessary to achieve PRC losses that are sufficient to measure precisely so the samplers can be accurately calibrated, and deployments into beds below water that is deeper than divers can go (>30 m) have not yet been sufficiently tested.