The objective of this project was to develop and standardize a procedure using field deployable solid phase microextraction (SPME) for the measurement of freely dissolved porewater concentrations of hydrophobic organics and demonstrate the relationship of these measurements to contaminant flux, bioavailability, and bioaccumulation.
Specific objectives of the polydimethylsiloxane (PDMS) technology for hydrophobic organic compounds include:
- Determination of mobile and available contaminants in sediments
- Assessment of bioaccumulation potential in benthic organisms
- Assessment of vertical chemical profiles in surficial sediments and sediment caps.
The work was conducted in sediments both in laboratory and field testing. Laboratory testing allows sediments to be collected and tested in the laboratory, avoiding problematic field deployments where placement and retrieval is too difficult or costly. This testing also allows coupling of availability measurements with laboratory bioassays under controlled conditions avoiding the difficulties and variability of field bioassays. The field testing allows determination of availability and cap performance under conditions that might not be reproducible in the laboratory. All porewater measurements were measured in situ (i.e., in sediments whether field or laboratory) and require no porewater separation from the sediments prior to analysis.
In situ SPME is a passive sampling approach for measuring hydrophobic organic contaminants in porewater and involves the insertion of a polymer sorbent into the sediments for a specific period of time and measuring the contaminants sorbed to the polymer. The contaminant concentration on the polymer is directly proportional to the dissolved contaminant concentration in the porewater. The technology demonstrated in this project uses PDMS as a polymer sorbent as a thin coating on a glass core but is essentially equivalent to SPME using other sorbents such as polyoxymethylene (POM) and polyethylene (PE). The primary advantages of PDMS are cylindrical geometry (for ease of insertion into sediments), somewhat lower sorption capacity than POM and PE (which aids the rapid achievement of equilibrium), and commercial availability in a variety of sizes and polymer coating thicknesses. Simple approaches to shield the PDMS fiber and to segment and analyze the fiber were developed as part of the demonstration. Conventional analyses are employed that require no special processing or analytical techniques so any laboratory that can conduct polycyclic aromatic hydrocarbon (PAH) and polychlorinated biphenyl (PCB) analyses can support the technology.
Demonstration of the technology was conducted in several phases:
- Laboratory demonstration of detection limits, accuracy, and reproducibility of PDMS-SPME for measurement of water concentrations. The technology could measure hydrophobic organic contaminants with accuracy and reproducibility equivalent to conventional techniques but with very low detection limits.
- Evaluation of kinetics of uptake of PDMS-SPME for water and porewater concentrations. Models capable of describing PDMS-SPME uptake kinetics were developed and methods for field evaluation demonstrated.
- Laboratory demonstration of the relationship between measured porewater concentrations and bioaccumulation in various benthic organisms. The demonstration showed that the potential for bioaccumulation was approximately given by the product of the octanol-water partition coefficient, Kow, of the compound and the measured porewater concentration.
- Laboratory demonstration of cap performance assessment using measured porewater concentration profiles. The demonstration showed that porewater concentrations in the biologically active zone of a sediment cap also indicated bioaccumulation in benthic organisms populating a cap. A cap that effectively reduced the porewater concentration to which benthic organisms were exposed was shown to be effective.
- Field measurement of porewater concentration profiles in sediments. The demonstration showed that vertical profiles in hydrophobic organic contaminants could be measured in situ, assisting in the evaluation of the mechanisms and rates of transport.
- Field measurement of relationship between bioaccumulation in benthic organisms and measured porewater concentrations. Field measurements of bioaccumulation in various benthic organisms and sediments were shown to correlate with measured porewater concentrations in the near surface sediments. Field measurements were complicated by the dynamics of uptake onto the sorbents, the dynamics of uptake in the organisms, and the presence of other stressors in the field.
The primary difficulties associated with the in situ PDMS measurement of porewater concentration is the time and cost of deployment and the complexities of interpretation of the results. Deployment may involve divers for both placement and retrieval (although sediment can also be retrieved by conventional means, e.g., coring, for laboratory testing) and long delay times between placement and retrieval (7-28 days). Expert knowledge is required to appropriately balance considerations such as achievable detection limit and rate of attainment of equilibrium. For more hydrophobic compounds, methods must be employed to assess attainment of equilibrium. The attainment of equilibrium is primarily controlled by site-specific processes and sediment properties. Failure to accurately assess polymer uptake kinetics and the degree of equilibration with a given exposure can significantly limit the applicability of the results.