The surf at Imperial Beach Pier, September 23, 2015
The results from the pink dye dropped into the surf between Imperial Beach and the border last year are in, and they're set to color the way we think about how sewage spills and dumped pollutants behave, according to Scripps Oceanography researchers.
"It is striking that it hugs the coast, especially when you compare it to the plume-tracker data," said Scripps grad student Derek Grimes. "The momentum the flow [of dyed water] has coming out of the river mouth can definitely have an impact, but it still hugs the shore."
Grimes presented the preliminary results at the International Water and Boundary Commission citizens forum meeting Thursday night (September 1).
The Scripps project is not focused on studying local problems so much as it is about learning the physics of how close-to-shore ocean water behaves, said Scripps assistant professor Sarah Giddings.
"The dye releases are a way for us to understand the physics," Giddings said. "Once we have a good understanding of the physics, we can actually do models that are predictive."
But for Roderick Michener, a former Surfrider official who surfs I.B. and O.B., the finding that dye hugs the coast is important — the county's array of water samplers are set up about a half mile off shore, he said.
"That means it's a mystery what's going on in close to shore. On days that the county says it's fine, things can smell bad and you come out of the water with an irritation. And on days they close the beach, the water smells clean," he said. "It's exciting to see how contaminants move so we can get to where we don't close the beach so often if the surf-line isn't contaminated."
Grimes and his crew released pink dye at three locations between September 23 and October 15 of last year: at a spot 300 meters south of the I.B. Pier, and then with the last release from inside the Tijuana Estuary river mouth. They had planned to make a fourth dye-drop in Mexico with permission from Mexican authorities but got stalled at a border-crossing backup and gave up, he said.
The other dye releases, which made for spectacular aerial photos, started out intensely pink but dissipated in about 48 hours. Dye went as far as halfway up the Silver Strand, Grimes said.
"It takes ten hours for the dye to get from the river mouth to the shores of Imperial Beach," he said. "For us, the rate at which it is spreading out is important."
On one dye release, the scientists watched the dye move north for ten hours — hugging the shore all the time; and then with wind and tide and swells changing, move back south and into Mexican waters, still hugging the shores. The factors affecting the dye movement — wind, tide, swells and air and water temperatures — are "incredibly complex," Grimes said.
The dye-releases eventually move across the surface, where the water is warmer, out to sea. But they stay close to shore longer than the Scripps team had anticipated, Grimes said. Unfortunately, there are no plans to continue the surf-line dye testing. Giddings said that the study, funded by the National Science Foundation, is aimed at learning the physics, not at studying the path of contaminants coming into the ocean from the Tijuana River and Mexican shores.
"Once we start talking about doing a local project, the funding sources shrink," she said. But, funding aside, I.B. city councilman Ed Spriggs said the project is important because it suggests that the cleanliness of the water could be measured more accurately. "This sort of data could actually limit beach closures to when it's appropriate," he said.