Quantifying In Situ Contaminant Mobility in Marine Sediments
Metal Contaminant Mobility Demonstration
Objectives of the Demonstration
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)
Organic Contaminant Mobility Demonstration
Contaminants enter shallow coastal waters from many sources, including ships, shoreside facilities, municipal outfalls, spills, and non-point source runoff. Sediments are typically considered a primary sink for these contaminants. Sediments in many bays, harbors, and coastal waters used by the Department of Defense (DoD) are contaminated with potentially harmful metal and organic compounds. DoD is required by the Comprehensive Environmental Resource Conservation and Liability Act, as amended by the Superfund Amendment and Reauthorization Act of 1986, to assess and if necessary remove and remediate these sites and discharges in order to protect the public health or welfare of the environment. To determine whether contaminants are moving into, out of, or remaining immobilized within the sediments, a determination of contaminant flux must be made.
Objectives of the Demonstration
The primary objective of the demonstrations of the Benthic Flux Sampling Device 2 (BFSD2) was to perform deployments at contaminated sites in San Diego Bay and Pearl Harbor under the observation of California Environmental Protection Agency (EPA) certification evaluators and other observers, including local, state, and federal regulators; remediation program managers; academics; industry; and other DoD representatives. Each site offered different validation opportunities. San Diego Bay was used to show instrument repeatability and comparison with historical trends, and Pearl Harbor was used to show site differences and geochemical trend analysis. The specific planned objectives of the demonstrations were to:
1. Evaluate the quality of water samples collected using the BFSD2, specifically for use in determining if a statistically significant flux was occurring at the test locations in comparison to the blank flux results for the BFSD2.
2. Evaluate the BFSD2 for repeatability.
3. Evaluate the logistical and economic resources necessary to operate the BFSD2.
4. Evaluate the range of conditions in which the BFSD2 can be operated.
Another objective was to expose various user communities to the technology to encourage continued interest and applications.
This project included demonstrations of the commercialized BFSD2 at sites in San Diego Bay (Paleta Creek) and Pearl Harbor (Middle Loch and Bishop Point). BFSD2 performance assurance indicators showed that (1) a proper seal was achieved during both sets of demonstration deployments and chamber isolation of test water was maintained; (2) oxygen levels were maintained close to ambient levels except for the Bishop Point organics test; and (3) silica, oxygen, and pH trends varied as expected. The samples collected were thus considered valid for laboratory analysis. The resulting flux calculations demonstrated statistically significant contamination mobility.
Earlier sets of deployments of BFSD2 at Paleta Creek and at Pearl Harbor demonstrated consistent performance and the ability to measure trace metal mobility at distinctly different sites. The subsequent single deployments at Paleta Creek and Pearl Harbor demonstrated continued consistent performance and the extended ability to measure trace levels of organic contaminants. The applicable performance capabilities and demonstration objectives were met. Ease of operation and reliability were also demonstrated. It was further concluded that BFSD2 provides accurate and repeatable measurements of the mobility of metal and organic contaminants to and from shallow water marine sediments when the prerequisite performance assurance indicators are met. These sediment flux rates can be established with high confidence when the routine procedures, standard methods, and protocols are followed. Comparison of measured sediment fluxes with blank-chamber fluxes provides a statistical benchmark for the significance of the measured flux rates. Where statistically significant fluxes are observed, evaluation of impacts on water quality can be carried out, or comparisons can be made to bioaccumulation measurements to help identify exposure pathways. The resulting analysis will provide a significant new tool in evaluating potential cleanup options at contaminated sediment sites.
These demonstrations were used by evaluators from California EPA as part of their Technology Certification Program process. Accurate, precise, and repeatable results were obtained at both locations. The San Diego sites were used to emphasize repeatable performance, and the Pearl Harbor sites were used to emphasize the range of conditions for operation. Routine and standardized methods and procedures were used throughout the operations.
The BFSD2 and its support equipment are mobile by air transport, field portable, and can be operated with a minimum of resources. One technician experienced with standard BFSD operational procedures and the part-time assistance of a deck hand plus a skilled small boat operator are required.
Technical performance, schedule, and cost data were collected during the demonstrations, and they document the utility of the BFSD2 technology to measure, in situ, the mobility of contaminants in marine sediments effectively, efficiently, and in a timely manner. This new approach has no directly comparable technology in current use and thus represents an innovative and new resource to the environmental community.
Points of Contact
Mr. Bradley Davidson
Space and Naval Warfare Systems Command (SPAWAR)
SERDP and ESTCP