Contaminant metals in coastal marine sediments

Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997

CONTAMINANT METALS IN COASTAL MARINE SEDIMENTS: A LEGACY FOR THE FUTURE AND A TRACER OF MODERN SEDIMENT DYNAMICS

By M. Buchholtz ten Brink,
M.H. Bothner,
F.T. Manheim, and
B. Butman
USGS, Geologic Division,
Coastal and Marine Program: Woods Hole Field Center,
Woods Hole, MA 02543

Introduction

The nearshore marine ecosystem is a dynamic environment that has highly variable bottom sediment type, diverse biological habitats, and localized depositional and transport processes. It is impacted by many activities, such as fishing, recreation, housing, sewage and waste disposal, shipping, and preservation. In many cases, especially adjacent to major metropolitan and agricultural areas, the sediments are polluted and the ecosystem has suffered detrimental effects. An understanding of the mobility, transport rates, paths, and sinks of sedimentary particles and associated contaminants is necessary for predicting both the fate of substances that have been introduced into the ocean and any risk associated with their presence. Success in managing a sustainable ecosystem will rely on our ability to 1) identify and characterize impacted areas, 2) develop predictive capabilities for the long-term fate of contaminated sediments; 3) develop monitoring strategies to document long-term change; and 4) effectively provide this information to managers and the public.

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.

Methodology

A wide variety tools and techniques are utilized in the studies of contaminated sediments to both identify contaminant distributions and mobility and to place this information in a geological and oceanographic context. These geochemical and geophysical methodologies include: 1) compilation of a comprehensive database of historical and new measurements of contaminant concentrations in sediments to synthesize current knowledge, estimate sediment toxicity, and select sampling locations; 2) bottom sampling with specialized coring equipment, submersibles and divers to obtain undisturbed and uncontaminated samples; 3) bottom photography to verify collection integrity, assess sediment heterogeneity, and classify habitats; 4) sediment traps and instrumented moorings (currents, temperature, salinity, waves, and light transmission) to collect suspended particulate matter, identify resuspension events, and measure contaminant remobilization; 5) x-radiography and core logging (gamma attenuation and magnetic susceptibility) of whole cores to assess textural, physical, and temporal variations in sediments and screen for subsampling; 6) extensive measurements of metal and bacterial contaminant concentrations in sediment samples and cores to define distributions, historical inputs, and transport paths; 7) radiochemical measurements to assign dates in cores and determine mixing and sedimentation rates; 8) high-resolution sidescan sonar imagery and bathymetry to determine spatial heterogeneity and sediment transport patterns; 8) High- resolution CHIRP seismic reflection profiling to identify large areas of anthropogenic sediments; 9) integration with the regional geologic framework, sea-floor mapping, and water-current data to place chemical results in an ecosystem context, 10) computer mapping and creation of digital data archives to aid in data analysis and information transfer; and 11) mathematical modeling to predict the long-term fate of contaminants in the marine environment.

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.

Boston Harbor / Massachusetts Bay study

The USGS Boston Harbor/Massachusetts Bay study began in 1989. Concentrations of metals in surface sediments of Boston Harbor have decreased during the period 1977-1993, based on analysis of: (1) surface sediments collected at monitoring stations in the outer harbor between 1977 and 1993; (2) metal concentration profiles in sediment cores from deposition areas of the harbor; and (3) historical data from a contaminated-sediment database, which includes about 3000 samples with information on metal and organic contaminants and sediment texture. The background and matrix-corrected concentrations of lead (Pb) measured in the surficial layer (0-2 cm) of cores decreased by an average of 46% ą 9% among four locations in the outer harbor during the 16 year period. Chromium (Cr), copper (Cu), mercury (Hg), silver (Ag), and zinc (Zn) exhibited similar trends. The decreases in metal concentrations that are observed in more recent years parallel a general decrease in the flux of metals to the harbor, such as: (1) ending of sewage sludge discharge to the Harbor in December, 1991; (2) greater source reduction (e.g. recovery of silver from photographic processing) and closing or moving of companies; (3) improvements in wastewater handling and sewage treatment; and (4) diminishing use of lead in gasoline beginning about 1973. Cleaner sediments near the surface and contaminated sediments at depth may be a nation-wide trend as source-reduction becomes widespread.

