DURHAM – The months-long debate over the Mill Pond Dam will come to a close next week when Durham residents flock to Oyster River High School for their annual election. Key points ranging from fish migration to historical relevance have been brought up by each side. But no single issue is as talked about as the polluted sediments that currently sit at the bottom of Mill Pond. 

Wil Wollheim, an associate professor of natural resources and the environment at the University of New Hampshire (UNH),  said that today, fewer sediments are reaching the Great Bay Estuary, which the Oyster River and Mill Pond feed into. This is because of dams across the region, he believes dam removal is the best route to clean up the Mill Pond area. 

“Having a few more free flowing places and having that sediment get in the estuary will be beneficial because it will eventually be redistributed in there,” Wollheim said. “It’s all going to spread out across different surfaces.” 

These sediments are crucial for a healthy watershed, especially as climate changes and the sea level rises. They contain inorganic matter, like sands or silts, which help emergent marshes grow alongside the rising oceans. 

In the case of the Mill Pond, it is listed as impaired under the New Hampshire Department of Environmental Service’s Section 303(d) Clean Water Act for low dissolved oxygen and pH levels, according to a town FAQ. This is mainly due to high phosphorus levels found attached to sediments, which Wollheim said are a limiting factor for most freshwater bodies. Conversely, nitrogen is a limiting factor in most salt water bodies – like the Great Bay Estuary – so most of the phosphorus flowing into the bay post-removal will be destroyed by the salt water.  

“The dam will always retain sediments – that’s just what they do. There’s always going to be phosphorus on those sediments, so there’s always going to be phosphorus going back into the water,” Wollheim said. “But phosphorus won’t be an issue in Great Bay if the dam is removed.” 

The sediments in Mill Pond are also high in various polycyclic aromatic hydrocarbons (PAHs), which are metals found naturally in coal, crude oil, and gasoline, according to the CDC. The 2020 Feasibility study found that 17 out of 18 sediment samples showed high levels of PAHs in the Mill Pond, and similar levels downstream of the dam, according to questions from the town council.  

One metal in particular – mercury – has been focused on heavily during the dam debate. According to a 2019 study by former UNH student Hannah Miller, “almost all sediment levels at Mill Pond had mercury content above the NOAA 1999 Upper Effects Threshold for mercury in freshwater.”  

Wollheim noted that only four samples were taken during the study, so more examination would be needed to determine the true levels in the pond. The studying would only be to determine if any special disposal methods would be required, not because of any threat to human or plant life. 

“The entire region is mercury impaired, in part because we’re downwind of a lot of [former] coal and fire plants,” he said. “This should be a concern right now, and the best way to deal with it is to actively remove highly contaminated sediments from the Mill Pond system.” 

The metal’s existence in Mill Pond is likely human-induced, with runoff from UNH science labs and incinerators – practices discontinued in the 1980s – going into the watershed. Wollheim also cited the Mill Plaza, which College Brook, a feeder stream to the Mill Pond, runs through. It is an impairment inducer due to the impervious surfaces around the area. 

The pro-dam group has been critical of UNH for its role in making College Brook, a feeder stream to the Mill Pond Impoundment, one of the most polluted streams in New Hampshire, according to a video featuring Wollheim. The group has often cited agricultural practices, mainly runoff from the Moore Fields, as a reason for the Brook and Mill Pond’s poor condition. 

But at a recent town council meeting, Dr. Anton Beckerman, director of New Hampshire Agricultural Experimental Station, presented on UNH Sustainable Fields Practices, aiming to clarify UNH’s role in Mill Pond’s impairment. 

“There may be some contribution [to Mill Pond’s low water quality], but it’s certainly well below the range of what has been set by NHDES,” he said. “It’s really hard for me to say that [there’s merit to those claims] because there’s such a long distance between Mill Pond and the Moore Fields. And there hasn’t been a scientific investigation into that cause and effect relationship.” 

Wollheim believes that the best way to clean up the Mill Pond is to remove the dam and take out the 3,000 cubic yards of sediment that the 2020 feasibility study suggests under the dam removal option. The remaining sediments will be stabilized as part of active channel restoration to “minimize potential sediment impacts downstream, as well as improve the stability and ecological integrity of the upstream area following dam removal.” 

According to April Talon, Durham’s town engineer, the town will have to comply with various environmental agencies in the process of dam removal, if that is what the residents vote on. Sediments are heavily regulated by agencies to ensure that no life-threatening pollution is left unstable. 

“The topic of sediments is one that’s strictly controlled, and we are going to follow the advice and guidance of the state agencies that issue that permit,” she said. “I hope people would be comfortable with not only the town, but NOAA, NHF&G, and even the Army Corp of Engineers giving us guidance through the process.”