Part of the Santa Cruz River runs red this week. Here's why

The Santa Cruz River on Tucson’s northwest side will take on a reddish hue Tuesday when UA and federal researchers launch an experiment to learn about sunlight’s effects on “trace chemicals” in the water.

At about 6:45 a.m. Tuesday, University of Arizona and U.S. Geological Survey researchers will release a couple of gallons of what they say is a nontoxic dye called rhodamine WT, diluted with stream water, into the river on the north side of the Ina Road ridge, near the Tres Rios Water Reclamation Facility.

As the water flows north toward Marana, the dye could become visible as far away as the Trico Road Bridge, 17.5 miles north of Ina Road.

The reddish color should last for a couple days, most likely turning a diluted pink before dissipating, said Joseph Cuffari, a program manager for the Pima County Regional Flood Control District, which manages rivers and washes.

“The dye should be quite visible in the river initially, perhaps dramatically so, and will fade as it moves and disperses going downstream,” said Eric Shepp, Flood Control District deputy director, adding that the researchers don’t yet know how far along the river the dye will remain visible.

To check the impacts of sunlight, researchers will conduct separate tests later for concentrations of personal care products, pharmaceuticals and other chemical compounds in the water, Cuffari said.

The river water at that site is mostly treated effluent from two Pima County sewage treatment plants.

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“You can never fully clean the water — you clean it to the best of your abilities,” said Cuffari. “Everything is 99.9% effective. They want to look at the last 1%.”

“The key takeaway: Does sunlight alter chemical compounds?” he added.

Since river water flows at different speeds in different locations, the use of the dye will let researchers ensure they’re collecting samples of the same sections of water as it moves downstream.

By taking samples at specific locations, researchers hope to be able to measure sunlight’s cumulative impact, as to whether it either destroys or chemically alters organic materials in water. That research will be done separately, in later laboratory experiments in which researchers will collect water samples at specific intervals downstream to measure sunlight’s impacts, flood control officials said.

Researchers will collect samples until about Friday.

The data could be useful for researchers around the country looking at water quality treatment efforts for effluent, Cuffari said.

The USGS requested the project to support its research. “The county and the flood control district are okay with USGS performing this work, but it is their work,” Shepp said.

30+ historic photos of the Santa Cruz River through Tucson