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One day in March 2017, Mitch McKay and his wife, Suzanne, took a walk on the sand near Imperial Beach, a small surf town south of San Diego where they’d raised their children. Suzanne liked to collect sea glass, and they often brought a spare grocery bag to pick up any trash they found amid the seaweed and driftwood. “It was our ritual,” Mitch said. Back home, Suzanne started to suffer from splitting headaches that seemed to emanate from the back of her neck, near the base of her skull. The headaches soon got bad enough that she went to the emergency room, where doctors performed a spinal tap. She had, they determined, spinal meningitis.Suzanne spent 12 days in the hospital, taking antibiotics and slowly regaining strength as doctors tried to deduce how she’d gotten sick. The U.S. Centers for Disease Control and Prevention sent a representative from Los Angeles to review her case. Ultimately, only one coherent explanation materialized: Bacteria living in fecal matter in seawater had entered her body through a small open blister on her foot. “That was my first slap in the face in terms of what’s going on down here,” Mitch recalled. “People can die from this.”The McKays’ fateful walk came at the end of a wet winter. That January, just over the U.S.-Mexico border, workers from Tijuana’s water utility had been called to an industrial stretch of the city, where a rapidly growing sinkhole claimed a bus shelter, then the sidewalk beside it, and soon threatened traffic along a major thoroughfare. The cause, foretold by the smell, was a break in an even more important artery: a sewage pipeline five feet in diameter carrying the feces and dishwater of hundreds of thousands of people.Residents of imperial beach smelled the change within days, as a plume of turgid, foamy sewage pushed out to sea. For many, the spill was a signal event, dividing life into “before” and “after.”This flow ordinarily made its way across the border and into California underground, to a treatment plant owned by the U.S. arm of the International Boundary and Water Commission, or IBWC, an agency that administers bilateral agreements for watersheds shared by both countries. After the pipeline broke, workers used an inflatable plug to stop the sewage and redirect it. But when repairs got underway, the pumps weren’t capable of sending the backed-up sewage to the plant by another route. Instead, the waste began to empty into the Tijuana River, which heads north through a concrete flood channel and crosses into California six miles from the Pacific Ocean. Residents of Imperial Beach smelled the change within days, as a plume of turgid, foamy sewage pushed out to sea. By the time the spill stopped, at the end of February, up to 256 million gallons had flowed through a protected estuary and out to the ocean, leaving a dark residue in the sand that technical reports refer to vaguely as “organic material.”For the McKays and many of their neighbors in Imperial Beach, including Serge Dedina, the mayor at the time, the spill was a signal event, dividing life in the town into “before” and “after.” For more than a week, Dedina tried to reach federal officials in the United States and Mexico to learn what was going on. Nobody answered his calls. “Like, literally, there was no response,” he said.Sewage overflow and beach closures are a long-standing problem on this part of the border—U.S. officials barred the sale of vegetables grown in the Tijuana River Valley as far back as the 1930s, fearing sewage contamination in the water there—but the 2017 spill heralded an era of cascading failures. Repairs to one section of the pipeline revealed more damage elsewhere. Pumps and valves failed. More pipes broke. Tijuana’s largest sewage treatment plant, five miles south of the border, was eventually degraded beyond repair, and soon began sending 40 million gallons a day of essentially untreated sewage straight out to sea. From there, it was carried north on summer swells to Imperial Beach and Coronado, one of the wealthiest communities in California—perhaps best-known for the iconic Hotel del Coronado, made famous by Marilyn Monroe in the film Some Like It Hot.Though a few bold surfers disregard the warnings, sections of Imperial Beach have been closed to swimmers for more than 900 consecutive days. Bars and restaurants have seen business dry up; lifeguards keep leaving for towns where they don’t have to tell people to get out of the water. A recent City Council meeting featured a debate on whether to cancel a popular summer sand-castle competition. Citing sales data on comparable housing elsewhere in coastal San Diego County, Norm Miller, an emeritus professor of real estate at the University of San Diego, estimated that homes in Imperial Beach are discounted by as much as 50 percent.For Dedina, the whole thing can feel like an exercise in futility. A lifelong surfer and geographer by training, Dedina runs the nonprofit Wildcoast, which works on coastal conservation on both sides of the border. He ran for mayor to put in sidewalks and pave alleyways, only to find himself suing the federal government under the Clean Water Act and “leading an international coalition to fix a sewer system,” he said. Why, he wondered, did it fall to “a small city with no money” to press for change?By some measures, Dedina’s lawsuit was a success, providing leverage that helped members of Congress secure $300 million in federal funding to address sewage pollution as part of the United States-Mexico-Canada Agreement, the successor to the North American Free Trade Agreement. It also helped spur a review by the Environmental Protection Agency, outlining a range of projects on both sides of the border needed to provide a more durable solution. Unfortunately, no one thinks $300 million, or even the additional $156 million secured earlier this year, will come close to resolving the issue.Earth is home to hundreds of border-spanning watersheds, and versions of this struggle exist all over the planet: where the Ganges carries