Hydraulic Dredging Is the Environmental Choice for the City of Waltham MA
The 42-ac. Hardy Pond has been a favorite recreational spot in Waltham, MA, a suburb of Boston, for as long as either city has existed. In fact, it’s easy to imagine it has been a favorite spot for much longer than our country has existed, but the last 50 years have not been kind to this body of water.
Research has shown that the great pond—a term used in Massachusetts to designate a pond with a surface area of 10 ac. or more—was once more than 30 ft. deep in places, but by 1970 it was no deeper than 4 ft., and its mean depth was only 2 ft. Swimmers no longer entered the pond, the once-sandy beach was eroded, fish were dying, and residents downwind from the pond were suffering through offending odors every summer.
|The barge pulls itself back and forth across the pond as it dredges the sediment and pumps it back to shore.
Searching for a Solution
A city-funded study indicated that phototropic eutrophication (excessive growth of aquatic plants) was the main problem and identified three main causes: runoff from surrounding farms, improperly sited and poorly maintained septic systems, and runoff from streets and parking lots. These sources brought in nutrient-rich sediment, which promoted rapid growth of algae and vegetation. As this material decayed, it filled the pond and robbed it of oxygen.
The study suggested eliminating the causes and then dredging the pond to a depth of at least 8 ft. Most aquatic weeds cannot survive in depths of more than 6 ft.
During the 1980s, Waltham managed to get $1.2 million from a federal program, provided that the city contributed $400,000 in matching funds, to begin remediation work on the pond. The city’s public works department hired the consulting firm of Metcalf and Eddy (M&E), which designed a series of gross particle separators for the storm sewer outfalls that emptied into the pond. In addition, M&E began the process of acquiring necessary permits from the various regulatory agencies involved.
|A horizontal auger chops the sediment and vegetation and feeds it to a pump that pushes the slurry ashore.
|After being removed from the pond, the dried sediment is loaded onto a truck and hauled away from the site.
“It took a number of years to go through the permitting process,” says Rob Adams, the M&E senior project engineer in charge of the Hardy Pond project. “There were a lot of concerns about the wetlands and the environmental impact on the pond and surrounding areas.”
One of the most serious early concerns dealt with the material that would be removed from the pond. Early testing indicated the sediment contained contaminants, which would require that the material go into a hazardous-waste landfill. Subsequently, more narrowly focused testing revealed that the materials suspected of being petroleum-based hydrocarbons were actually waxy organic hydrocarbons and presented no problem.
In the meantime, the farmland in the pond’s watershed was converted for residential use, and the remaining septic tanks were abandoned in favor of connections to the city’s sanitary sewer collection system. These changes, combined with the gross particle separators, significantly reduced the amount of nutrients reaching the pond.
Finally in 1998, specifications for the first phase of the dredging work were written. They called for mechanical dredging with a clamshell, then transporting the dredged material on a barge to shore where it would be stockpiled until it dewatered naturally. Once the mountain of material dried, it would be trucked away to a landfill.
A lakeside park with a baseball field was available for stockpiling and dewatering the material, but it could only accommodate 2,000 yd.3 of material at one time. Because at least 120,000 yd.3 of material had to be removed from the pond, it was obvious that residents would not be able to use the park and ball field for two years or more.
Choosing a Dredging Method
Despite of its drawbacks, the project was bid and awarded. The low-bidding contractor expressed several concerns, however: the small disposal and stockpile area; the drying time required; barge transportation; community reaction to the dust, mud, and noise; and the turbidity requirements of the permit. It was specified that turbidity 100 ft. from the dredge could not rise more than 30% above its normal level. As a result, the primary contractor was receptive when another contractor, Mobile Dredging & Pumping Co. of Chester, PA, offered to subcontract the work using hydraulic dredging.
“We have years of experience with hydraulic dredging,” says Jerry Vetter, executive vice president of Mobile Dredging, a company of the Carylon Corporation. “The turbidity requirements are no problem for us. Whatever the auger stirs up on the bottom of the pond is pumped into the dredge immediately, so you don’t have any solids floating to the surface. We only require a 120- by 60-foot working area on shore, and we do not stockpile any material. All of the solids dredged from the pond are dried within minutes and trucked away; only clean water is returned to the pond.” He convinced those involved, including the four permitting authorities, that hydraulic dredging would be faster, cleaner, quieter, and more environmentally friendly than mechanical dredging.
Permitting authorities worried that polymers planned to be used in the dewatering process might have a negative effect on the lake, the aquatic habitat, or the resulting solids. To address these concerns, polymers used for potable-water production were proposed for this project. Both the dredged material and the liquid removed from the material were tested after the polymers were used. When no harmful effects were found, the specifications and permits were changed to allow Mobile Dredging to proceed with hydraulic dredging.
The Processing Line
Ken Waterfield, Mobile Dredging’s supervisor at Hardy Pond, explains that an 8-ft.-wide auger lowered from a barge to the lake bottom chops sediment and vegetation and feeds it to a pump, which lifts the slurry and pushes it through a flexible, floating pipeline to mixing tanks on shore. The tanks keep the slurry at a milkshakelike consistency until it is pumped into one of the four trailer-mounted, four-belt filter presses. Just before the slurry is released into a filter press, it is mixed with a polymer that causes the solids to coalesce and separate from the water.
The filter presses quickly and progressively compress the solids into a continuous “cake” of nearly dry material, which looks very much like a chocolate brownie.
As the material leaves the filter presses, it falls onto a conveyor belt that moves it up into the box of a waiting 30-yd. dump truck. Because this material is nearly 50% organic, it is being used by a local farm management facility. Between 10 and 12 truckloads of “cake” are hauled from the site each day. The nearly clear water squeezed from the sediment is discharged into an area of the pond protected by silt curtains, which capture any residual solids.
“It takes less than five minutes for the material to go from the head of the dredge to the bed of the truck,” Waterfield says. “Sometimes we hold the material longer than that in the mixing tanks, but even the slowest transport is pretty fast.”
The self-propelled barge that carries the dredge pulls itself along on a cable stretched across the pond. As it moves forward, it removes about 4 ft. of sediment from the bottom of the pond. An equal amount is removed on the return pass. The cable then is moved about 6 ft. to one side, and the two-pass process is repeated.
The project was planned as two phases to allow for time to analyze the result of the first phase and acquire sufficient funding for the second phase. Mobile Dredging successfully bid on the second phase as the primary contractor. That work began in 2002, but dredging operations were stopped in November 2002 due to the onset of freezing temperatures and did not start again until the end of March 2003.
|Four trailer-mounted filter presses are kept busy dewatering the sediment as it is pumped from the pond.
As the project nears completion, John M. Bradley, the City of Waltham’s public works director, notes the project has been very successful. “We’re very close to having most of the pond dredged to a depth of approximately 9 feet. Over 150,000 cubic yards of material were removed in the first phase of the operation, and almost that much more was removed in the second phase. The hydraulic dredging technique has worked wonderfully.”
“This has been a remarkably unintrusive project from the beginning,” agrees Marc Rudnick, president of the Hardy Pond Association, an active group of concerned residents. “We monitor the work closely, and we find it exemplary. The machinery is quiet, and everything is extremely clean. Hardly a speck of dirt hits the ground, and when it does, it’s cleaned up immediately. I believe the project has been handled in an environmentally appropriate way. It really has been remarkable.”