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USGS OFR 2005-1250, Processes influencing the transport and fate of contaminated sediments in the coastal ocean - Boston Harbor and Massachusetts Bay Skip navigational side bar

Section 11: Summary of Important Findings

by Michael H. Bothner and contributing authors

Boston Harbor and Massachusetts Bay are the focus of a multidisciplinary scientific investigation having two major objectives. The first is to gain fundamental understanding about the behavior of contaminants introduced to coastal waters. The second objective is to provide relevant and accessible information to state and federal managers responsible for managing coastal environmental issues.

The coastal region off Boston is a particularly appropriate area for this focus. In the late 1980s, Boston Harbor was known as the most contaminated harbor in the nation (Section 1). At that time, the harbor was also the site of a $4 billion court-ordered cleanup program that involved cessation of sludge discharge, upgrading sewage treatment to secondary, moving the outfall for sewage effluent 9.5 miles seaward of the harbor mouth, and eliminating combined sewage overflows. The execution of the cleanup program was proceeding with limited knowledge about the nature of ocean currents, bottom sediment types, and contaminant concentrations in sediments of the harbor and bays. The need for such information fit the mandate and capability of the USGS to apply earth science principles to environmental problems in coastal areas. Within that mandate, we developed a multidisciplinary research program, which began in Massachusetts Bay in 1989 following a smaller effort in Boston Harbor initiated in 1977. This summary highlights results from seafloor mapping, circulation and sediment transport modeling, and geochemical investigations that were undertaken in the region from 1977 to the present. The full bibliography of the project is available on the project Web page,

Seafloor mapping

New seafloor maps have been completed for much of the western Massachusetts Bay, Stellwagen Basin, and the Stellwagen Bank National Marine Sanctuary (Section 2). They are based on a high-resolution multibeam echo-sounding system that provides complete coverage of the topography and backscatter intensity of the seafloor. The maps revealed for the first time the high variability in sediment texture and seafloor morphology over small spatial scales. Clear images are provided of trenches in the sea bed caused by iceberg groundings, dredge spoil disposal sites, and mounds of excavated material discharged while drilling connecting risers to the outfall tunnel. Where sidescan sonar data are available, trawl marks resulting from fishing gear are also observed. Earlier versions of these maps were used to choose between alternative sites for the outfall diffuser and to select sampling sites for the outfall monitoring program. The maps continue to provide the geologic framework for commercial, scientific, and management activities.

Modeling circulation, sewage outfall plumes, and sediment transport

Understanding the ocean currents in Massachusetts Bay has been a critical component of our research on the transport and fate of contaminants introduced by man's activities. The currents have been studied using field observations from moored instruments, drifter buoys, and hydrographic surveys. Numerical hydrodynamic models have been developed that have provided simulations of currents under seasonally varying conditions of water column density, wind speed, river runoff, and boundary conditions in the Gulf of Maine (Section 3). Simulations of residual current speeds and patterns for winter (defined as November through February) and summer (June through August), averaged for the years 1990-1992, are quite consistent with the observed flow from limited field observations (Section 4). A water particle discharged from the Massachusetts Bay outfall in winter would be carried southeastward toward Cape Cod Bay over a transit time of about 30 days. During the summer, when stratified conditions exist, surface waters from near the outfall travel east across Stellwagen Bank. At 25 m depth, below the summer thermocline, there is a southerly flow into Cape Cod Bay. In summer and winter, the residual flows in Massachusetts Bay are very slow, less than 0.05 ms-1

Numerical circulation models in Massachusetts Bay have had three applications worth special note. The first dealt with the major controversies regarding the relocation of the sewage outfall from the harbor mouth to a new location 9.5 miles seaward in Massachusetts Bay (Section 5). A key concern was that the new outfall would transfer the contaminated conditions in Boston Harbor to the eastern shores of Massachusetts Bay, Cape Cod Bay, and to the Stellwagen Bank National Marine Sanctuary, where endangered whales would be further threatened. The model simulations compared the distribution of sewage plumes originating from the (then) existing harbor outfall and the planned Massachusetts Bay outfall. The model predicted that discharge through the new bay outfall would greatly reduce effluent concentrations in Boston Harbor without significantly increasing concentrations in most of Massachusetts Bay (Signell, and others, 2000). Visualizations of these results were widely distributed and helped the public and decision makers understand the predictions, which has been verified by using the measured distribution of ammonia from the new outfall as an effluent indicator. Maps showing effluent distribution from the new outfall generated by the model and by actual measurements are in good agreement (Section 5).

A second application of the circulation model concerned the final design of the secondary sewage treatment plant. Based on model predictions, a smaller treatment system was approved, saving about $160 million (Section 5).

The third application has been the coupling of the circulation model with a sediment transport model (Section 6). Sediment maps, contaminant distributions, and current observations were used to test the combined model simulations that were carried out to explore the effects of Northeast storms on the transport of sediments in Massachusetts Bay. The model simulations show how a mixture of sediments (medium sand through medium silt) placed uniformly in a layer on the seafloor throughout Massachusetts Bay would be redistributed under a series of northeast storms, similar to the largest storm of December 1992. There is a qualitative agreement between the model result and the observed present distribution of sediments. This agreement is consistent with the hypothesis that sediment transport caused by northeast storms plays a key role in sediment distribution in Massachusetts Bay. The model allows variation in wind direction and reveals that winds from 0-60 degrees cause particles originating at the mouth of Boston Harbor or from the new outfall site to be transported southward and eastward toward Cape Cod Bay and Stellwagen Basin. The transport pathways of sediments from Boston Harbor during northeast storms are consistent with Boston being the long-term source of anthropogenic silver found in the surficial sediments of Cape Cod Bay and Stellwagen Basin. An understanding of the complex topography and sediment distribution and the long time-series record of oceanographic data in Massachusetts Bay makes the region a natural laboratory for developing and testing sediment transport models.

