Biodiversity Returns in Dried-Up Desert Riverbeds When Treated Effluent Water Is Added

Good News: dried-up desert riverbeds such as the Santa Cruz River in Tucson recently witnessed the resurgence of biodiversity when fed with treated effluent water.

Drying up the Rivers

In the late 1800s, the southwest U.S. saw rivers being diverted and parceled out. Dams and irrigation canals stopped and diverted river water flow. As human populations grew, there came more pressure on groundwater, and with the changing climate, previously perennial streams started running dry, and water tables began to sink.

Southeastern Arizona's Santa Cruz River had the same fate. Its banks used to have poplars, plantains, ash-trees, walnut trees, oak trees, and willows in the mid-1800s. By the mid 20th century, however, all these were gone.

Almost 70 years after Santa Cruz River's downtown reach ran dry, it finally tasted water once again, as reclaimed water amounting to 2.8 million gallons was released each day, thanks to Tucson City's Santa Cruz Heritage Project.

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Return of Biodiversity

As the water valves were opened to release millions of gallons of treated water, the University of Arizona School of Natural Resources & the Environment researchers asked themselves how and when the biodiversity returned.

According to the College of Agriculture and Life Sciences natural resources assistant professor Michael Bogan, they already saw seven dragonfly species on the first day of the water's return. Then, after just ten months, they already found more than 40 species. They concluded that putting water back to de-watered river systems will entice aquatic life to return even after such a long time.

They published their results after a year of the river being fed with treated effluent. It was published this week in PeerJ.

Better Treatment of Wastewater

School of Natural Resources & the Environment graduate student Hamdhani said that streams being fed with poorly treated effluent become degraded and have lower biodiversity due to the poor water quality from treatment facilities. His published study review of water quality in streams fed by effluent worldwide gave him insights on the matter. The review was published in the Freshwater Biology journal.

He says that water quality's critical parameters closest to effluent outfalls are lacking, such as nutrient content, high temperature, low dissolved oxygen, and trace levels of organic contaminants.

However, many wastewater treatment plants can better reclaim the water quality with upgraded facilities, which can surprisingly support a high level of biodiversity once they are fed to streams.

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This is true with Santa Cruz River. According to Bogan, this is due to Tucson's many changes around eight years ago in the two treatment plants feeding Santa Cruz's northern reaches. Before 2012, low-quality water was discharged, and the river can be smelled by drivers passing on Interstate-10. They were upgraded during the 2010s, improving the water.

Bogan said that the smell disappeared after that, and the biodiversity came back. In Santa Cruz, they found more than 150 different insect species, thanks to better water treatment and higher quality of water.

Arid lands resource sciences doctoral student Drew Eppehimer said that the presence of caddisflies and mayflies shocked them, and the level of biodiversity now in the effluent-dependent river is encouraging. Caddisflies & mayflies indicate a healthy and regenerating water system.

This shows promise in regenerating and reviving the biodiversity of other dried riverbeds that can be fed with treated effluent water.

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