Marine sediments serve as a repository for contaminants from a wide variety of sources. The environmental risks posed by these contaminants are determined largely by the degree to which they remobilize into the environment. This project demonstrated the Benthic Flux Sampling Device (BFSD), an instrument adapted from benthic flux chamber technology developed for oceanographic studies of the cycles of major elements and nutrients on the seafloor, to directly quantify the mobility and bioavailability of trace metals contaminants in marine sediments. The BFSD is an autonomous instrument for in situ measurement of flux rates of sediment contaminants like heavy metals (e.g., lead, mercury, chromium, zinc, and copper), polychlorinated biphenyls (PCBs), dioxins, and petroleum products. A flux out of, or into, sediment is measured by isolating a volume of water above the sediment, drawing off samples from this volume over time, and analyzing the samples for increase or decrease in toxicant concentration. The BFSD system performs autonomous collection of samples from a sealed volume at the sediment water interface during a period of 2 to 4 days, providing a basis for risk-based decision making and potential cost savings by measuring bioavailability of contaminated sediment, reducing cleanup requirements where contaminants are not remobilizing, evaluating the integrity of sediment caps, and documenting the actual contaminant contribution of sediments.
Independent evaluation of BFSD trace metals performance by California EPA has resulted in formal certification of performance. Performance in three replicate blank tests and four field demonstrations verified the trace metals performance claims for precision, accuracy and repeatability. Standardized procedures were established in support of efficient and affordable field applications. Full reports detailing the results have been published and certification results can be found on the CA EPA website (www.calepa.ca.gov/EnviroTech/).
Where it can be demonstrated that remobilization of contaminants is limited because many contaminants are strongly sequestered within the sediment and not likely to leach out, significant cost avoidance may be achieved through reduction of cleanup costs. Estimated disposal costs for contaminated sediments range from $100 to $1,000 per cubic yard. A survey of U.S. Navy shoreside facilities (NRaD, 1995) indicated that of the 31 facilities that responded, 29 reported the presence of contaminated sediments. The actual volume of contaminated sediment at these sites is not well documented; however, even conservative estimates suggest that millions of cubic yards of material may exceed sediment quality guidelines.
The mobility of metals contaminants from sediments into the overlying water can now be accurately measured with technology certified formally by a regulatory agency. This added dimension to the assessment of contaminated sediments should greatly influence future cleanup decisions. In many cases a lower mobility than that indicated by samples extracted from sediments may lead to faster, cheaper and better cleanups. Capping and in-place treatments may be more acceptable alternatives with the additional information derived from BFSD mobility results. (Project Completed - 2000)