At Bare Hill Pond, the pump began operating on June 21. It is running at low speed to remove water from 10 feet below the surface, where anoxic conditions (having no dissolved oxygen) begin to occur. This trial operation is slated to continue until Aug. 19. Data from this trial will help the Bare Hill Pond Watershed Management Committee determine if pumping out oxygen-depleted water from the lower depths could help reduce anoxic conditions and potentially algae growth.
Bruce Leicher, chair of the watershed committee, and Wendy Gendron, a wetlands biologist consultant, proposed removing anoxic water through low-rate pumping at the June 2 meeting of the Conservation Commission. The commission unanimously approved the proposal at its June 16 meeting.
Gendron noted that this is an inexpensive option compared to other mitigation methods such as dredging, oxygenation, or phosphorus inactivation. “This is very low risk with low consequences. It’s worth a try to do this selective withdrawal with the capital equipment you already have,” said Gendron.
A leading trigger of algae blooms is the accumulation of phosphorus at the bottom layers. At lower depths, anoxic conditions release phosphorus usually trapped in the sediment. Free phosphorus is gobbled up by algae, enabling them to grow exponentially, and facilitating algae blooms.
Low-volume, low-rate pumping will remove water from the lower water layers where anoxia occurs, presently at depths greater than 10 feet below the surface. Removing the anoxic water makes it possible to replenish oxygen levels through the input of fresh groundwater, essentially diluting leftover anoxic water. Gendron noted fresh groundwater is typically spring fed, with low mineral content and low phosphorus, and is fast-moving, which helps with oxygenation.
Pumping water from the pond through an annual drawdown has been a standard practice at Bare Hill Pond for almost 15 years. The drawdown acts as a control for invasive plant species by exposing invasive weeds to freezing and drying conditions. The annual process reduces the water level in winter and systematically restores the pond depth come spring.
All algae blooms have occurred in Bare Hill Pond during August, whereas the majority of algae blooms throughout the state have occurred in July. “It may be that the pumping we’ve done each winter has helped reduce the anoxic level to begin with, and that a little more in the summer, if it doesn’t prevent it, may delay it further. We don’t know unless we try,” said Leicher during the June 2 meeting. Gendron said that her data appeared to support this theory, and that algae blooms and anoxic conditions likely would have been a more severe problem sooner without annual drawdowns.
The Bare Hill Pond Watershed Management Committee said it will cease pumping if the water level dips below its normal summertime level. But Leicher, in conversation with the Press, said it was highly unlikely that running the pump at this low speed would change the level of the pond. The pond level will be monitored regularly. Gendron confirmed that the stratification of the water’s layers would not be altered by this trial. Gendron’s firm, Aquatic Restoration Consulting, will continue to monitor the temperature and levels of dissolved oxygen and phosphorus.
