U.S. Geological Survey

Contaminated-Sediment Database Development and Assessment in Boston Harbor

"Sediments Have Memories"
--J.R. Schubel1

Bottom sediments have been regarded as memory banks for contaminant inputs to urban waterways. Bottom sediments accumulate by the addition of particles that enter the waterway from many sources (U.S. National Research Council, 1989). Sediments include solid contaminants, as well as contaminants from the water column that are adsorbed on organic matter or soil (rock) particles. Sediments from Boston Harbor and adjacent parts of Massachusetts Bay are now among the most intensively studied in the Nation in terms of chemical composition.

Scientists at the U.S. Geological Survey (USGS), Woods Hole, Mass., undertook a pioneering effort to assemble a database from all available sources of information on chemicals in sediments of the Boston Harbor study area. The database was created to help understand the distribution of sediment contaminants and their sources, transport, and other processes and to support environmental management. The work involved collaboration with the U.S. Environmental Protection Agency (USEPA), the U.S. Army Corps of Engineers (USACOE), the Massachusetts Water Resources Authority, and other organizations. More than 100,000 sediment measurements from more than 3,000 samples were gleaned from 1,500 references. New approaches were developed to scientifically edit and validate data from heterogeneous sources. The compiled data included inorganic and organic chemical information and physical parameters like sediment texture (grain size), as well as documentation to minimize a userıs need to track down hard-to-find reports.

Contaminant Distribution and Toxicity Potentials

Fifty percent of the total sediment samples available from the harbor area show concentration levels that exceed screening-level, bulk-sediment toxicity criteria for zinc, copper, lead, and mercury. The zinc concentration in surficial sediments of Boston Harbor is shown in figure 1, along with locations of known waste inputs.

Figure 1
Figure 1. Zinc concentrations in Boston Harbor sediments (total database including all depths). The color coding uses the screening-level, bulk-sediment toxicity criteria of Long and others (1995). The concentration levels denoted by red, yellow, and blue correspond to "Effects Range Medium," "Effects Range Low," and concentrations below those expected to produce toxic effects on bottom organisms, respectively. Natural background concentrations of zinc in average sediments are approximately 90 µg/g. "Outfall" means a submarine pipe (present or past) discharging sewage effluent or sludge; combined sewage outfalls (CSO's) combine street runoff with household wastewater and may discharge raw sewage to the harbor during heavy rains.

Bulk-sediment toxicity guidelines of the kind used in figure 1 (Long and Morgan, 1990; Long and others, 1995) are among the few means available to interpret the toxicity status of samples described by historical data. The criteria based on concentration levels are generally agreed to be conservative and help constrain areas that may require more specific (higher tier) toxicity evaluations. These evaluations include site-specific animal toxicity tests, AVS (acid-volatile sulfide) measurement, bioaccumulation tests, and bottom organism community analysis. The criteria of Long and Morgan (1990) and Long and others (1995) were developed by the National Standards and Trends benthic monitoring program of the National Oceanic and Atmospheric Administration.

Characteristic Patterns in Contaminant Distributions

Contaminants tend to follow distinctive statistical patterns when sufficient numbers of samples are available. Compilation of total data on seven elements from the 1993 version of the Boston Harbor database (fig. 2) suggests that contaminants in sediments follow fractal patterns. Fractal distributions share a common symmetry in their frequency patterns, regardless of the absolute concentrations of the contaminants This independence of distribution from concentration is termed "scale invariance." The current expanded data set shows patterns similar to those in figure 2. Zinc (top histogram in fig. 2) has the largest concentrations, followed by lead, chromium, and copper. The median values of contaminant metals range from 4 to more than 20 times estimated background (pre-anthropogenic) concentrations.

Figure 2
Figure 2. Sample frequency distribution for seven potentially toxic metals in sediments from Boston Harbor and inner Massachusetts Bay (1993 data). Colors correspond to those in figure 1.

Arsenic and silver (not shown) and other metals are also found at higher levels than background values for naturally occurring estuarine sediments. Organic contaminants such as polychlorinated biphenyls (PCBıs), polyaromatic hydrocarbons (PAHıs), and pesticides like DDT and chlordane are also widely distributed in the sediments, although the proportions of the sediments that fall into toxic ranges are smaller.

In spite of these coherent statistical patterns, the spatial distribution of contaminants shows a high degree of patchiness, as can be inferred from figure 1. Local variability becomes more extreme in Boston Inner Harbor close to the coasts. Here, samples 100 meters apart may show markedly different concentration levels, a condition that is partly related to proximity to point sources of contaminants. In harbor environments, sediment composition at any given point may not be representative of larger areas; the often extreme spatial variability emphasizes the importance of extensive spatial data coverage.

Estimating Sediment Accumulation Rates on Dredged Bottoms

Knowledge of the physical and chemical composition of bottom sediments is needed to guide dredging and sediment disposal for deepening of shipping channels. Recent studies by the USACOE have been used with USGS sediment data to estimate the thickness of sediments deposited since the last major dredging of the channels, which occurred around 1948. Accumulated sediment thicknesses were estimated from cores and acoustic profiles for sites in the Chelsea and Mystic Rivers and for portions of the Inner Harbor (fig. 3). The computed accumulation rates range from 0 to 1.87 cm/yr, with an average of about 1 cm/yr. This average agrees with USACOE estimates of about 1.3 cm/yr calculated independently from areal volumes.

Figure 3
Figure 3. Thickness of sediments in various locations, Boston Inner Harbor bottom.

Such data allow estimation and prediction of sediment supply and burial rates. Accumulation rates help define specific contaminant loading per unit time. Burial rates are important to estimate the time needed to cover severely contaminated sediment layers with cleaner sediments that result from improvement in wastewater management.

Results and New Applications

Concurrent with updating of the Boston Harbor sediment database and its assessment, the results and methods from this project have been utilized to compile similar chemical databases in the Gulf of Maine and in southern Louisiana estuaries. A Long Island Sound database has been started, and a cooperative compilation and an assessment are planned to complement field studies in the New York Bight area. These research efforts have so far resulted in expansion of electronically accessible data by five to ten times. They extend knowledge of the geographic distribution of contaminants in coastal and estuarine waters and contribute to national assessment efforts by the USEPA and other agencies, as well as by local environmental managers.

References Cited

Long, E.R., MacDonald, D.D., Smith, S.L., and Calder, F.D., 1995, Incidence of adverse biological effects within ranges of chemical concentrations in marine and estuarine sediments: Environmental Management, v. 19, no. 1, p. 81-97.

Long, E.R., and Morgan, L.G., 1990, The potential for biological effects of sediment-sorbed contaminants tested in the National Status and Trends Program: Seattle, Wash., National Oceanic and Atmospheric Administration, NOAA Technical Memorandum NOS OMA 62, 175 p.

U.S. National Research Council, Committee on Contaminated Marine Sediments, 1989, Contaminated marine sediments -- Assessment and remediation: Washington, D.C., National Academy Press, 493 p.

For more information, please contact:

Frank Manheim, Marilyn Buchholtz ten Brink, Polly Hastings, or Ellen Mecray
U.S. Geological Survey
Woods Hole Field Center
384 Woods Hole Road
Woods Hole, MA 02543-1598
phone: (508) 548-8700
E-mail address: fmanheim@usgs.gov

URL: http://coast-enviro.er.usgs.gov/boseco/ and linked sites

U.S. Department of the Interior
U.S. Geological Survey
                                                  USGS Fact Sheet FS-078-99
March 1999

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