Sediment and Dam Removal


Sediment and Dam Removal Feasibility

In September 2010, the Army Corps of Engineers published on an extensive study of the sediments of the Assabet River and the feasibility of dam removal and other options to remediate nutrient pollution problems in the river. The final report and modeling reports are available on: Army Corps of Engineers Assabet Project webpage. This is a direct link to the report: Assabet River, Massachusetts: Sediment and Dam Removal Feasibility Study. (24 MB) Read more about Assabet dams.

"The purpose of the study was to provide planning assistance (planning level engineering and scientific information) to MassDEP on the potential feasibility of sediment and dam removal to reduce internal recycling of phosphorus (sediment phosphorus flux) in the Assabet River. The first part of the study focused on reductions in internal phosphorus recycling from sediment for sediment and dam removal measures. The second part of the study focused on engineering and environmental considerations for hypothetical dam removal. If in the future a proponent steps forward who wishes to pursue dam removal, then there would be a detailed environmental assessment and permitting process involved at all levels of government – local, state, and Federal.

The following six dams on the river, and the associated sediment behind them, were considered in the planning study:
- Aluminum City Dam, Northborough
- Allen Street Dam, Northborough
- Hudson Dam, Hudson
- Gleasondale Dam, Stow
- Ben Smith Dam, Maynard
- Powdermill Dam, Acton"

(Study, p. ES-1)

Previous Work

USGS studied sediment volume, quality (including metals and toxic inorganic and organic contaminants), and nutrient flux in the main the Assabet River impoundments.
Sediment Studies in the Assabet River, Central Massachusetts, 2003. SIR 2005-5131.

From this study, USGS concluded (excerpted from the study abstract):
"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."

"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."