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Massachusetts-Rhode Island Water Science Center

Sediment Studies in the Assabet River, Central Massachusetts, 2003

U.S. Geological Survey Scientific Investigations Report 2005-5131

By Marc J. Zimmerman and Jason R. Sorenson

In cooperation with the Massachusetts Department of Environmental Protection and the U.S. Environmental Protection Agency

This report is available as a pdf.


From its headwaters in Westborough, Massachusetts, to its confluence with the Sudbury River, the 53-kilometer-long Assabet River passes through a series of small towns and mixed land-use areas. Along the way, wastewater-treatment plants release nutrient-rich effluents that contribute to the eutrophic state of this waterway. This condition is most obvious where the river is impounded by a series of dams that have sequestered large amounts of sediment and support rooted and floating macrophytes and epiphytic algae. The water in parts of these impoundments may also have low concentrations of dissolved oxygen, another symptom of eutrophication.

All of the impoundments had relatively shallow maximum water depths, which ranged from approximately 2.4 to 3.4 meters, and all had extensive shallow areas. Sediment volumes estimated for the six impoundments ranged from approximately 380 cubic meters in the Aluminum City impoundment to 580,000 cubic meters in the Ben Smith impoundment. The other impoundments had sediment volumes of 120,000 cubic meters (Powdermill), 67,000 cubic meters (Gleasondale), 55,000 cubic meters (Hudson), and 42,000 cubic meters (Allen Street).

The principal objective of this study was the determination of sediment volume, extent, and chemistry, in particular, the characterization of toxic inorganic and organic chemicals in the sediments. To determine the bulk-sediment chemical-constituent concentrations, more than one hundred sediment cores were collected in pairs from the six impoundments. One core from each pair was sampled for inorganic constituents and the other for organic constituents. Most of the cores analyzed for inorganics were sectioned to provide information on the vertical distribution of analytes; a subset of the cores analyzed for organics was also sectioned. Approximately 200 samples were analyzed for inorganic constituents and 100 for organics; more than 10 percent were quality-control replicate or blank samples.

Maximum bulk-sediment phosphorus concentrations in surface samples from the impoundments increased along a downstream gradient, with the exception of samples from the last impoundment, where the concentrations decreased. In addition, the highest phosphorus concentrations were generally in the surface samples; this finding may prove helpful if surface dredging is selected as a means to control phosphorus release from sediments. There is no known relation, however, between bulk-sediment concentration of phosphorus and the concentrations of phosphorus available to biota.

Potentially toxic metals, including arsenic, cadmium, chromium, copper, nickel, lead, and zinc were frequently measured at concentrations that exceeded U.S. Environmental Protection Agency sediment-quality guidelines for the protection of aquatic life and that occasionally exceeded Massachusetts Department of Environmental Protection guidelines governing landfill disposal (reuse). Due to the effects of matrix interference and sample dilution on laboratory analyses, neither pesticides nor volatile organic compounds were detected at any sites. However, samples collected in other studies from nearby streams indicated the possibility that pesticides might have been detected in the impoundments if not for these analytical problems. Although polychlorinated biphenyl concentrations, as individual Aroclors, generally exceeded published U.S. Environmental Protection Agency guideline concentrations for potential effects on aquatic life, the U.S. Environmental Protection Agency guideline concentrations for human contact or the Massachusetts guidelines for landfill reuse were rarely exceeded. Concentrations of polycyclic aromatic hydrocarbons, both individually and total, frequently were greater than guideline concentrations. Concentrations of total extractable petroleum hydrocarbons did not exceed Massachusetts guideline concentrations in any samples.

When the sediment analytes from surface samples are considered together to compare their potential toxicity to aquatic organisms, it is clear that sediment exposure is likely to have harmful effects. By most measures, samples from the first and last impoundments in the sequence had the poorest sediment quality with respect to the degree and the frequency with which guideline concentrations were exceeded.

A consortium of towns along the river has investigated a range of options aimed at controlling concentrations of nutrients, phosphorus in particular, released from wastewater-treatment plants into the river and its impoundments. The second part of this study examined the changes in phosphorus concentrations in surface and pore waters in one impoundment during a 2-month period. The results were used to determine whether the sediments could release sufficient amounts of phosphorus to render ineffective decreased phosphorus releases from treatment plants. Knowledge of sediment characteristics and chemistry will allow local water managers to choose among the sediment-management options that may be considered for the Assabet River: decreasing concentrations of phosphorus released from treatment plants, partial or complete dredging, dam removal or modification, phosphorus sequestration by chemical treatment, and making no changes.

To investigate the dynamics of phosphorus concentrations in the impoundment in Hudson, Massachusetts, water samples were collected weekly at five locations. In addition to ortho- and total phosphorus, some samples were analyzed for chloride, which served as a conservative tracer for waters released from upstream wastewater-treatment plants. At each location, data were collected to provide water-column profiles of temperature, dissolved oxygen, and specific conductance.

In the study of phosphorus dynamics in the impoundment in Hudson, Massachusetts, phosphorus concentrations varied over time, primarily in bed-sediment samples, at the off-channel sampling locations where the water column was stratified. These variations, including some substantial increases in phosphorus concentrations in pore-water samples, however, did not appear to lead to increases in phosphorus exported from the impoundment. This observation suggests that most of the phosphorus released from the sediments was recycled internally in the impoundment

Although net phosphorus storage did vary, it did not generally increase with time as total phosphorus entered the water column from sediments in stagnant areas in the Hudson impoundment. Physical and redox processes seem to control phosphorus release from sediments to the water column. These findings and the results of other studies indicate that the amount of phosphorus released from sediments over a yearly cycle was relatively small compared to the amount of phosphorus entering the system from upstream wastewater-treatment plants.

Table of Contents


The Assabet River and its Sediments

Part 1. Sediment Distribution and Chemistry in Six Impoundments in the Assabet River, Central Massachusetts

Study Design

Mapping of Bathymetry and Sediment Thickness

Selection of Sediment-Sampling Sites

Collection of Sediment Cores

Sample Preparation

Analytical Methods

Quality Assurance and Quality Control

Sediment Thickness and Bathymetric Maps

Sediment Chemistry of the Assabet River Impoundments

Elemental Analyses


Toxic Trace Elements

Reactive Sulfide

Organic Analyses

Volatile Organic Compounds and Organochlorine Pesticides

Polycyclic Aromatic Hydrocarbons

Polychlorinated Biphenyls

Extractable Petroleum Hydrocarbons

Potential for Adverse Biological Effects

Sediment Distribution and Chemistry—Possible Implications for Sediment Management

Part 2. Phosphorus Dynamics in a Wastewater-Dominated Impoundment, Hudson, Massachusetts


Site Selection and Sample Collection

Reconnaissance and Site Selection

Field Measurements and Collection of Phosphorus and Chloride Samples

Quality Control

Field Study of Phosphorus Dynamics

Site-Specific Profiles

Changes in Phosphorus Storage

Mass Balance (Chloride:Total Phosphorus)

Phosphorus Dynamics in the Hudson Impoundment





Suggested Citation:


Zimmerman, M.J., and Sorenson, J.R., 2005, Sediment studies in the Assabet River, central Massachusetts, 2003: U.S. Geological Survey Scientific Investigations Report 2005-5131, 87 p.


This document is available in Portable Document Format (PDF): SIR2005_5131.pdf (20.7 MB)

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