Science Snapshot: Tucson’s Effluent-Fed Santa Cruz River Fertile Ground for Microplastic Research

Tuesday, May 25, 2021

From synthetic fibers in clothing to the packaging of manufactured goods, plastic debris comes in all shapes and sizes. As the name suggests, microplastics are particles that are less than five millimeters—roughly the size of a sesame seed—and, although they may be small, they are a big concern for aquatic ecologists. 

There are two types of microplastics, primary and secondary.

Microbeads are a great example of primary microplastics and were the subject of the Microbead-Free Waters Act of 2015 that prohibited the manufacturing, packaging, and distribution of rinse-off cosmetics containing exfoliating plastic microbeads.

The three important types of secondary plastics include fragments, microplastic films, and microfibers.  

Fragments are irregular shaped pieces that are broken down from larger polymers.

Microplastic films are introduced into the environment from degrading plastic bags, cellophane wrapping, and other similar products.

Microfibers primarily come from synthetic textiles. When washed, synthetic clothing can slough off and shed synthetic fibers into municipal wastewater.  

As an emerging field of study, much of how microplastics affect ecological systems is still unknown but previous research has focused on the world’s oceans, where aquatic life and birds can mistake plastic particles for food.  

“In recent years, the research has shifted to include freshwater systems because our rivers and streams often and eventually drain into those marine environments and are the source of a great deal of microplastic pollution,” said Drew Eppehimer, who recently graduated from the Arid Lands Resource Sciences interdisciplinary program with a Ph.D.   

Formerly a researcher with the U.S. Geological Survey in the Great Lakes region, Eppehimer spent much of his Ph.D. program studying freshwater systems in the arid Southwest and, most recently, tracking the biodiversity resurgence in Tucson’s effluent-dependent Santa Cruz river.

As was the fate of many freshwater systems across the Southwest, the Santa Cruz River ran dry nearly 70 years ago. Today, stretches of the river flow once more thanks to the release of millions of gallons of treated effluent from Tucson’s surrounding wastewater treatment plants.

For the past several years, Eppehimer and a team of researchers from the College of Agriculture and Life Sciences have waded into the river to track the return of aquatic life.  

“We were looking just for the insects, sampling aquatic invertebrates, but in doing so, we were finding lots of colorful microplastic pieces in the water,” Eppehimer said. “It got us thinking about microplastic pollution in the lower Santa Cruz River and that led us to design a project to specifically look at the potential dynamics driving the microplastic pollution we were observing.”  

The Santa Cruz River’s unique hydrodynamics proved to be fertile ground for microplastic research.

Eppehimer’s study, recently published in Environmental Science and Pollution Research, is the first to focus on microplastic pollution in an effluent-dependent stream, where 100% of the baseflow is dependent upon treated wastewater.  

While effluent discharge from wastewater treatment plants can be a substantial source of microplastics, previous studies have shown an average of 94% of microplastics are removed by the tertiary wastewater treatment phase.

As a 100% effluent-fed river, the Santa Cruz afforded the researchers the opportunity to study the distribution and concentration of microplastics as water flowed down river from treated wastewater outflows.

Infrequent but heavy precipitation events in the Tucson valley, primarily occurring within the region’s monsoon season, also allowed the team to explore how microplastic concentrations fluctuated as rainwater runoff and flooding surged through the river.  

Wading into the research

The research team examined microplastic concentrations in the water column and benthic sediment, as well as microplastic consumed by mosquitofish, Gambusia affinis, at 10 sites along a roughly 40 kilometer stretch of the lower Santa Cruz River.

“We collected samples from two important time periods. The first was baseflow, so from a period of 100 days of continuous effluent flow. In other words, there was no runoff, rain inputs, or monsoon flooding,” Eppehimer explained. “That allowed us to focus on what we assumed would be most representative of what was coming straight from the wastewater treatment plants.”

Their second sampling was collected following the first large monsoon storm to hit Tucson, in which flood flow discharge was greater than 100 cubic meters per second.

“We sampled about one day after the large floods, very soon after the flows receded back to normal levels,” Eppehimer said. “The idea being we would incorporate any runoff inputs that came from the urban landscape around Tucson, as well as any changes that might have resulted from simply having a large magnitude flood wash through the system and stir up the riverbed.”

In addition to water and benthic core samples, the team also collected roughly 400 mosquitofish throughout the two sampling periods. Back in the lab, the team carefully dissected the fish to examine the contents of their stomachs.

“Mosquitofish are a small, nonnative fish species that's very abundant throughout the Santa Cruz River and we collected those to do subsequent diet analysis back in the lab and to see if we could observe any microplastics that they had consumed,” Eppehimer said. “We’re talking about fish that are roughly 1 to 2 inches long, so it took a steady hand and some patience to open up their stomachs.”

Dredging the results

While the research team hypothesized microplastic concentrations in both the water column and sediment would decrease as water flowed further away from wastewater outpours, the team saw no correlation between outflow and concentrations along the river.

Instead, monsoon flooding and municipal runoff turned out to be the major culprit in microplastic pollution within the Santa Cruz.

The study found:

  • microplastic fragments in the water column (i.e. floating) increase in abundance after monsoon floods

  • microplastic fibers in the sediment (i.e. riverbed) actually decrease in abundance after monsoon floods (they get scoured out by surging flood waters)

  • mosquitofish in the river are eating some microplastic (on average 6% of fish sampled had observably consumed microplastics) but their consumption of plastic is higher after monsoon floods

The study is a novel contribution to microplastic research in that it stresses the importance in considering the unique characteristics of freshwater systems, particularly in arid and semi-arid climates.

“It's important not just to look at the concentrations of microplastics, but also how they relate to the hydrodynamics of the system, which we know are going to vary region to region,” Eppehimer said. “And as our research shows, that can have a huge impact on the amount of microplastics we’re seeing.”

“The amount of plastic in the Santa Cruz is actually relatively low, considering that it's 100% effluent-fed,” said Michael Bogan, a professor in the School of Natural Resources and the Environment and co-author of the study. “We were kind of expecting our values to be on the high end of what you would expect, but both for the concentrations in the water and in the fish, we're basically seeing run of the mill to even lower-end plastic concentrations than you would see in a natural river system.”  

Bogan counts that as a success story: wastewater treatment is in fact removing a great deal of microplastic pollution.

“This isn't like a sea of microplastics flowing down the Santa Cruz River channel. It's good quality water and it's acting a lot like a natural river would,” Bogan said.

Looking at the role litter and rainwater runoff plays in microplastic pollution in the Southwest, the researchers stress residents should properly dispose of waste and reduce-reuse-recycle to help keep our waterways healthy.

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