untreated effluent and industrial runoff from India into the lowland farms and coastal swamps of Bangladesh; along the Zambezi, the Mekong, the Danube. One merciful quality of the Tijuana River is that it’s not longer, limiting the scope of the conflict to two metro areas in two countries, as opposed to, say, the 11 nations whose disputes span the 4,000-mile course of the Nile.At the heart of the sewage crisis in Tijuana is the question of who bears responsibility for keeping up with the city’s growth. As Carlos de la Parra, an environmental planner from the city, put it, “We have no business being this large, except for the fact that we border California, and that Mexico and the U.S. signed the North American Free Trade Agreement 30 years ago.”Parts of NAFTA anticipated this dynamic; the agreement included a provision to set aside $100 million a year for environmental infrastructure along the border. As time went on, though, Congress lost its appetite for funding public health upgrades in Mexican cities. How about building a wall and making Mexico pay for it?That’s the kind of solution that appeals to the American political psyche, but it suffers from a basic misunderstanding. You can draw the border as a line on a map, but you still have to deal with the world on the other side. A sewage crisis in Mexico can’t be solved with pipes in California any more than a migration crisis that spans the hemisphere can be solved with a wall across Texas and Arizona.Extreme cases like Suzanne McKay’s haunt Imperial Beach: the surfer with the lung abscess, the Border Patrol agent with a flesh-eating bacterial infection. But no one quite knows how many people get sick from the water south of San Diego.Kimberly Dickson and her husband, Matt, both doctors, moved to town in 2011 to open an urgent care clinic. Over the years, they estimated that ailments associated with sewage made up 10 to 15 percent of their business, the way a clinic in a popular hiking area might see more than its share of scrapes and sprains. But they hadn’t tracked patterns in the data until August 2023, when Tropical Storm Hilary lashed the Pacific Coast with torrential rains from the tip of the Baja Peninsula as far north as Los Angeles.Suddenly, instead of seeing five or six cases of diarrhea in a week, they counted 34. “We just started noticing, ‘Gosh, we have just a full clinic in the middle of summer with people with vomiting and diarrhea and abdominal cramping.’ And the thing is, none of these people were going in the water,” Matt recalled. “That was the really startling thing for us … where are they getting this?”One answer lay at the south end of town, where sewage overflowed along Hollister Street, leaving a layer of foul-smelling mud to dry into dust on the roadway. “Kids go to school on that sidewalk,” Matt explained. “They walk into class, maybe they touch their feet, and then they eat lunch. Now they’re sick. Or, you know, you drive through it and you drive into your garage. Well, now you’ve tracked sewage into your garage.”It wasn’t only diarrhea. People complained of skin infections, sinusitis, sore throats, headaches, asthma flare-ups, and general cloudiness, all of which the Dicksons associated with what Kimberly called the “whiff test”—as in, “If you open your window and you smell, there’s your whiff test.” Many ailments seemed to track the San Diego County Air Pollution Control District’s measurements of hydrogen sulfide, the rotten-egg smell released when organic material breaks down with insufficient oxygen—say, during a sewage spill. The Dicksons are now collaborating with the epidemiology unit of the county health department to evaluate both routes of infection: water and air.In 2021, Falk Feddersen, an oceanographer at the Scripps Institution of Oceanography, led a study funded by the EPA to model the path of sewage plumes along the coast in order to evaluate which infrastructure upgrades would deliver the most benefit. Nestled among the findings was a startling estimate: Based on the prevalence of norovirus in the waters off Imperial Beach, nearly one out of every 25 swimmers could be getting sick—potentially thousands of people a year. Heather Buonomo, who leads the unit responsible for water testing at the County Department of Environmental Health and Quality, declined to comment directly on that projection, because, she said, the county was not involved in the research. But she suggested that the system of closures and health advisories triggered by evidence of sewage spills has been an effective deterrent: “People aren’t going in the water,” she said. “So the work that we’re doing to protect public health is working.”The Dicksons aren’t so sure. “We’re only seeing the tip of the iceberg,” Kimberly said. “There’s probably more out there, and it’s flying under the radar because it’s not reportable.” The real worry, Matt said, comes if Tijuana experiences a more virulent disease that sheds into sewage flows that cross the border: cholera, for instance, or shigella, a leading cause of diarrhea globally. (Though both pathogens are rare in Mexico, they are often spread through untreated water and can cause fatal illness.) “That’s where we’re gonna have a big problem,” he said. “It all depends on what’s coming across in that water.”One afternoon in April, I accompanied Rosario Norzagaray, who works with Dedina at Wildcoast, to visit Los Laureles, a neighborhood where a small share of the sewage that makes its way to the Tijuana River—and, ultimately, Imperial Beach—begins its journey. Norzagaray took a circuitous route through steep ravines along Tijuana’s western flank, bringing us to a ramshackle neighborhood in the headlands of a canyon whose waters drain into the United States above the river’s floodplain. She waved her arm at the sweep of pastel-colored homes clinging to the eroded slopes above us. “All this is invasion,” she said, explaining that the so-called colonias were settled by people who built homes without title to the land. Then again, she added, chuckling, “half of Tijuana is an invasion.”Tijuana’s