Nara Bopp was working at a thrift store in Moab, Utah the morning of March 4 when her desk started moving.

“I immediately assumed that it was a garbage truck,” Bopp said.

She looked out the window. No garbage truck. No construction nearby either. So she did the same thing she does every time something weird happens in Moab: she logged onto the town’s unofficial Facebook page to see what was up.

“Pretty much everyone was saying: ‘Did you just feel that earthquake?’ or, ‘Did you just feel something shaking? Was that an earthquake? Does Moab even get earthquakes? This is crazy,’” Bopp said.

Moab doesn’t normally have earthquakes people can feel. This one -- at a magnitude 4.5 -- didn’t cause any damage. But it was enough to get people’s attention in communities all along the Utah-Colorado border. Many took to social media to post about the uncharacteristic shaking.

We felt this in Grand Junction, barely. Thought it was workers on the roof moving equipment. #earthquake #mesacounty https://t.co/kTBGq0HB7G — Scott Braden (@greenriverscott) March 4, 2019

Whoa! We just felt an earthquake shake Moab for a few seconds. — Ashley Bunton (@ashbunton) March 4, 2019

Earthquakes can feel like a freak of nature, something that strikes at random. But not this one. There’s no question where it came from and that human activity caused it.

Since the turn of the 20th century, the Colorado River and its tributaries have been dammed and diverted to sustain the growth of massive cities and large-scale farming in the American Southwest. Attempts to bend the river system to humanity’s will have also led to all kinds of unintended consequences. In Colorado’s Paradox Valley, those unintended consequences take the form of earthquakes.

Credit Luke Runyon / KUNC / KUNC The Paradox Valley Unit in western Colorado has caused small earthquakes along the Colorado, Utah border since it first began disposing of brine in a well deep underground in 1996.

The earthquake that shook desks in Moab had its epicenter near a little-known federal wastewater facility near Bedrock, Colorado.

“Welcome to Paradox,” said Andy Nicholas, at the headquarters for the U.S. Bureau of Reclamation’s Paradox Valley Unit. The office is situated near the banks of the Dolores River, an important Colorado River tributary. The waterway cuts across the valley perpendicularly, rather than flowing down it. An early explorer noted the valley’s uncanny, paradoxical topography and the name stuck.

Credit Luke Runyon / KUNC / KUNC Andy Nicholas manages the Paradox Valley Unit for the U.S. Bureau of Reclamation.

Nicholas, the project’s operations specialist, has felt quite a few earthquakes since he moved to the valley in the 1990s.

“The one in 2013, it woke me up and I was definitely startled,” Nicholas said.

It was particularly startling because Nicholas knew he had played a role in making that earthquake -- and the dozens of others that have occurred at the site -- happen.

“I'm pretty closely connected with the project so that probably had some sort of an effect on how it made me feel,” Nicholas said.

In a pickup truck, Nicholas drives along the latte-colored Dolores River. After the Paradox Valley, the river meets up with the San Miguel and wends its way through a remote stretch of western Colorado and eastern Utah’s canyon country before emptying into the Colorado River. It’s a water source for about 40 million people in the Southwest.

Since 1996, the facility’s sole goal has been to keep the Dolores from getting too salty, part of a watershed-wide effort to keep the entire Colorado River from picking up too much salt. In 1973 the U.S. signed an agreement with Mexico called the "Permanent and Definitive Solution to the International Problem of the Salinity of the Colorado River." As part of that agreement, the U.S. said it would take steps to reduce the river’s salt load. That led to projects like Yuma, Arizona’s inland desalination plant, and the Paradox Valley project.

If the Colorado River runs too salty at the U.S.-Mexico border, the U.S. could be in violation of a longstanding treaty over the river’s quality, further raising the stakes to figure out what’s happening in Paradox.

