What actually reaches the ocean

Sewage reaches the ocean near Imperial Beach from three places: the mouth of the Tijuana River, the shoreline at Punta Bandera in Mexico, and the small canyons along the border. Which one matters most depends on the season.

The short version

MGD means million gallons per day. These are approximate averages from public reports; real flows vary a lot from day to day and storm to storm.

In dry months: Punta Bandera

When it isn't raining, the Tijuana River is mostly diverted before it crosses the border, as long as the PB-CILA pump station in Mexico is working. The bigger source is the San Antonio de los Buenos treatment plant at Punta Bandera, about five miles south of the border. It receives roughly twice the sewage it can treat, and the rest is discharged onto the shoreline. Ocean currents carry that plume north.

Modeling by the Scripps Institution of Oceanography, summarized by the EPA, found that Punta Bandera is the main source of the pollution behind beach closures in dry months. At about 18 million gallons a day, that adds up to something on the order of 6 to 7 billion gallons of untreated sewage a year. That annual figure is our own estimate from the published daily numbers, not an official one.

In storms: the river

When it rains, the river rises past what PB-CILA can handle, and the pumps shut off to protect themselves from sand and trash. EPA data show storm flows crossing the border average about 109 million gallons a day, all of it untreated. It runs down the river, through the estuary, and into the ocean at the south end of Imperial Beach.

In the rainy season, the Scripps modeling attributes about two-thirds of closure-causing pollution to the river and about one-third to Punta Bandera. Since 2018, more than 100 billion gallons of sewage-laden water have crossed the border in the river, according to the U.S. International Boundary and Water Commission.

The border canyons

Collectors in Goat Canyon, Smuggler's Gulch, and three smaller canyons catch dry-weather flows and pipe them to the South Bay treatment plant. When a pump station in Tijuana fails, or a storm overwhelms them, sewage and trash flow through the valley to the estuary. How much reaches the ocean this way is poorly documented.

What's in it

It isn't only human waste. Researchers have found viruses, antibiotic-resistant bacteria, and hundreds of chemicals in the water, including pesticides, solvents, and flame retardants. Much of this comes from industrial discharges, and there is no known industrial pretreatment program in Tijuana's sewer system. The river also carries tons of trash and sediment: the trash boom just north of the border has stopped about 2,000 tons of trash and debris over two storm seasons.

From the water into the air

What reaches the ocean doesn't stay there. Research led by Scripps and San Diego State University has shown that bacteria and chemicals from sewage-polluted water can become airborne in sea spray and travel inland. Along the river, churning sewage releases hydrogen sulfide gas. Health covers what that means for people nearby.

What would change this

Fixing dry-season closures depends mostly on Punta Bandera, where an offshore outfall and plant expansion are being studied but aren't funded. Fixing rainy-season closures depends on handling storm flows, which no current facility is built for. The project tracker shows where each project stands.