Whoa, Mexico, that water clean?

Four South Bay cities have signed off on a letter that calls on the North American Development Bank to pay attention to the sewage-tainted seawater that may go into the Rosarito desalination plant the bank is funding.

San Diego city councilman David Alvarez, who represents south San Diego, including Nestor and San Ysidro, also signed the letter.

The letter focuses on both the flows in the Tijuana River and on the "treated" wastewater pumped directly into the ocean from Tijuana's San Antonio de Los Buenos plant (commonly called Punta Banderas) to a pipe that spits it out at Las Playas.

"The discharge of partially treated wastewater on the beach at Punta Banderas is now recognized as a major cause of contaminated water in Imperial Beach and Coronado as identified by a Scripps Institute of Oceanography research project. The study indicates that pollutant transport times are only 6-14 hours for sewage plumes to reach Imperial Beach and the south end of Coronado during prolonged southerly swells and winds," the letter says.

A recent visitor to the plant, which treats sewage to the primary level — not to U.S. secondary-level standards — observed that of the 150 aerators at the plant, only three were working.

"We are getting killed by the 15 million gallons of treated wastewater they dump every day," Imperial Beach mayor Serge Dedina said at a city-council meeting last night (January 18).

The letter counts annual beach-closure days because of fecal bacteria in the water at 233 for Border Field, 74 at the I.B. pier, and 41 days in parts of Coronado, where the Silver Strand is worst hit. Many of those beach closures happen when the Tijuana River isn't flowing, and I.B. lifeguards have long blamed closures on the Punta Banderas plant.

The letter comes months after Dedina called for the resignation of International Boundary and Water Commission chief Edward Drusina over the continued flows of untreated stormwater down the Tijuana River from Mexico — the only time the river runs.

The city, which decided to focus on ecotourism as part of its growth strategy, has been shackled by poor water quality. One of the key ecotourist draws to the region, the parks and open space of the neighboring Tijuana River Valley, have been bedeviled by the river, other downhill flows from Tijuana, and the incredible amount of sediment, trash, and bacteria that pour in with the sewage-tainted storm water.

With Mexico's decision to build a desalination plant at Rosarito — and $1 billion in funding from the North American Development Bank — the South Bay cities saw an opportunity to apply pressure to the agencies they say haven't kept Punta Banderas in good order.

"I'm confident because of the need for the desalinated water in Baja, there's more political will in Mexico than ever before," Dedina said.

The desalination plant wants to peddle the water to South Bay cities, according to bank documents.

"We made the argument that if you're going to sell desalinated water from Baja with 50 million gallons a day of sewage going into the ocean next to the plant, you better make sure that water is cleaned up first," he said. "We made it very clear we will do everything we can to make sure that water isn't sold in the U.S. [if there may be sewage in it]."

Calls to the North American Development Bank and Alvarez's office for comment were not returned.

The desalination plant is being built in two phases. The first phase is meant to provide 50 million gallons of desalinated drinking water — equal to the Carlsbad Poseidon plant output — to Baja by 2019. The second phase should be completed by 2026. It will add another 50 million gallons a day to the plant's production. Between 10 and 30 million gallons of that will be diverted to the Otay Water District, which serves South Bay cities.

Construction on the plant began in November 2014 and phase one is about 70 percent complete, according to the bank website.