Tuesday evening, a putrid black liquid was pouring into the Trinity River just south of downtown. Just as it had been for a week.

Officials in Dallas Water Utilities and the Office of Environmental Quality said Tuesday evening they believe the fetid fluid was "organic material," perhaps spoiled food coming from a nearby industrial site. Sarah Standifer, assistant director of Dallas Water Utilities, said the material could be rotten fruit and vegetables.

Whatever the substance, it's entering the river as a steady stream along a channel that runs parallel to the remnants of the old Atchison, Topeka & Santa Fe trestle bridge. The length of that channel, from the sewage pipe to the river, is about 1,500 feet.

The gunk was entering the river at the exact spot where crews are continuing to remove what remains of the whitewater feature once known as the Standing Wave. That's the failed $4-million city project that cost $2 million to remove. A few feet downstream is a put-in for river-riders using canoes and kayaks.

Standifer said tests so far have revealed only what the substance is not: raw sewage. That was the immediate concern after DWU and OEQ workers noticed the color change on July 23. It's also not animal waste, she said, ruling out concerns that this flow was an echo of the 2012 incident that involved the dumping of pig's blood into the Trinity.

"It's not a hazardous material," Standifer said, citing a week's worth of test results. She said OEQ and DWU employees had seen frog, turtles and fish swimming in the river, near the channel discharging the fluid into the Trinity, and that tests had determined oxygen levels were still safe.

"There is nothing in the water detrimental to the wildlife," she said.

That said, the city has notified the Texas Commission of Environmental Quality. And when the source is located, the city or state could cite the company responsible for causing an illicit discharge. That citation could range from a reinspection of facilities to a fine.

That's because whoever is responsible, said Standifer, "would be in violation of their TCEQ permit for waste and stormwater."

Ben Sandifer walks past a culvert of gray, putrid water where an illicit discharge flows into the Trinity River near the Santa Fe Trestle Trail on Tuesday. (Smiley N. Pool / Staff Photographer)

The public only found out about the pollution Tuesday after naturalist Ben Sandifer — the Trinity River and Great Trinity Forest's unofficial watchdog — tweeted about it. Sandifer initially thought he was looking at raw sewage because of the color and, especially, the smell, which is potent depending on the breeze.

"That's incredible. It's been going on for a week, and it's still flowing like that," he said. "And it boggles my mind that someone has known about this for a week, and there hasn't been a way to find out who's responsible."

Tuesday afternoon, after temperatures cracked 100 degrees for the first time this summer, city workers were still chasing the source. At one point they left the channel, which was filled with the dark gray fluid, and drove to a nearby manhole on Dallas Area Rapid Transit property to see if a pipe was leaking.

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Along the Trinity, a few men stood with fishing poles in the water. Occasionally they would wash their hands in the river.

Sandifer said even if what's in the water is organic, "it's pollution any way you look at it."

"It's basically turning the Trinity into an open sewer, and it will remain that way until someone fixes it. ... And it's disgusting," Sandifer said. "If this was flowing down the street of Dallas, no one would put up with it. We need to treat the river better than this."