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.

Gulf of Maine Contaminated- Sediment Database Project

Information on contaminant concentrations in sediments is available from a wide variety of sources; however, data access and regional synthesis of historical measurements has traditionally been stymied by the wide variety of sources, variable levels of documentation, and lack of organized and accessible databases. Each of the regional pollution studies in the Marine and Coastal Program has a data rescue ccomponent that surmounts these difficulties. When assembled and validated, the data provide an opportunity to develop a description in space and time of contaminant levels that would be difficult to obtain otherwise. The regional Contaminated-Sediment Database could well serve as a model in building a National Sediment Quality Inventory.

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.

Long Island Sound Environmental Study

Semi-enclosed areas, such as Long Island Sound (LIS), may be particularly sensitive to anthropogenic activities. LIS has several large cities along its shores and major rivers that drain inland areas. This potential for adverse environmental effects, plus environmental management concerns, prompted a multidisciplinary study of environmental health of the estuary. One component of this ongoing study is an appraisal of the distribution of selected contaminants in LIS sediments and determination of their mobility and probable fates.

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.

Pollutant Transport in the Hudson Shelf Valley

New York City has generated millions of tons of sewage and industrial waste in the last century. Between 1923 and 1987, most of the sewage sludge was dumped in 30 m of water at a site 12 miles offshore. The distribution of contaminants in sediments and changes in their patterns observed over time have been used to identify dispersal and deposition patterns, transport rates for fine grained sediment, and to ascertain the potential of the impacted sediments for inducing toxic effects in biota both at this site and at a deep water dump site (106 mile). This information is critical for future environmental management decisions, such as where and how to dispose of material dredged from NY and NJ shipping channels and how best to regulate certain fisheries.

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.

Conclusions

In summary, the geochemical studies in a variety of coastal environments show that:

Contaminants are widespread in coastal sediments. When remediation is needed, the large volume of impacted sediment presents a challenge. In many cases, the accumulations of the last hundred yearsŐ industrial and urban waste will continue to be reworked by biological and geological processes and thus remain accessible to biota well into the future.
Urban centers and sites of past ocean disposal of wastes continue to be sources of contaminants to the coastal environment despite regulatory and practical advances in source reduction. Dated sediment cores provide a ruler with which to measure changes in many parameters over time.
Databases of contaminant measurements in sediment that are assembled from diverse sources provide information about the spatial and temporal distribution of contaminants. Synthesis of regional data allows characterization of sediment quality, guidance in sampling strategies, determination of concentration ranges, identification of accumulation sites, indicators of recovery, and evidence of sediment dynamics.
Patchy distribution of contaminants in sediments are associated with both source proximity and natural variations in sediment deposition and transport processes. Every region studied has locations of negligible contamination accumulation and other locations where high contaminant concentrations occur in "modern" sediment deposits that are tens of cm thick. The degree of uncertainty due to spatial variability must be considered in all environmental assessments.
Many contaminants are associated with fine-grained material and/or are characteristic of specific sources or input periods. Changes in contaminants patterns can consequently trace transport and cycling processes and help measure rates of modern geological and geochemical processes in the marine environment.
An integrated systems approach that includes geochemistry, geology, oceanography, and biology is necessary to develop predictive capabilities for contaminant fate and transport in coastal environments.
Regional studies benefit from the active participation of collaborators from other federal, state, and academic entities. Studies within the Coastal and Marine Program would welcome additional collaboration with colleagues interested in 1) studying effects of contaminants or human activity on biota in the coastal ecosystem, 2) assessing continental, atmospheric, and riverine sources of contaminants to coastal sedimentary systems, and 3) creating databases of contaminant data. Geochemical tools and techniques utilized in the work discussed here can also be applied equally well to continental waters.

ten Brink, Marilyn Buchholtz, U.S. G.S., Woods Hole, MA.

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.

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