Despite state wildlife agency reports that more than 5,600 fish died last month in the Big Thompson River upstream from Loveland’s water treatment plant, city officials stress residents’ drinking water remained safe.

Colorado Parks and Wildlife officials confirmed Wednesday that a significant fish kill occurred March 7 on the Lower North Fork of the Big Thompson and mainstream Big Thompson River from Drake downstream to the canyon mouth in west Loveland.

Initial reports of fish kill from residents launched a multi-agency response, and Parks and Wildlife officials said when they found out it was limited to rainbow and brown trout, suckers and dace, the agency started investigating its cause and extent.

Officials said that while they are still analyzing the fish kill and its cause, it seems that chemicals from concrete work on the County Road 43 project and replacement of Storm Mountain Road bridge went into the water, dramatically increasing its pH levels.

“To date, the project, which is slated for completion in late summer 2016, has replaced multiple bridges and constructed grouted … walls along many sections of road and river without issue. Unfortunately, site conditions, weather, soils, topography and other factors at the Storm Mountain bridge created conditions that allowed movement of chemicals from concrete to enter the stream, causing a dramatic increase in pH … which when moving downstream sickened or killed fish in its path,” the news release stated.

Darrick Turner, senior environmental specialist for the county, and other county staff members, after hearing from residents of the fish kill, followed up with construction crews but found that the crews’ environmental mitigation practices were in compliance at the time.

“At that time, (Colorado Parks and Wildlife) staff was doing a fish assessment on the river, and we coordinated with them and just made sure that we had an accurate response to what was going on,” Turner said.

It’s hard to determine the exact cause of the problem, Turner said, especially because the appropriate hazard mitigation methods were in place.

But the trout in the river are “very sensitive species,” according to Turner, so even a slight deviation in pH could have caused the kill. But that doesn’t necessarily mean people need to put away their fishing equipment.

“As far as sustained chemistry in the river that would make people not want to eat fish, I don’t think we have that,” he said.

The state will continue to monitor water quality standards and fish in the river, Turner said, and the county will continue to work closely with construction contractors to make sure all proper measures are taken to mitigate hazards to the river, land and air.

Loveland Water Treatment Plant Manager Scott Dickmeyer said the city shut off intake valves from the Big Thompson River and stopped treating that water on March 7 when they found out about the fish kill.

Dickmeyer said the U.S. Geological Survey was conducting a preplanned sampling of the river March 7 for the Big Thompson Watershed Coalition when they noticed the dead fish.

Almost instantaneously, Dickmeyer said, the fish in the Loveland Water Treatment Plant swimming in a tank in water from the Big Thompson River also died.

So, Loveland residents are getting their drinking water from Green Ridge Glade Reservoir, which is currently filling with spring runoff.

“There’s no pressure for us to be treating Big Thompson water,” Dickmeyer said, especially with all the construction going on along the canyon.

In the water treatment plant, the fish have been replaced in river water and seem to be doing fine.

Colorado Parks and Wildlife spokeswoman Jennifer Churchill said while the agency plans to replenish the fish supply, it’s not an easy or short task — it could take up to a year, she said.

The agency’s aquatic biologist for the Big Thompson drainage Ben Swigle determined that sections of the river between Drake and Estes Park were not affected, according to the news release, and “healthy populations of both native and sportfish species in the upper sections will partially serve to repopulate sections of river comprised as part of this unfortunate event.”

Despite the fish kill, Dickmeyer said turning off the water from the Big Thompson doesn’t necessarily mean the water would have been harmful to people or that it couldn’t have been treated.

“We turned our intake off out of an abundance of precaution,” he said.

Interim Water Utilities Manager Roger Berg said the water treatment plant staff members are constantly monitoring the water quality, and at no point in time, did they see a change in levels. And, he said, staff members would have noticed even a slight change because the system would have alerted them of it before something serious occurred.

Dickmeyer said the Colorado Department of Public Health and Environment was notified of the issue, but officials had no concerns and didn’t ask the city of Loveland to change anything.

The city is still waiting for official results, but Dickmeyer said from the information he’s received, it seems that it was just a “slug” of bad water, and once that flushed through, it was no longer an issue.

Turner shared a similar perspective, saying it seemed to be a single-time “illicit discharge” event.

Still, Dickmeyer said, until he sees hard numbers, he won’t be suggesting Loveland’s drinking source be moved back to the Big Thompson River.

Saja Hindi: 970-669-5050, ext. 521, hindis@reporter-herald.com, twitter.com/SajaHindiRH.