Three environmental groups have filed a formal notice indicating that they intend to sue the Washington Suburban Sanitary Commission, alleging that its water treatment plant near Seneca, Md., has been illegally discharging pollutants into the Potomac River for years.

The organizations, citing a 2003 letter from a commission official and the plant’s own records, assert that a malfunctioning system installed 10 years ago has regularly forced operators to release mud containing aluminum and phosphorus into the river.

According to the notice, which under the Clean Water Act must come 60 days before a lawsuit, the releases violated the plant’s state-issued permit. The WSSC maintains that the permit allowed the discharges, according to a statement from a spokeswoman.

At the plant, water drawn from the Potomac is cleaned with chemicals. The purified water is piped to most residents in Montgomery County, while the chemicals and the sediment that have been removed are generally trucked away and disposed of.

The plant’s capacity for handling solid material is limited, and operators are allowed to release some of the sediment, along with the chemicals used to purify the water, into the river after heavy rains. Otherwise, the plant would be overwhelmed with sludge.

The three organizations — the Environmental Integrity Project, Potomac Riverkeeper and the Chesapeake Bay Foundation — assert that the solid-material treatment operation has been running under capacity. If the plant were operating according to specifications, they claim, the discharges would not have been necessary.

Based on the plant’s records, the groups calculate that the facility added between 18 million and 30 million pounds of material to the river over the past four years, beyond what was in the water withdrawn for treatment.

“We do like the nice clean water that comes out of the tap,” said Eric Schaeffer, director of the Environmental Integrity Project. “You can put a little less of the residue from treating it into the river.”

Cloudy water prevents sunlight from reaching grasses growing on the riverbed, while sediment floating in the river makes it harder for fish to breathe and can smother their eggs when it settles. Aluminum, commonly used in water treatment, can be toxic to fish as well.

The plant’s records also document phosphorus discharges, Schaeffer said. Algae feed on phosphorus, leading to the blooms that have blighted the Chesapeake in recent years.

Because the sanitary commission is a public utility, its customers will probably have to pay for any improvements at the plant through their water bills. Schaeffer said residents would be able to afford slightly higher rates, pointing to a similar plant in Baltimore where pollution has not been a problem recently. “Baltimore is not the richest city in America,” he said. “They’ve gotten it together and managed to treat their water without the same side effects, and we think Montgomery County can rise to the occasion.”

He also noted that plants downriver, such as the Army Corps of Engineers facility at MacArthur Boulevard, which provides drinking water to District residents, must treat the water that the Seneca plant discharges.