Passive Sampling in Contaminated Sediments: Improving Assessment of Exposure and Bioavailability
Hydrophobic organic compounds like PCBs contaminate sediments at many Department of Defense (DoD) sites. Assessment traditionally requires expensive sampling campaigns and laboratory analyses. High costs are driven by the need to obtain enough samples to define the scope of the problem and to overcome analytical difficulties arising from complex mixtures. Composite samples reduce costs, but they also reduce our understanding of the site, potentially even masking “hot spots”. Designing and implementing successful in situ remediation requires knowledge of the vertical and horizontal extent of contamination to ensure completely addressing the problem area, while avoiding expending resources on clean materials.
Much research has shown the inaccuracy of using sediment concentrations to infer exposures of receptors and corresponding risks. The equilibrium partitioning (EqP) approach for determining sediment concentrations is limited because it does not consider soots and chars (together called black carbon or BC) that are now known to be in all sediments examined to date.
One way to circumvent the problem of using sediment concentrations to estimate organism exposures would be to use passive samplers. Such samplers accumulate contaminants from the environmental matrix in proportion to the chemical activities in the sampled medium. Polyethylene (PE) passive sampling is well suited for determining the distribution of contaminants like PCBs in sediments, thereby facilitating remedial designs and long-term monitoring. These passive samplers are simply sheets of low-density polyethylene held in an aluminum frame. Performance reference compounds (PRCs) are impregnated into the polyethylene before use to allow evaluation of each deployment. The polyethylene strips are inserted into the sediment and retrieved at a later date. They are extracted with organic solvent and the extracts are analyzed by gas chromatography–mass spectrometry. Using appropriate polyethylene-water partition coefficients, the porewater or bottom water concentrations of target contaminants are deduced. These data are used to show bed-water column gradients driving continuing contamination of overlying ecosystems, as well as bioavailable contaminant levels in the sediment. Compared to traditional sediment site characterization, the PE technology provides cost reductions in manpower, days in the field, equipment, and shipping. This technology is also safer than traditional sediment sampling and handling. For sites already being remediated, long-term monitoring using PE samplers can be readily implemented in place of alternatives like mussel deployments.
Under ESTCP project ER-200915, Dr. Phil Gschwend and his team from the Massachusetts Institute of Technology, have recently completed a series of Standard Operating Procedures for the preparation, deployment, and analysis of PE passive samplers based on lessons learned during their demonstration of the sampler. In addition, the project team has developed a Performance Reference Compound (PRC) Calculator to allow users to easily process PRC data from PE passive sampler deployments.
If commercially developed and accepted by regulators, the PE passive sampling technique could be cost-effectively employed at virtually all DoD contaminated sediment sites. By increasing the data quantity and improving the data quality through the use of PE passive samplers, it is possible that some sites that would otherwise undergo dredging or ex situ remediation could be directed to a more cost-effective in situ remedy, resulting in potential cost savings on the order of millions of dollars. In addition to life-cycle cost reduction, risk is substantially reduced by the increase in certainty derived from more representative samples and improved understanding of the bioavailability of contaminants in the sediments.