The Determination of Sediment Polycyclic Aromatic Hydrocarbon Bioavailability Using Direct Pore Water Analysis by Solid-Phase Microextraction
Dr. Stephen Geiger | AECOM
The Department of Defense (DoD) currently has environmental liabilities associated with polycyclic aromatic hydrocarbon (PAH) impacted sediments at many of its sites. The magnitude of these liabilities is determined by assessments of the ecological risks that are represented by these impacted sediments. One of the primary goals of these ecological risk assessments is to determine what concentration of PAHs can remain in the sediments without causing an unacceptable risk to the environment. Studies of the bioavailability of sediment-bound PAHs and their toxicity to aquatic organisms have shown that the use of the total PAH concentrations in the sediment to predict toxicity often overestimates the ecological risk, which can lead to additional remediation costs with no additional reduction in risk. The observed lack of toxicity at elevated total PAH concentrations in sediments has been attributed to the fact that these compounds are much more strongly bound to sediment organic carbon than is assumed by the standard equilibrium partitioning model, reducing their bioavailability to the receptor organisms.
Objectives of the Demonstration
Specific objectives of this demonstration were to (1) use solid-phase microextraction (SPME) analyses of sediment pore water to predict the bioavailability of PAHs in freshwater sediments collected from the Washington Navy Yard (WNY) site, (2) compare the predicted bioavailability of the impacted sediment PAHs to the actual measured toxicity of the sediment to the freshwater amphipod Hyalella azteca, and (3) use these data to develop technical/regulatory guidance for the management of PAH-impacted sediments that incorporates the use of the site-specific estimates of PAH bioavailability for the purpose of predicting toxicity, assessing risk, and making more informed sediment management decisions.
To eliminate the uncertainty in pore water concentrations using published equilibrium partitioning coefficients, a method to directly analyze PAHs in pore water has been developed. An industry-led organization, the Sediment Contaminant Bioavailability Alliance (SCBA), was formed to develop an analytical method that (1) directly determines dissolved-phase PAH sediment pore water concentrations, thus alleviating the need to model or predict pore water PAH concentrations based on equilibrium partition from multiple phases of carbon that might be present; (2) enables the collection of a very small quantity of sediment (20 to 40 mL); and (3) provides low detection limits in the analysis of PAHs in pore water (pg/mL or ppt). The method developed by the SCBA—EPA SW-846 Method SW-8272 and Association of Standard and Testing Materials (ASTM) Method D-7363-07—utilizes SPME.
SPME has been used to directly estimate PAH bioavailability and predict the toxicity of PAHs in contaminated sediments with greater accuracy than can be achieved by comparing total PAH concentrations to sediment screening guidelines. The pore water concentrations measured by SPME are expressed in terms of toxic units (TUs): pore water concentrations divided by final chronic values. The TUs strongly correlate with survival of the freshwater amphipod H. azteca, the freshwater midge Chironomous dilutus, and the marine amphipod Leptochirus plumulosus. The individual PAH TUs are summed over a sediment sample to arrive at a total PAH (TPAH) TU. A TPAH TU<1 indicates no probable impact to benthic organisms, while a PAH TU>1 indicates a potential impact to benthic organisms.
The demonstration tested the applicability of a direct pore water analysis method for PAHs, the results of which were compared to TUs that result in hydrocarbon narcosis from PAHs to benthic organisms. As such, this method provides a site-specific estimate of PAH bioavailability in the lower Anacostia River sediments and provides a direct comparison to aquatic toxicity test results utilizing H. azteca as a model benthic invertebrate species. This project served three purposes:
- The project data supports the development of site-specific management strategies for sediments in the Anacostia River immediately adjacent to the WNY.
- The project demonstrated the SPME analytical methodology to be an important and cost-effective sediment characterization tool for DoD site managers at and beyond the WNY.
- The project data was combined with the existing SCBA database to support changes in the current regulatory paradigm for managing PAH-impacted sediments, which compares total PAH concentrations to published sediment quality guidelines.
The EPA white paper entitled Procedures for the Derivation of Equilibrium Benchmarks (ESBs) for the Protection of Benthic Organisms: PAH Mixtures ( EPA-600-R-02-013) established the effectiveness of using direct PAH pore water measurements when assessing the effects of sediment-bound PAHs to benthic organisms. However, many state and federal agencies still require that aquatic toxicity tests be conducted alongside pore water analysis before making site decisions. Therefore, the acceptability of the SPME direct PAH pore water method to regulatory agencies will need to be negotiated on a site-by-site basis. The 2011 Interstate Technology & Regulatory Council (ITRC) document on contaminated sediment bioavailability will help improve understanding of the use of bioavailability measurements for site assessment and characterization.
The SPME direct pore water method has been accepted as an EPA SW-846 Method SW-8272. It also has been given an ASTM provisional method designation ( D-7363-07). As of the completion of this project, the round-robin laboratory testing required for full ASTM method establishment was under way. The pore water analysis for this study was conducted in a combination research/commercial analytical laboratory (EERC). Other research laboratories have conducted this analysis (UMBC, University of New Hampshire). There are three commercial laboratories participating in the ASTM method approval (Test America, Alpha Woods Hole, and META) and therefore able to provide this analytical service. It is hoped that other commercial laboratories will provide analytical services on this method once the full ASTM method assignation is provided. A web-based tool that provides an overview of this method can be found at http://t2.serdp-estcp.org/t2template.html#tool=SPME&page=Introduction.