To dredge or not to dredge, that is the question.
After hiring consultants to conduct a $16,000 bathymetric study of Burke Centre’s six ponds, Conservancy staff and elected Board of Trustees members are faced with new decisions: whether to dredge the ponds biologically or mechanically, and whether to do it now or later.
“It seems like a no-brainer, but we’re going to need more information and time to make the right decision,” said Kala Quintana, the Woods neighborhood trustee.
The no-brainer Quintana refers to is choosing to biologically dredge the ponds. David Cutlip, of DSC Aquatic Solutions in Springfield, told Burke Centre staff and trustees at their Tuesday, Jan. 30 work session meeting, that the ponds’ make-up of organic material is significant enough for biological dredging to be effective in about 50 percent of all six ponds' total volume.
“If [sediment] is mostly organic, then biological is the way to go,” said Cutlip.
So rather than lugging in heavy equipment and tearing up grass and roads, a large amount of bacteria could be added to the ponds in a liquid form that would naturally take care of nutrient levels in the ponds, said Cutlip. The other plus to biological dredging is that it’s significantly less costly than mechanical dredging, said Seth Brown, of the water resource consulting firm GKY & Associates, the firm that conducted the study. The mechanical dredging is estimated to cost around $102,000, he said.
The bathymetric study determined, as accurately as possible, what the original depth levels of the ponds were when they were built 30 years ago. Field workers inserted a push-pole into the water at several locations to determine the amount of organic material, or muck, at the bottom. The composition of muck in Meadow Pond — the top priority pond for dredging because of an invasive plant living in it — is about 80 percent, according to the study.
An invasive aquatic plant, the water chestnut, took over the Meadow Pond late last summer. The plant blocks sunlight from entering the pond’s surface, thus preventing photosynthesis from occurring, said Cutlip.
“The plant is robbing the oxygen in there,” he said.
Brown’s firm recommends eliminating the water chestnut, which could take years, before dredging the pond. Since each plant produces 10 seeds, and each seed produces 10 plants, simply containing it is challenging, said Cutlip.
“We have to decide what’s best for these ponds,” said Patrick Gloyd, executive director at the Burke Centre Conservancy. “The water chestnut threw a monkey wrench into [the dredging project].”
One water chestnut plant was also found in Bass Pond, and treatment was applied immediately. It isn’t clear yet whether the plant will become a problem for that pond as well. Bass, Burke and Spring Ponds, in addition to Meadow, all had low dissolved oxygen levels too. Those four ponds are recommended for dredging, but Bryce and Lynch Ponds appear to be doing fine, said Brown.
It took about 30 years for the sediment levels in the ponds to reach the point where dredging is necessary. Since the ponds are mature now, said Brown, they won't erode as much as they did in the past. The Conservancy can take preventative measures to make future dredging projects easier and less costly.
Installing an underwater berm, or sediment forebay, is a way to concentrate future dredging to one area. The berm encourages sedimentation in one area, which would allow for easier maintenance, said Brown.
The Conservancy staff and board members now have to decide how long they want to wait to dredge. If the water chestnut takes five years to eradicate, which Cutlip said is likely, then the GKY & Associates recommendation to postpone dredging until the chestnut is gone has to be weighed. Quintana suspects it will take staff a couple of months to determine which route to take. They also have the option to install aerators into the ponds to try and improve oxygen levels in the mean time.