“In the West, water flows uphill towards money,” Marc Reisner writes in “Cadillac Desert.” His observation rings even truer today. Just south of Tijuana, for example, plans are underway to build a $600 million ocean desalination plant that will increase Tijuana’s water supply by a whopping 50%. While Tijuana arguably needs more water to feed its growing population and to counter cuts from the Colorado River, the project raises an important question: Will that additional supply of drinking water result in more sewage coming across the border?
Mexico is under a treaty obligation to keep all “treated and untreated wastewater” out of the United States. Most interpret that agreement to mean that no flows should enter the U.S. via the Tijuana River that have not been generated by rainfall. To meet this obligation, Mexico operates a large river diversion structure and pumping plant just south of the border. During the dry season, the river is supposed to be pumped out of the channel in its entirety and sent to the coast, 6 miles south of the border. Unfortunately, failing infrastructure over the last decade has resulted in increasingly frequent violations. Using rainfall and river flow data, I calculated that in 2015, the U.S. saw only two days of flows not caused by rainfall. In 2025, that number jumped to 149. Volume-wise, in 2015, 27 million gallons of contaminated “dry-weather” flows crossed into the U.S. By 2025, this number had increased to 1.8 billion gallons.
In 2015, while touring a reverse osmosis membrane-manufacturing facility in Oceanside, I asked our guide why seawater desalination was so popular, given that wastewater can be converted into the same ultra-pure water using only 25% of the energy. The response: “Of course wastewater reuse makes more sense, but it’s much easier to convince the public to drink treated seawater than treated sewage.”
Many don’t realize they already drink treated wastewater. Colorado River users downstream of Las Vegas, for example, consume such water (not everything that happens in Vegas stays in Vegas). And Las Vegas’s treatment isn’t as protective as reverse osmosis. Nor do people realize how expensive desalinated ocean water is. Water from Poseidon’s Carlsbad plant costs San Diegans three times the water pumped from Northern California or the Colorado River.
Spending millions to produce desalinated water only to later discharge it back into the ocean after a single use seems crazy. Crazier still: Tijuana’s main wastewater treatment plant discharges into the ocean only a few miles north of where the proposed Rosarito desalination plant will pull its seawater from. Scripps Institution of Oceanography’s “Pathogen Forecast Model” shows the wastewater can travel long distances without much dilution, meaning this wastewater will often contribute to the desal facility’s source water. Why not, at a fraction of the cost, cut out the middleman (a.k.a., the ocean) and apply reverse osmosis directly to the treated wastewater?
The International Boundary and Water Commission’s recent binational agreement, Minute 333, recognizes the desalination plant’s potential impact but only requires Mexico to “share information.” Mexico will likely argue that the additional water will only serve as a substitute for Colorado River water — a dubious claim that doesn’t answer a more fundamental question: How can it afford to spend over half a billion dollars on desalination but can’t find the remaining $90 million to fix the critical wastewater infrastructure projects they committed to four years ago in Minute 328?
Both the U.S. and Mexico should follow San Diego’s “Pure Water” example and reuse all Tijuana’s wastewater before they look to more energy-intensive and environmentally destructive sources such as ocean desalination. Doing so will help solve both the problem of too little water in the Colorado River and too much in the Tijuana River. By creating an economic incentive for Mexico to capture and treat its sewage, reuse is the only sustainable long-term solution to this crisis.
If Reisner’s statement still holds true, however, the mega-desalination project will be built, leaving Mexico with no appetite for reuse and San Diegans with just more salt in the wound.
Liden is a retired engineer from the U.S. Environmental Protection Agency who spent the last two decades working on Tijuana River issues from EPA’s San Diego Border Office, the U.S. Consulate in Tijuana and the U.S. Embassy in Mexico City. He lives in San Diego. His views and opinions do not necessarily reflect the official policy or position of the U.S. government.