The objective of the project is to validate the application of a High Resolution Passive Profiler (HRPP) that is capable of (1) evaluating the bioavailable distribution of contaminants (metals and organics) in sediments via passive sampling, while simultaneously measuring (2) dominant redox processes, (3) key gene/microbial densities, and (4) pore water velocity as a function of sediment depth at cm resolution. This project will demonstrate the application of a cost-effective, commercially deployable unit that can be applied to evaluate the performance of remedial efforts and corresponding reduction in long term risk, and to increase our understanding of the overall impact of typical and novel remediation measures on sediment geochemistry, microbial processing, and sediment pore water transport.
The HRPP is a passive sampler that can be directly inserted into a saturated environment (e.g., aquifer, marine or freshwater sediments) to simultaneously determine the concentrations of biogeochemical species and pollutants, the composition of microbial communities, microbial activity, and pore water velocity as a function of depth. The HRPP uses equilibrium diffusional sampling to evaluate dissolved biogeochemical and metal species in the pore water and polymeric sorbents for passive sampling of hydrophobic organic compounds (HOCs) such as PCBs and PAHs. The microbial community is evaluated using a micro-scale Bio-Trap consisting of Bio-Sep beads placed within a modified peeper chamber separated from the sediments using a stainless steel mesh. In some instances, contaminants adsorbed on the Bio-Sep beads can also be evaluated via Compound Specific Stable Isotope Analysis (CSIA) to assess their transformation potential. Pore water velocity is estimated using the loss of a conservative tracer and a relationship between the mass transfer co-efficient and velocity developed under project SERDP ER-2419. The HRPP can be used to evaluate a wide range of contaminants including metals, anions, chlorinated volatile organic compounds (CVOCs), HOCs, and explosives, among others. The true innovation of the HRPP for contaminated sediment characterization (sediment HRPP; sHRPP) is its ability to produce high resolution data sets that combine contaminant concentrations, geochemical conditions, microbial community composition, and transport in a single co-located depth profile. Individual methods exist for each of these components but currently cannot be simultaneously evaluated in a single profile. Furthermore, the efficacy of remediation efforts whether based on microbial transformation and/or sorption is difficult to evaluate, particularly if based upon separate samples due to spatial heterogeneity in sediments. The sHRPP can evaluate changes in contaminant bioavailability, transformation, and impact of remediation efforts on biogeochemistry and transport. The combined data will lead to more accurate models of contaminant bioavailability, transformation, and fate and transport as well as better estimates of the long term reductions in contaminant bioavailability by producing higher fidelity sediment characterization.
Passive samplers for measuring pollutant bioavailability in sediments have been employed at a number of contaminated sediment sites but their use has been largely viewed as applied research that requires academic involvement. There has been limited regulatory acceptance and limited use of this approach by consulting firms and commercial laboratories. Through this project, passive sampling will be integrated into a commercial field characterization tool (sHRPP) transitioning the results from previous SERDP efforts to the field for simultaneous characterization of key geochemical processes, microbial genes/densities, pore water velocity, and contaminant bioavailability. The ability to obtain co-located profiles of these key parameters will provide better site models and better evaluation of natural attenuation and active remedial efforts. This in turn will improve site management at lower costs and higher reliability. The use of a standardized data collection tool and commercial laboratories for interpretation and analysis will provide the long-awaited transition for passive sampling to routine use in the field to inform better, more efficient site remedies. (Anticipated Project Completion - 2021)