In 2015, scientists made that happen on purpose off Southern California. Imagine a beach where the waves suddenly turn bright pink. The dye was used to simulate pollution. Scripps reports that the dye moved north through the surf zone at nearly eight miles per day during a strong south swell.
Researchers conducted three experiments near Imperial Beach, releasing non-toxic, bright pink fluorescent dye to see how it moved along the coast, according to Scripps Institution of Oceanography .
Because you often cannot see it, pollution is hard to track once it enters the sea,. Dye takes care of that problem. It glows, so researchers can track it with sensors in the water and imaging equipment on an airplane. The work was part of a project called Cross-Surfzone/Inner-shelf Dye Exchange, or CSIDE, funded by the National Science Foundation.
Waves did most of the work. When waves roll toward a beach at an angle, they create a current that runs along the shore. The study found that the dye moved north in the direction predicted by the incoming waves. That matches the Scripps account of dye moving north in a strong south swell, a route that leads toward the entrance of San Diego Bay. The study attributes much of that drop to rip currents, narrow, fast streams that pull water away from shore, which carried dye from the surf zone to deeper water.
A detailed scientific account comes from a 2021 study by Derek Grimes, Falk Feddersen, and Sarah Giddings published in the Journal of Geophysical Research: Oceans . The team followed a release made on October 8, 2015, about 0.6 miles north of Imperial Beach. Over about four hours, they pumped in about 30 gallons of dye solution and tracked it for about 30 hours, with instruments covering about 4 miles of coastline. The leading edge of the dye crossed that stretch in roughly 11 hours. The strongest dye reading in the surf, near the release point, was about 90 parts per billion. About 4 miles further up the coast, it was under 5. The authors estimate that surf-zone water was exchanged with deeper water about every 2.3 hours.
What this means for beachgoers
Exchange between the surf zone and deeper water reduces the dye in the surf zone, where most people are exposed, decreasing the downstream water quality impact of a pollution event. But there are limits. The study looks at one release under one set of conditions, and the authors say similar long-range observations are needed to test whether their model holds up elsewhere. Even so, the pink water gave scientists a clear view of something normally hidden, and it helped explain how shoreline pollution may travel.
Scripps also says more studies and model work are needed for better real-time forecasts of where pollution is going for beaches in San Diego, Imperial Beach and Coronado.
