An unknown amount of untreated sewage flowed into the Cedar River Monday and Tuesday after gushing from a pipeline on the hillside near Royal Hills.

City officials aren’t sure how long the sewage was coming from a manhole serving a 14-inch sewer interceptor before it was discovered by walkers on an asphalt trail at about 9 p.m. Monday.

Emergency crews found the pipeline was clogged with debris – some as big as a small log – and removed it using special equipment. However, on Tuesday the sewage spilled out again after the pipeline become clogged with debris, including dirt and rocks.

Both times, crews “found a significant backup of sewage surcharging out of the manhole,” said Gregg Zimmerman, Renton’s public works director.

The clog was again removed and crews were keeping an eye on the pipeline to ensure the sewage was still flowing.

The seven-foot-deep manhole was at a point where the pipeline had narrowed from 24 inches to 20 inches and then to 14 inches. It’s likely the debris couldn’t pass through the narrower width and sewage was pushed up the manhole.

The source of the debris was the subject of intense research and investigation by city officials during the week.

One possible explanation is that a manhole cover was knocked off farther away during a land-clearing operation and the debris was pushed into the manhole and eventually the pipeline, said Zimmerman.

The city was searching inspection and permit records to find such an operation. Zimmerman and other city workers planned to walk the area during the week in search of such evidence.

Zimmerman has pretty much ruled out vandalism because of the amount of debris in the pipeline, which carries sewage from the Benson Hill area, under the Cedar River to a large Metro sewage interceptor.

A break in the pipeline is unlikely, too, he said. He and others found large chunks of wood in the pipeline, one a log about four-feet long.

The pipeline roughly follows the powerline right of way down the hill toward the Cedar River near Royal Hills Drive. The manhole is about half-way down the hill. The sewage flowed down the hillside, missing any residential areas.

“It’s in a fairly obscure area,” Zimmerman said. The pipeline, part of city’s sewage system, isn’t monitored by equipment, so the only way to detect a problem is to see one.

The sewage flowed into the river just downriver from fish-spawning channel. City officials are working with county and state agencies to determine what impact, if any, the sewage had on fish and other wildlife.

The spawning channel wasn’t “much affected,” Zimmerman said.

No sewage had been reported floating in the Cedar. Groundwater and recent rains helped to dilute the material, Zimmerman said.