In 2023, San Diego researchers released bright pink dye into an estuary plume to study how runoff moves through breaking surf

Imagine a typical Southern California beach day: blue water, white foam, kids building sandcastles. Now imagine that water turning bright pink right in front of everyone. That is exactly what happened at Torrey Pines State Beach on January 20, 2023. It was a real science experiment, not a prank or a movie set. According to the UC San Diego report, researchers wanted to track something most beachgoers probably don’t think about: where storm runoff actually goes when it hits the waves.

What actually happened at the beach
The experiment was called Plumes in Nearshore Conditions, or PiNC. A report from UC San Diego says the project was led by the Scripps Institution of Oceanography and the University of Washington, which released a bright pink, non-toxic dye at the mouth of Los Peñasquitos Lagoon, a small estuary located between San Diego and Del Mar. There were three releases planned, the first on January 20, 2023, and two more in early February. Each was timed to the outgoing, or ebb, tide so the pink water flowed straight from the lagoon into the surf. The dye could be seen for several hours and detected by instruments for about a day. It posed no threat to people or wildlife, but beachgoers were asked to swim elsewhere near the lagoon on release days.

The technology behind the color
The team used drones, sensors on poles in the sand and a jet ski with a fluorometer, a device that measures the light that is emitted by the dye. Meanwhile, wave height, tides, salinity and temperature were recorded by seafloor moorings. Alex Simpson, a Scripps postdoctoral scholar, said he was excited to fly a drone camera that could pick up nearly 100 times more of the visible light spectrum than a typical phone camera, according to the UC San Diego report.

Not the first pink wave in San Diego
Scripps Oceanography had used pink dye before. The report from UC San Diego cites an international study that tracked beach pollution in 2015 near the U.S.-Mexico border, and earlier dye work at Imperial Beach and Huntington State Beach. PiNC extends that approach to Los Peñasquitos Lagoon, a small intermittently closed estuary where researchers can study a compact plume meeting the surf zone along a relatively uniform stretch of coast.

Why small waterways matter more than they seem
For years, ocean scientists mostly studied large rivers with high outflow, far bigger than most American creeks and storm drains. Scripps coastal oceanographer Sarah Giddings, who led the PiNC study, noted that far less research existed on smaller plumes that interact directly with breaking waves, even though these smaller flows are common along the California coast.

A peer-reviewed study explains why that void matters. The 2018 paper, ‘Impacts of Nearshore Wave-Current Interaction on Transport and Mixing of Small-Scale Buoyant Plumes,’ in Geophysical Research Letters by Angelica Rodriguez, Sarah Giddings and Nirnimesh Kumar states that small freshwater plumes behave differently once they encounter breaking waves. Instead of spreading out to sea, waves tend to trap the water close to shore, pushing it sideways along the beach instead. Whatever that water is bringing can hang around the shore longer than one would think.

What this means for your next beach day
That freshwater is rarely just rain. It often carries sediment, bacteria, fertilizer, oil, and pet waste collected from streets and storm drains, and this is not just a California story. A 2024 EPA national beach report covering the 2023 beach season shows that rainy day stormwater runoff is one of the leading causes of U.S. beaches getting swimming advisories or closures. That flash of pink at Torrey Pines was a stand-in for something unseen that happens along coastlines all over the country every time it rains.

The bigger picture
This UC San Diego report notes that PiNC was supported by National Science Foundation award #1924005 and additional support from the California Department of Parks and Recreation for related work at the same site. The project also involved researchers from UC Irvine, the Mechanical Engineering department at UC San Diego, the Naval Postgraduate School, and Chile's Universidad Técnica Federico Santa María. Giddings said the plan was to combine this fieldwork with results from an earlier, separate field study, plus computer models, to understand how these plumes spread both at Los Peñasquitos Lagoon and at other similar small river mouths around the world.

The takeaway
So next time you see a headline that says scientists are dyeing the ocean pink, you know it wasn't just a stunt. It was a clever way to answer a question that matters to anyone who lives near the coast: when the rain stops, where does the water really go? Thanks to this experiment, we have a clearer picture of the answer, one wave at a time.