Chemists, oceanographers, and geologists in the Coastal and Marine Program are presently conducting studies to determine the distribution, transport and fate of contaminated sediments in impacted estuaries around the nation. The work is conducted in collaboration with colleagues in academia, state and other federal agencies, and each regional study is tailored to meet the scientific and management needs of the region. Programs in the Gulf of Maine, Boston Harbor and Massachusetts Bays, Long Island Sound, New York Bight, the 106-Mile Dump Site off New Jersey, Florida Bay, Lake Pontchartrain, the Farralon Islands, Honolulu Harbor, and Puget Sound are at various stages of maturity (see examples below). Additional regions are under study by staff from other field centers or are planned for the future.
Environmental managers in many of these regions frequently need to know the contaminant levels and sediment volumes in areas to be dredged or remediated, how long it will take for contaminated sediments to be isolated by burial with cleaner sediment as waters are cleaned, whether sediments left in place will clean themselves, whether material migrates or accumulates preferentially in certain locations, where best to locate monitoring sites, and what kind of risk the presence of contaminants in sediment may pose to human health, fisheries, ecosystem sustainability, or recreational use of an area. Our studies of the distribution, fate and transport of contaminants in coastal sediments has allowed us to help answer many of these questions. For others, such as issues of toxicity and characterization of sources of contaminants to the marine system, the Marine and Coastal Program can benefit from increased collaboration with specialists in complimentary fields.
These tools have been used to study pollutant transport and fate in a number of studies. Results from the Boston Harbor/ Massachusetts Bays Study, the Gulf of Maine Contaminated- Sediment Database Project, the Long Island Sound Environmental Study, and the investigation of Pollutant Transport in the Hudson Shelf Valley will be given as examples.
Despite the general decrease in metal concentrations in Boston Harbor surface sediments, contaminant levels are still of concern. The highest concentrations in the surface sediments are in the Inner Harbor; values decrease in the Outer Harbor and in Massachusetts Bay. The concentrations of Ag and Hg (two characteristic contaminants) measured at some outer harbor stations in 1993 were still above the level associated with frequent adverse effects to marine organisms and concentrations of additional metals were in the range considered to occasionally induce adverse biological effects. USGS measurements have also helped define the scale and impact of natural variability in measurements of contaminants; which is helping managers identify anthropogenic impacts and better assess the need for remediation.
The highest metal concentrations in surface sediments occur where high sediment accumulation rates coincide with a nearby source. Boston Harbor is a source of contaminated sediment to Massachusetts Bay and offshore areas; the halo effect that is seen around all urban areas that we have studied is typified by elevated metal concentrations in surface sediments where both Boston and Salem Harbors enter the Bay. A layer of sediment (30-160 cm in thickness) that has metal concentrations nearly an order of magnitude above background values is present in depositional areas throughout the region. In erosional or non-depositional areas, patchy contaminant concentrations reflect natural variability in the sedimentary substrate. The distribution of Ag and Clostridium perfringens in sediment cores suggests that Cape Cod Bay, 50 km southeast of the harbor mouth, is a long-term sink for fine-grained sediment and sewage-derived contaminants. Sediments are resuspended and carried by currents southwest to Cape Cod Bay, where they are deposited and protected by the tip of Cape Cod from strom waves that casue subsequent resuspension.
Our database of existing data on chemical contaminant concentrations in sediment for the Gulf of Maine region has been compiled with the collaboration and cooperation of many scientists, agencies, and institutions. The database contains data on chemical constituents and sample identification from published and gray literature sources plus comments about the "quality" and degree of certainty of the data. Its usefulness relies on: collaborative effort among data generators and data users, "user-friendlly" computer tools, documentation that meets the needs of a variety of users, graphics and search capabilities for active use as a scientific tool, and 5) the ability to grow and evolve. Over 6200 sediment samples are documented.
The concentrations of metal and organic contaminants are being used to create maps and interpretations of contaminant distributions in the region. Sample density is not uniform in the Gulf of Maine: samples are concentrated in urban, coastal locations and targeted study areas, such as U.S.-EPA designated National Estuaries. Concentrations of metal contaminants in surface sediments range from background values to anthropogenically enhanced values that are up to three orders of magnitude above natural concentrations. The highest contaminant values in surface sediments are located near urban areas, with decreasing values seaward. The patchy distribution of contaminant concentrations for bulk sediments at many spatial scales in the Gulf of Maine reflects patchiness in both contaminant sources and sedimentary regimes. The database and related sedimentary -environment maps provide a growing body of data that is being be used for scientific and management purposes, such as locating monitoring sites for EPA.
