Waterford — Ask a group of local anglers about the best spots to catch fish, and at least one is likely to mention the waters of Niantic Bay near where the Millstone Power Station discharges the water used to cool the plant.

The 2.2 billion gallons per day discharged from the plant - virtually the same amount the two operating plants there draw in to absorb the excess heat of the nuclear reactors - create a warm current in Long Island Sound that, fishermen say, attracts flounder, striped bass and other species to the area.

While the heated discharge water, known as the plant's "thermal plume," is considered by some to be a desirable side effect of Millstone on local waters, state regulators are now taking a closer look at it, questioning how it impacts the near-shore marine environment and whether oxygen levels in the plume are high enough to support aquatic life.

"We see a need for Millstone to collect more robust data on the near-shore effects in its zone of influence," Oswald Inglese Jr., director of the Water Permit and Enforcement Division at the state Department of Energy and Environmental Protection, said in an interview last week. The "zone of influence," he explained, is the area 8,000 feet out from the discharge pipes for the two operating reactors, which defines the main area impacted by the plume of heated water.

This summer, DEEP directed the plant to undertake a new study of its thermal plume to assess impacts of the water temperature and dissolved oxygen levels. The agency has requested the study, Inglese said, because the Nuclear Regulatory Commission now allows the plant to start with warmer water in its cooling process.

This spring, the NRC granted Millstone's request to use water from the Sound up to 80 degrees, a 5 degree increase the plant said it needed because of a long-term warming trend in the estuary. Two years ago, Unit 2 had to shut down when the water it drew for cooling exceeded 75 degrees. Unit 3, which draws water from a colder, deeper part of the Sound, remained in operation.

Cathy Taylor, director of electric environmental services for Dominion, the company that owns Millstone, said parameters of the thermal plume study are being developed, including measurements at various seasons, tides and depths. She expects to be able to present DEEP with a proposed scope of study by December. Once DEEP and Millstone agree on the scope, an outside consultant would be hired to do the analysis, she said.

The study, she said, would consider how the warmer discharge water is affecting shallow areas in the zone of influence throughout the course of the day.

"There's a lot of mixing with various tides and conditions," she said.

Millstone spokesman Ken Holt said the water released from the plant is usually 20 degrees warmer than the surrounding waters, and that it has never exceeded the maximum temperature allowed under its DEEP discharge permit. The permit states that Unit 3 cannot discharge water more than 28 degrees above the surrounding waters, and Unit 2 discharge water must be no more than 32 degrees warmer.

Results of the thermal plume study will be one of the key pieces of information DEEP will consider when Millstone applies early next year for a renewal of its water intake and discharge permits, which expire in September 2015. Taylor said Millstone plans to submit the permit application in March.

It will be the first time DEEP has evaluated the plant's water use permit since the EPA's ruling in May giving state agencies more authority to factor in local conditions when interpreting the federal requirements that power plants minimize harm to marine life.Environmental groups, including the Connecticut Fund for the Environment, had been pushing for the EPA to require plants to use cooling towers rather than constant withdrawals of large quantities of water that kill fish eggs, larvae and other aquatic organisms in the process. When the EPA's decision was released, CFE officials and other environmental groups said they were disappointed, but would continue to pressure state regulators to require Millstone and other plants to build cooling towers.

Millstone officials said its analysis has concluded that retrofitting the plant with the massive 500-foot towers would cost $2.6 billion and reduce energy output. The alternative of building a series of smaller, "mechanical draft" towers up to 150 feet tall would also be impractical, Millstone officials concluded, because they would also be too costly to build and operate and would occupy too large a portion of available property at the site. Holt also noted that the plant in the past few years began using "variable draft" pumping that allows it to reduce saltwater use during spawning season for winter flounder.

Inglese said as part of the permit renewal, DEEP will also ask Millstone to analyze whether it can employ the "variable draft" system more frequently. The system reduces water use by up to 33 percent, he said.

Overall, he said, EPA's ruling means that state regulators must require plants to find ways to reduce destruction of fish, larvae and other aquatic life both in taking in water and discharging it. Rather than mandating a specific technology or equipment, he said, it allows for states to work with plants to determine the mix of techniques to accomplish that. In Millstone's case, he said, the intake system could use a combination of additional screens to trap organisms, reducing pressure on the pipes to enable fish to swim away, and other new equipment.

"There are new types of screens and operational controls that could be added to what they're doing already," Inglese said.

j.benson@theday.com

Twitter: @BensonJudy