PORTSMOUTH, R.I. — State environmental officials are zeroing in on a plan to clean up one of Aquidneck Island’s popular recreation areas.

For nearly 50 years, the Melville Ponds watershed, consisting of Upper Melville Pond (also known as Thurston Gray Pond), Lower Melville Pond, and the brook (also known as Matapurcetti Brook) that links the ponds to the ocean, has provided local residents with opportunities for swimming, kayaking, and an RV park for camping.

But full use of the ponds has been held back due to water quality problems: the bodies have long reported low dissolved oxygen levels, a chief requirement for sustaining life, and have been plagued by algal blooms such as cyanobacteria, which can be toxic to humans and animals alike.

Rhode Island Department of Environmental Management officials have listed both ponds on the state’s impaired waters list since 2010. The list, a requirement under the Clean Water Act, is a list of every known waterbody in the state that doesn’t meet federal water quality standards.

Water quality sampling of the ponds has been performed by the University of Rhode Island’s Watershed Watch program since 1997, but DEM scientists from the Office of Water Resources began studying the pond since 2021.

Their key goal was to write a TMDL, or total maxim daily load, plan for the ponds, which lists each pollutant flowing into the pond and how much of that pollutant can enter the pond before it compromises water quality. In the case of the Melville ponds, DEM officials have zeroed in on phosphorus.

“We estimate that Upper Melville Pond is getting close to 80 pounds of phosphorus per year,” said Brian Zalewsky, an environmental scientist in DEM’s Office of Water Resources. “Lower Melville Pond, because it has a bigger watershed, is getting around 150 pounds per year.”

Nutrients such as phosphorus overload freshwater bodies in Rhode Island all the time. They enter waterways typically via stormwater runoff, as rainfall is channeled into drainage systems that transport it usually to the nearest waterbody.

Along the way it will pick up whatever else is on the ground between where that rain fell and where it enters a brook, stream, or pond. Lawn fertilizers, pet waste, and road salt, among other pollutants, get carried along with the stormwater.

The runoff is just the first step in a toxic cycle: nutrients like phosphorus provide a food buffet for aquatic plants like algae, cyanobacteria, and duckweed. That buffet of nutrients supercharges their growth, letting them grow bigger and faster than they would otherwise naturally, limiting the amount of sunlight that penetrates to the water’s bottom, and limiting photosynthesis and oxygen production.

When the aquatic plants begin to die, they decompose in the water and bacteria increases, further choking off oxygen levels for fish and other species within the waterbody.

“Stuff that’s living there that can’t move to another waterbody is pretty much out of luck,” Zalewsky said. “It creates a vicious cycle, and it just happens again, year after year after year.”

Development plays a key role in nutrient overloading of waterbodies. Of the 734 acres that make up the Melville ponds watershed, 58% is developed. Another 25% is forestland or brushland, with 9% consisting of wetlands.

Since 2011, the Department of Health, in tandem with DEM, have issued 17 no-contact advisories for either Lower or Upper Melville Pond or both due to cyanobacteria blooms.

But paradoxically, in the case of the Melville ponds, development also created the waterbody system in the first place. The ponds were created between 1942 and 1950 by the Navy as a drinking water supply source and for fire suppression. Seven small ponds in total were built between the Upper and Lower Melville ponds to act as sedimentation basins, but those ponds later filled in as sediment built up over decades. Today they resemble wetlands rather than actual ponds.

Ownership of the pond system was transferred to the town of Portsmouth in the 1970s, and much of the original infrastructure — dams, intake structures, outlets — remain but are in poor or non-working conditions. Meanwhile, the town created Melville Park around the ponds, including hiking trails, a dog park, and a campground.

In its TMDL study of the ponds, DEM estimates that 95% of the phosphorus entering Upper Melville Pond is coming from point sources — identifiable origins of pollutants like discharge or sewer outfalls — with 90% of phosphorus in Lower Melville Pond coming from point sources.

Jeff Richard, a member of the Melville Park Committee, emphasized the pond’s importance as a source of recreation in the park.

“If something as serious as this isn’t dealt with,” he said, “we won’t be able to use the pond.”

So what would it take to clean the ponds enough for them to come off the impaired waters list?

“What this is saying is you need to control urban runoff, developed land runoff,” Zalewsky said. “We’re looking at around a 60% to 66% reduction in total phosphorus loads to the reservoirs.”

DEM’s draft TMDL plan, if approved by the Environmental Protection Agency, would require more stringent runoff controls for the town of Portsmouth, Naval Station Newport, and the Rhode Island Department of Transportation, with an overall goal of installing more stormwater controls in the catchment areas of outfalls.

Permittees under the new requirements would be required to sweep streets and clean catch basins twice a year, once after leaves fall in autumn and again in the spring after any snow melts. RIDOT and the town would be required to investigate catchment areas, identify and interconnect them. The town would also have to evaluate its options for reducing erosion in specific areas around the ponds.

Privately owned roads and runoff infrastructure, like at Clocktower Square or the Springfield Group neighborhood, would be required to increase catch basin cleaning and street sweeping.

DEM is accepting public comment on the draft TMDL plan until Nov. 7, after which it will submit it to the EPA for federal review and approval.