In the past five years, more than 510 million gallons of untreated sewage has spilled into the Tennessee River and nearby streams and streets from Moccasin Bend Treatment Plant's combined sewer overflow facilities, an environmental group says.

"These are staggering numbers that reflect significant and chronic noncompliance" with the Clean Water Act, states a letter to Chattanooga and Mayor Ron Littlefield from an environmental action group, the Tennessee Clean Water Network.

The network, which analyzed city and state records to compute the spills, now is threatening to sue the city to force environmental compliance.

"This is a fixable solution," said Renée Hoyos, executive director of the Knoxville-based group. "There's a solution to all of this. But the city will have to put work into it, and money."

The group has sent the city a certified notice of intent to sue, giving Chattanooga a 60-day window of opportunity to address the problem, she said. She said the group has filed similar lawsuits in Memphis and Knoxville, and the suits sped up those city's efforts to work with state and federal regulators to tighten standards and stop spills.

"(The city) can do it infinitely faster when faced with court deadlines," she said.

Richard Beeland, a spokesman for the mayor, declined comment.

Jerry Stewart, director of the city's waste resources division, which includes the Moccasin Bend sewage treatment plant, said he was aware of the nonprofit group's plan to sue, but did not want to comment.

"We're discussing it with our attorney," he said. "We got their notification last week. We're formulating our approach."

The city already is expected to face state and possibly federal sanctions over a 100,000-gallon spill last week that killed at least 3,600 fish and forced several residents from their floating homes at Browns Ferry Marina in Tiftonia.

City officials also have acknowledged that investigators with the U.S. Environmental Protection Agency and the U.S. Department of Justice have inquired about the city's sewer operations. Earlier this year, federal officials demanded boxes of data and documents about the Moccasin Bend plant's operation.

EPA spokeswoman Dawn Harris Young would not comment Tuesday about the investigation or the environmental group's notice of intent to sue.

Rising pollution

Sewage spills to the Tennessee River have increased each year for the past five years, according to Tennessee Clean Water Network's analysis of regulatory records.

* 2006 - 8,720 gallons

* 2007 - 14.5 million gallons

* 2008 - 35.5 million gallons

* 2009 - 122.3 million gallons

* 2010 - 146.5 million gallons

Source: Tennessee Clean Water Network

Locating spills over 5 years

* Almost 319 million gallons of untreated sewage was reported spilled directly into the Tennessee River through the sewage storage facilities built in recent decades to stop leaks from combined sewer overflows

* More than 156 million was reported spilled into Chattanooga Creek.

* Another 35 million was reported spilled into other local streams, streets and private property.

She said the Tennessee Department of Environment and Conservation "is the primary enforcement arm" for all pollution concerns, including sewage spills.

State spokeswoman Tisha Calabrese-Benton said TDEC is overseeing the cleanup at Browns Ferry Marina.

"We are in communications with EPA on the issue, which is normal operating procedure for an event such as this," she said. "We will be issuing a notice of violation, which will require a report from the city on exactly what went wrong, when, where and what the responses were. That will be used as the department determines appropriate next steps."

She said state regulators also have been coordinating with EPA to determine what enforcement action should be taken in response to sanitary sewer violations, including overflows, in the city of Chattanooga.

The timing of the Tennessee Clean Water Network's announcement Tuesday at the Chattanooga Pier coincides with last weekend's spill from the Tiftonia substation and sewage overflow storage facility.

A lightning strike Thursday caused a brief power outage across the treatment system. Workers scurrying to get the main plant back online failed to notice that the Tiftonia substation also had no power and did not come back up, Stewart said Monday.

The substation spilled about 100,000 gallons of untreated sewage into a bottle-necked marina bay on the Tennessee River during a period of 12 to 14 hours, Stewart said.

The Tennessee Wildlife Resources Agency reported that the spill depleted the oxygen in the bay and killed at least 3,600 fish. Live-aboard residents at the marina also were forced to leave their boats.

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