2017-06-15 / Front Page

Sewage spill in river lasted just 30 minutes, no contamination found

Photo courtesy National Park Service

This is the rockslide on Highway 140 east of El Portal. Officials have not determined when the road might reopen.Photo courtesy National Park Service This is the rockslide on Highway 140 east of El Portal. Officials have not determined when the road might reopen.By GREG LITTLE


A sewage spill likely related to the rockslide on Highway 140 in Yosemite National Park lasted just 30 minutes on Wednesday.

The spill resulted in about 1,000 gallons of wastewater going into the Merced River, according to Dave Conway, director of the Environmental Health Division for the Mariposa County Health Department.

Conway said the spill happened in the Cascades area, where a dam used to be located. That is at the intersection of Highway 140 and Big Oak Flat Road, which is an extension of Highway 120.

Conway said the slide is “assumed” to have caused the backup, though that won’t be known until the rocks are removed and an assessment can be done on the area.

He said the main sewer line out of Yosemite Valley runs underneath Highway 140 and goes to the sewage treatment plant in El Portal.

Conway said in the area of the slide, the main sewer line runs closer to the hillside than the river side, where more of the rocks landed.

“It did not break the line or expose it, but it may have pinched it a little bit,” said Conway.

When that happened, the line backed up and caused the spill from the manhole at the area around the Cascades.

Conway said within 30 minutes, crews had bolted down the lid of the manhole and it stopped the flow.

He said wastewater plant officials in El Portal are reporting “they are getting the majority in daily flows down at the plant.”

He also said officials are monitoring the area around the slide closely. Park officials have asked visitors and residents to practice water conservation until more information can be determined.

Another factor about the spill is where it happened, said Conway. That area provides aeration because of the topography.

“By the time it gets down to El Portal, it is pretty cleaned up,” said Conway.

Another key element is how fast the Merced River is currently running. At the Pohono Bridge on Wednesday, the flow was 2,250 cubic feet per second. That equates to 16,500 gallons of water per second. That is enough to fill a swimming pool passing each second.

He said 1,000 gallons is “a relatively small spill” when the factors of aeration, natural UV treatment and flow of the river are considered.

The National Park Service did erect warning signs downstream of the spill after it happened. By Thursday, those signs were taken down as Conway said testing wasn't expected to reveal an "detectable contamination." He said it takes about four days for the lab to complete the testing but because of the low volume of the spill, those results were expected to show not contamination.

Conway said National Park Service officials were bringing heavy equipment to the site on Thursday as they begin the process of removing the debris. As part of that process, he said equipment capable of creating a wastewater bypass line would be on site. 

He said if crews determine a section of the line has to be replaced, they can use the bypass equipment and no wastewater would escape.

“If they have to cut the line, they will,” said Conway.

That determination will not be made until the rocks can be cleared from the road.

Though Conway said Mariposa County isn’t responsible for overseeing the repairs, it is “responsible for the public health” in the County.

He said anytime Park officials discover a spill, they have to immediately notify Mariposa County. Conway said there have been similar spills in the past and the County has always been notified when it happens.

He also said, since he has been with the County, none of the spills have ever resulted in elevated contamination levels in the El Portal area.

As for Highway 140, park service officials have not made any announcements about when the road might be reopened. Their latest statement said it would be closed at least through this weekend.

The slide happened at around noon on Monday about two miles east of El Portal and the road has been close since.

(We’ll continue to update this story as information becomes available.)

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