Geochemistry - metal concentrations in sediments influenced by inputs and post-depositional processes

The metal inputs to Boston Harbor have been declining in recent decades as the result of legislation that has restricted both point and non-point sources. Major reductions in contaminant inputs resulted from the elimination of sludge discharge to the harbor in 1991 and by moving the outfall to its new location in Massachusetts Bay. The concentrations of silver, copper and lead in the surface sediments in Boston Harbor have decreased by about 50% over the last 25 years. Concentrations of metals considered toxic to benthic organisms still remain buried beneath surface sediments and may provide a continuing source of contaminants to the environment (Section 7).

To assess the chemical impact of the new Massachusetts Bay outfall, we determined concentrations of a suite of potentially toxic metals in suspended matter and in bottom sediments collected before and after outfall startup. In the suspended matter, collected using sediment traps, the only constituents found to increase in post-outfall samples were silver and Clostridium perfringens, a bacterium spore found in sewage (Section 7). The highest levels of silver in the trap samples were below the toxicity warning level. In fine-grained bottom sediments about 2 km west of the outfall, the average post-outfall concentrations were not different from pre-outfall values. The largest increase in silver and C. perfringens within the bottom sediment followed an exceptionally strong storm December 11-16, 1992 that generated 7.3 meter high waves in western Massachusetts Bay. The long time-series of data that were collected prior to the outfall startup was important because it established the range of natural variability for metal concentrations at the site. This long baseline improves our ability to properly interpret any changes that may follow as discharge from the outfall continues.

Special geochemical studies of oxygen, radioactive isotopes, and metal cycling are being conducted to define the rates and processes that control the fate of metals after deposition on the seabed. Oxygen penetration depths and diffusive oxygen fluxes into sediments (calculated from O2 depth profiles) were measured at the time of each sampling cruise between 1995-2005 (Section 8). These parameters provide another test of the outfall's impact on local sediment chemistry. If organic matter deposition from the outfall was significant, O2 penetration depths would decrease, and diffusive O2 flux into the sediment would increase. A comparison of results from the in situ oxygen profiler collected before and after the outfall startup provides no evidence for a change in these parameters following discharge of treated sewage. Even a substantial (~25%) non-diffusive O2 flux, not detectable with the method used here, seems unlikely to alter this conclusion. However, a very slow reaction rate for outfall derived organic matter could delay the onset of detectable changes.

Seasonal differences in O2 penetration and flux into the sediment were observed. The average O2 penetration and flux during the Feb-March deployments were 5.2±1.3 mm and 481±126 µM cm-2 yr-1. During September and October deployments, more shallow O2 penetration depths (3.6±0.9 mm) and greater O2 fluxes (617±163 µM cm-2 yr-1) were measured. Similar seasonal differences are observed in other coastal environments in response to warmer temperatures and oxidation of organic matter added by the spring-summer bloom.

Depth profiles of the radioactive isotopes 210Pb and 239+240Pu have revealed evidence of removal of material from the sediment surface followed by deposition a few centimeters below the surface on a time scale of months to a couple of years (Section 9). This process is attributed to the feeding and defecation of benthic organisms such as cirratulid polychaete worms. This interpretation contributes three main points to the understanding of contaminant additions to coastal sediments. First, an environmental benefit might be realized from bioturbation because the rapid mixing dilutes the new contaminant. Second, for contaminants that are dangerous at even low concentration, the downward mixing could be an environmental detriment because it transports the contaminants below the zone of normal resuspension and subsequent transport away from the nearshore environment. Third, the mixing process implies that surficial sediments may not accurately or sensitively record recent changes in contaminant deposition.

Additional new insights are developing through studies of the cycling of metals within contaminated sediments and of the mechanisms and rates by which metals in sediments are released to overlying water (Section 10). Measurements of pore water composition in sediment cores indicate that the cycling of Ag, Cu, and Pb between dissolved and solid phases is closely linked to the formation and dissolution of iron oxides and metal sulfides. These reactions occur within the top 10-20 cm of the sediment column, and sediment mixing by benthic animals can involve contaminated sediments found in deeper older sediment layers in these modern cycling processes. At the Massachusetts Bay site, Ag is often present and Cu is generally present at higher concentrations in pore water than in overlying water, leading to the release of these metals into the overlying water. At the Boston Harbor site, the diffusion of these dissolved metals is higher in winter than in summer. Erosion experiments at both locations indicate that resuspension of particles preferentially enriched in Ag, Cu, and Pb is an important source of particulate metals to bottom waters. In addition, the particles have been shown to release dissolved metals (Ag and Cu) to the water in which they are suspended. Kalnejais (2005) has shown that the release of Cu from contaminated harbor sediments by diffusion and resuspension represents about 80% of the annual input of Cu by all other sources. This highlights the relative importance of sediments as a lingering source of contamination to the coastal environment.

Future Work

This project is a Federal/State partnership that continues to address scientific and management questions in coastal waters. The next steps of the program are to

  1. further synthesize, interpret, and publish the data collected in 2005,
  2. refine and expand the application of sediment transport models,
  3. conduct additional experiments to define the release of metals from contaminated sediments to overlying water, and
  4. evaluate the potential of mollusk shell growth rings as recorders of metal concentrations in seawater over the past few decades.

The extensive new knowledge generated by this multidisciplinary program makes the coastal waters of Massachusetts a natural laboratory for further research on contamination that will have wide application in other coastal areas.

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