Sediment cores were collected for geochemical analysis in 1996 at 55 stations along N-S transects; grab samples provided additional coverage. Core and surface samples are being analyzed for magnetic susceptibility, density, texture and concentrations of Clostridium perfringens , major elements, and contaminants (e.g. Ag, Pb, Cu, Zn, Cd, Hg, and selected organics). Eventually, radiochemical dates will allow historical horizons to be identified so that contaminant accumulation rates and mobility can be determined.
LIS sediments are generally muddy in the west with a transition to coarser textures in the east; elevated contaminant concentrations follow the pattern of being present in muddier sediments. Sediment accumulation rates in some nearshore areas are enhanced by anthropogenic activity. Since C. perfringens is a bacteria present in the intestinal tract of mammals, its spores are an excellent tracer of past and present sewage input into an ecosystem. C. perfringens profiles in cores indicate that values up to 1000 times greater than background occur to sediment depths of 30 cm at many relatively muddy sites offshore of urban centers. Cores located in coarser sediments under areas that have strong bottom currents have nearly constant contaminant values over this depth interval, suggesting that heavy sediment reworking and winnowing occurs. Sewage is often a major source of other contaminants in coastal waters so the C. perfringens measurements provide a valuable screening tool for the magnitude and distribution of other contaminants.
Contaminant concentrations measured in LIS sediments indicate widespread and long-term addition of anthropogenic components to this sedimentary system. Values in surface sediments are well above background everywhere in the sound except the easternmost sands and gravels and down-core profiles indicate that the highest concentrations often occur near the surface. These findings indicate that local management strategies will impact the overall quality of LIS and that efforts to reduce contaminant sources are not yet resulting in decreased loading to the sediments.
Concentrations of contaminants (e.g., metals such as Ag, Cu and Zn, C. perfringens spores, and organic carbon) and physical properties (magnetic susceptibility, density, grain size, color, odor) measured in cores and surface-grab samples taken in 1993- 1996 indicate that a sewage signal is present in Hudson Shelf Valley (HSV) surface sediments nearly 100 km from the dumpsite. Measurements in cores show that a lens of sludge-derived sediment was deposited in the upper 25 km of the HSV and is now buried in some places under a thin layer of cleaner sediment. The spatial and depth extent of this lens of anthropogenic sediment were mapped using sub-bottom profiler data; it contains a minimum of 6 cubic km of sediment having metal concentrations sufficiently high to cause toxic effects in test organisms.
Sedimentation rates in the uppermost portion of the shelf valley are extremely high, up to 2 cm/yr, apparently due to winnowing of fine-grained material from the shallower (and sandy) surrounding shelf and deposition of the fines in the deepening valley. Mixed layer depths extend into the "sludge" lens and allow this material to remain accessible to benthic organisms and available for remobilization despite cessation of dumping.
Metal concentrations measured over time and "downstream" confirm that sediment is actively migrating down the HSV. Sediment profiles of metals and textural laminations suggest that episodic resuspension occurs during storm events and allows the HSV to be a conduit for movement of modern fine-grained sediment and urban wastes to the deep ocean. The deposition of sewage material, and its accompanying contaminants, in the NY Bight occurs on a much larger scale than near smaller urban centers. However, identification of the relative importance of various transport and remobilization processes for contaminants here can help guide studies and regulation elsewhere.
ten Brink, Marilyn Buchholtz, U.S. G.S., Woods Hole,
For the last 5 years, Dr. ten Brink has been a geochemist in the Coastal and Marine Program of the Geological Division. Her work in the marine system has focused on mobility of radioisotopic tracers across the sediment water interface, processes related to formation and diagenesis of marine cherts, and the fate and transport of pollutants in coastal waters and sediments. Other work addressed transport of radioactive wastes in tuffaceous rock. Her expertise on trace elements and contaminant transport processes, sediment-water interactions, and interdisciplinary studies of natural systems has developed during 20 years of stiving to understand the exchange processes for these compounds between solid, colloid, aqueous and biological phases; diagenesis at the sediment/water interface in marine and lacustrine environments; and integration of water and sediment dynamics with geochemical processes.