A collection of sewage — filled with human feces, garbage, and disease-ridden substances — being dumped into a body of water is clearly not the most attractive image.
But according to scientists from the University of Arizona, wastewater has the potential to restore threatened ecosystems — so long as it’s used correctly.
In fact, new research in the journal Restoration Ecology details how scientists have unlocked a breakthrough in their work to save the Santa Cruz River, which runs 180 miles through Arizona and Mexico.
Earlier this year, the Santa Cruz was named one of the most endangered rivers in the United States, drying up due to climate change, irresponsible wastewater disposal, and groundwater pumping.
For decades, local Tucson residents thought the river was a dry wash — a body of water that momentarily fills up following heavy rain but otherwise remains dry. But as it turns out, the river flowed year-round prior to the early 1900s and was used sustainably by Indigenous populations for thousands of years.
In 1913, however, the river’s water dried up, causing countless aquatic species to go extinct in the area. Additionally, the dry bed was then mined for sand and gravel — and became a dumping ground for trash.
“Thankfully, the Santa Cruz has been reborn in recent years,” a University of Arizona web page explains, “and several parts of the river once again thrive with swimming fish, hopping toads, and dazzling dragonflies.”
So, how did they do it?
Using wastewater to restore the Santa Cruz River’s flow
Scientists strategically introduced effluent — sewage that is discharged into a body of water from an industrial plant — to restore the river’s flow, over 100 years after it dried up.
Of course, the wastewater was treated prior to being poured into the riverbank, removing disease and pollutants. From there, it could bring life back to the dried-up stretch of the Santa Cruz.
Over a period of two years, scientists found that hundreds of invertebrate species, as well as plenty of fish, toads, and turtles, rebounded.
“I think the biggest surprise in our study was just how quickly species returned to the Santa Cruz River when flow was restored — it was astounding how fast biodiversity could recover when given a chance,” the study’s co-author Michael Bogan, a professor of aquatic ecology at the University of Arizona, told Salon.
“That stretch of the Santa Cruz River had lost year-round flow due to groundwater pumping more than 100 years earlier — and yet within weeks of flow being restored, there were literally dozens of aquatic species living in the reborn river,” Bogan continued.
He said that nearly 150 species returned after a year of adding the wastewater to the river, including plenty of insects, which help maintain a natural food cycle, feeding birds and other local critters.
“The number of bird species in the restoration site has gone up dramatically because they love eating all the aquatic insects emerging from the river,” Bogan added.
Not only did these populations boom, but they were sustained two years after the effluent was introduced.
While the conservation efforts were being introduced and monitored, scientists faced a “massive disturbance,” in which the county’s regional flood control district decided to stop the river’s flow to protect Tucson and resume riverbed dredging.
It was initially a frustrating surprise, but as Bogan explained to Salon, became a helpful addition to the scientists’ research, as they were able to “quantify the impacts of urban channel maintenance activities on recovering communities,” per their research abstract.
As it turns out, when the effluent flow was reintroduced after this temporary pause, the species returned — “just as fast the second time,” Bogan said.
Much of this is progress thanks to the improvement of wastewater treatment facilities in the area. The county invested $600 million to upgrade two wastewater treatment plants in the Tucson area — its largest public works project ever.
“Pima County chose to invest in upgrading the treatment facilities, and as a result we have very clean effluent to use for ecosystem benefit,” Bogan said.
This progress also requires ongoing maintenance and care for water to continue to be released back into the Santa Cruz, especially as recent news reports of a raw sewage spill could have major implications for the river and the surrounding environment.
“I think it’s important for people to know that each aspect of decision-making in their local city and county governments can have huge impacts on the wildlife and ecosystems of their city — and in return, the livability of their city,” Bogan said.
He added that this project uses less than 1% of the treated wastewater generated in Tucson every day, meaning a small fraction of sewage treatment can create an enormous ripple effect for local biodiversity.
A new hope for a once-dry river
Evolving investments in water treatment and partnerships with the right stakeholders mean the Santa Cruz River has a promising future.
Moving forward, effluent releases from Pima County will support two sections of the river in north Tucson and Marana as part of Tucson Water’s Santa Cruz River Heritage Project, which the university claims “has created a new ribbon of green in downtown Tucson.”
The university has also encouraged locals to track wildlife using apps like iNaturalist to maintain a vested interest in these improvements.
And beyond Arizona, this research could have broad implications for water management boards across the country.
“Our results demonstrate that adding water to a previously dry channel can quickly lead to a diverse macroinvertebrate community in urban streams,” the researchers’ published work reads.
“Effluent is an important and underappreciated tool for creating new habitat for many riverine species in arid and semiarid regions.”
In other words, using effluent as a tool to breathe life back into drying waterways could change the trajectory of a number of bodies of water resigned to fates like that of the Santa Cruz.
“We can undo some of the damage that we’ve done to rivers and their inhabitants across the western USA,” Bogan said.
“For more than a century, the growth of cities and towns has resulted in rivers running dry and aquatic and riparian species going extinct as their habitat disappears.”
Finally, he said, there is hope.
“This is a rare ‘good news’ story in ecology and wildlife, where human development is actually helping to restore an ecosystem,” Bogan said, “In this case, specifically by using our city's treated wastewater to do so.”
Header image courtesy of Elnogalense (CC BY-SA 4.0)
