Santa Cruz River – Red Dye Testing

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Pima County releases notification:

Researchers use red dye to study Santa Cruz River

PIMA COUNTY, March 4, 2022 – Beginning March 8, the Santa Cruz River will run red. The changing color of the river is part of a scheduled research project between partner agencies including the University of Arizona and the U.S. Geological Survey.

Researchers plan to apply the dye to river flows near the Tres Ríos Water Reclamation Facility located near Ina Road. The non-toxic dye, rhodamine WT, will be released into the river and be monitored as it flows downstream in order to aid researchers as they take water samples at selected downstream locations. Rhodamine WT, the fluorescent red dye used in this study, and commonly used in other hydrologic studies, is non-toxic and safe for the environment.

Since the water in the river flows at different velocities at different locations, the application of the dye will allow researchers to time the variable rates of flow to ensure they are collecting samples of the same sections of water as it moves downstream.

The dye injection is being done by the USGS in coordination with University of Arizona researchers, who plan to use the water-travel data in later laboratory experiments. For that project, researchers will collect water samples at specific intervals downstream to measure the effects of sunlight on certain organic materials. Some of the materials researchers are testing for include personal care products, pharmaceuticals, and other chemical compounds.

By collecting water samples at specific locations, researchers hope to measure the cumulative impact of continued exposure to sunlight on destroying or chemically altering the residual organic materials in effluent.

The dye may be visible for several days if ponding at a location were to happen. University staff should be collecting samples until about March 11. The Regional Flood Control District and Regional Wastewater Reclamation Department will be monitoring the release of the dye and will be interested to see what the data yields. Pima County supports community research conducted within our facilities to contribute to knowledge that can benefit sustainability. The Regional Flood Control District and Regional Wastewater Reclamation Department will retain copies of all research papers and articles generated by this study for use by future researchers. 

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