In 2022, NASA’s Jet Propulsion Laboratory launched a satellite sensor to map minerals in the Earth’s dusty, arid regions. But that’s not all it’s useful for—in a new study, scientists used the spectroscopic tool to study massive amounts of sewage flowing into the sea off the Southern California coast.

Every year, millions of gallons of untreated and treated wastewater are unceremoniously dumped into the Tijuana River, ferrying pollution through communities on both sides of the U.S.-Mexico border before reaching the ocean. In a study published in the journal Science of the Total Environment, researchers used Earth Surface Mineral Dust Source Investigation (EMIT), a hyperspectral imaging instrument aboard the International Space Station, to examine this nauseating wastewater plume at the Tijuana River delta.

Pollution from wastewater isn’t pretty. It can bring toxic chemicals, harmful algal blooms, and unfriendly bacteria, which can impact the health of beachgoers.

People typically rely on water-quality dashboards to tell them when it’s unsafe to swim, but these often rely on field samples, which don’t always cover the full extent of the pollution. Collecting those samples also takes time and money, especially in heavily contaminated areas.

That’s where EMIT comes in. It uses imaging spectroscopy, a technique that captures both spatial and spectral information by measuring sunlight reflected off the Earth’s surface. Each image can capture hundreds of wavelengths per pixel, giving scientists detailed insights into the molecular composition and material properties of what’s on the ground.

This allowed researchers to measure the spectral signature of the wastewater plume. The researchers compared the hyperspectral satellite images taken by EMIT with the spectra of untreated wastewater, diluted wastewater, and seawater sampled near the plume. They also compared EMIT’s images with readings from a spectrometer on the ground. They found that water with high wastewater contamination emits a distinct red spectral feature that could allow researchers to monitor the extent of pollution in the future—though the source of that feature is still unclear.

The researchers hope that the data can complement efforts to sample water quality near the river’s mouth by telling water scientists where to sample.

“From orbit you are able to look down and see that a wastewater plume is extending into places you haven’t sampled,” study coauthor Christine Lee, a scientist at JPL in Southern California, said in a statement. “It’s like a diagnostic at the doctor’s office that tells you, ‘Hey, let’s take a closer look at this.’”

EMIT has also proven useful for detecting emissions of methane and carbon dioxide from gas leaks, assessing forest health, and estimating snowpack melting rates.

Now, EMIT has yet another job. “The fact that EMIT’s findings over the coast are consistent with measurements in the field is compelling to water scientists,” Eva Scrivner, a doctoral student at the University of Connecticut and the study’s lead author, said in a statement. “It’s really exciting.”