Snowmelt from the nearby La Sal mountains filters through the Paradox Valley’s salty soils, draining toward the valley’s low point, the Dolores River. As it drains, the snowmelt transforms into brine, eight times saltier than sea water, and seeps into the Dolores, loading it with salt. In dry years the brine collects on the valley floor in black, sulphur-smelling pools.

Salt degrades water quality and makes it harder for people to use, Nicholas says. Using a network of shallow groundwater pumps, the project intercepts the naturally-occuring brine before it can seep into the river.

“It's just a natural pollution,” Nicholas said. “It's polluting the river and contaminating the river.”

“But if it's naturally occurring is it pollution?” I asked.

“Well it doesn't make it any better just because it's natural. Right?” he said.

Credit Luke Runyon / KUNC / KUNC "Everybody that I've talked to said it was a pretty good shaker," said Paradox Valley Unit operations specialist Andy Nicholas about a March 2019 earthquake that registered a 4.5 magnitude.

Salt enters the Colorado River system in many ways. Some springs that feed western rivers are naturally saturated with salt. Desert agriculture requires irrigation. As fresh river water moves over salty, desert soils it dissolves salt, and some portion of that irrigation water returns to the river channel. The Paradox Valley, with its thick subsurface salt layer, is unique in the basin, Nicholas said.

“Paradox is kind of unusual because it is about a two-mile stretch where you get 150,000 tons a year of salt potentially getting into the river system,” he said.

After the brine is intercepted, pipes carry it to a treatment facility and then on to a deep injection well. When that brine lubricates a faultline it can slip and cause an earthquake — like the magnitude 4.5 quake Nara Bopp felt in March.

“It probably got attention for maybe a day and then probably was something that people would kind of mention in passing over the course of the week,” Bopp said. “But then we kind of all forgot about it.”

Moab residents moved on quickly, but the earthquake has had a lasting effect on the project. It was the largest earthquake the project ever caused.

The project’s potential ability to cause earthquakes was well known before its current injection well was drilled. To prepare for the creation of the Paradox Valley Unit, in 1983 the Bureau of Reclamation put in place a network of seismic sensors to keep an eye on earthquake activity in the region.

In the project’s early years the earthquakes were relatively small and infrequent. In 1999 an earthquake from the facility breached 3.0 magnitude. Since then, 3.0 and higher magnitude quakes have taken place in 2000, 2002, 2004, 2013 and March 2019.

“The earthquake in March, it was a fairly considerable-sized fault plane that slipped and that's what allowed the larger magnitude,” Nicholas said.

Credit Luke Runyon / KUNC / KUNC After leaving the Paradox Valley, the Dolores River wends its way through canyon country in western Colorado and eastern Utah before emptying into the Colorado River.

Since early March, the facility has been turned off. In the past four months about 27 other small earthquakes have registered in or near the Paradox Valley.

Now, with its injection well reaching the end of its life, the Bureau of Reclamation is forced to look at how it wants to handle the Dolores River’s saltiness decades before it thought it would need to revisit it.

“I think they hoped (the injection well) would last 50 years,” Nicholas said. “And as it turns out we're about 25 now and so we know that we're approaching the end of its useful life just based on its performance.”

Reclamation announced it was putting together a new Environmental Impact Statement (EIS) for options to dispose of the brine in 2012. The agency has yet to release a draft EIS, but has listed a set of possible paths.

One option is to try the same tactic, with a new injection well drilled somewhere near the Paradox Valley facility and into rock layers less likely to see seismic activity. Another option would remove hydrogen sulfide from the brine, and then pump the salty water into an open pit to evaporate. One more option would use technology to crystallize the brine and then dispose of the waste.

The final option in the EIS is that Reclamation might choose to retire the project altogether, let the Dolores return to its salty natural state, and find other places in the Colorado River basin to clean up the water in order to keep the U.S. from violating its treaty obligations to Mexico.

This story is part of a project covering the Colorado River, produced by KUNC and supported through a Walton Family Foundation grant. KUNC is solely responsible for its editorial content.