population has followed a path of near exponential growth over the last century, ballooning from a community of 21,000 in 1940 to nearly two million at last count. The city’s first sewer infrastructure, a septic tank for 500 people, was built in 1928; within a decade, it served 10 times that number. U.S. officials made their first effort to stop untreated sewage fouling the coast in the 1930s, with an underwater pipe, or “outfall,” that discharged around 140 feet offshore. By then, Tijuana had built another, larger tank, to serve 5,000, but it was quickly oversubscribed to the point of obsolescence. It wasn’t until 1983 that another tank was built; at that time, Tijuana’s population was passing half a million, with dozens of maquiladoras, or foreign-owned manufacturing plants, attracting new transplants each year. By the late 1980s, Tijuana had become the world capital of television manufacturing, producing 30 million TVs a year. The passage of NAFTA in 1994 only accelerated the city’s growth.“Tijuana is a stop, not a destination,” Norzagaray said. “People don’t come to Tijuana thinking they’ll stay; they come thinking they’ll go find their American dream, but when they can’t get there, they come back here.”Though some houses in Los Laureles were built half a century ago, the neighborhood today reminded me of a packed open-air concert, where each group of new arrivals crowding onto the grass forces those who came earlier to rearrange their picnic blankets. Unpermitted homes, built and expanded in stages, jam the hillsides. We got out of the truck at the entrance to a concrete flood control structure. A trash boom, installed in 2021, stretched across the ravine like an oversize necklace made of corrugated plastic piping and steel. When it rains, the boom floats up with the floodwaters, skimming off piles of plastic bags, milk bottles, and Styrofoam as the runoff continues downstream. This is the linchpin of Wildcoast’s work in the area, a community recycling program that has removed more than 100,000 pounds of plastic waste in the last four years. But plastic is only the most tractable part of the equation. The other major components of runoff—sediment and sewage—require more than a trash boom.All around us were signs of development that had outpaced the infrastructure to support it. A garden hose snaked along 50 yards of concrete wall, splitting a single paid water connection among several houses. Raw sewage trickled into the street from exposed, broken drainage pipes that zigzagged down from homes high above us. Narrow stairways and retaining walls made of used tires ran up the slopes. “They’re trying to control all this with tires; but the water takes it,” Norzagaray explained about the eroding hillside. “And this situation is replicated in every canyon in Tijuana. Wherever there’s not supposed to be construction and there is—there are problems with sewage.” She pointed out an empty expanse on the slope above us where 20 homes had stood until 2015, when they were damaged during a landslide brought on by heavy rains.Over the years, the government has extended services to Tijuana’s colonias piecemeal. But many residents have no choice but to make do with latrines and DIY septic tanks: Though they’d be willing to pay the connection fee, they aren’t eligible for a new sewer line without title to the land. Maria del Pilar Márquez Gómez and her husband, Manuel López Paz, live in a modest white and blue cement house on a shared lot that backs up against the flood control channel. Each moved to the city during the boom years of the 1990s. Pilar Márquez Gómez came to Tijuana on a lark, and stayed when she found steady work cleaning beachfront apartments owned by wealthy Mexicans and American expats. A mason, López Paz recalled construction foremen driving trucks around the city and calling out for workers from their windows. His brother was the first to settle in Los Laureles, and he gradually brought in new families to share a 5,000-square-meter lot. “When I got here, all this didn’t exist,” López Paz said, looking up at the homes around us. “It was only farms.”Though none of the occupants had formal title, the arrangement came to feel settled, even sanctioned by the city. When the government announced plans to channelize the arroyo out back, the family lost most of their backyard, but the authorities didn’t touch the houses. Not long afterward, they returned from an extended trip to visit family in Guanajuato to see that their street was being paved, and their neighbors’ homes had been connected to water and sewer service as construction proceeded up the canyon. It turned out to be something like a onetime amnesty. Thirteen years later, they are still trying to get the right paperwork through City Hall to acquire title, and still using their septic tank. “We missed our chance,” Pilar Márquez Gómez said.In this instance, however, it’s not clear whether a sewer connection would make much difference. A mile uphill, the pavement stopped and the concrete channel gave way to an overgrown ditch lined with trash, a canyon in miniature etched by a small stream that ran downhill. Two children in pigtails skipped home across a makeshift bridge made of shipping pallets. Nearby, the stream’s “headwaters” spouted from a manhole cover atop a sewer main, where a persistent blockage sent raw sewage bubbling over in a man-made waterfall, destined for the beaches of California.Even as Dedina saw his view of the sewage crisis vindicated through his lawsuit—the federal judge in the case went so far as to visit the Tijuana River estuary to smell the stench in person—the underlying conditions were growing more dire. In late 2019, another major pipeline broke in Tijuana; nine million gallons of sewage crossed into California in two days. As the pandemic set in and the border was closed to nonessential travel, the combination of wet weather and failing infrastructure, Dedina said, seemed to create a new baseline. “It’s just polluted every day. The river’s going to flow, and there doesn’t have to be a response or timeline to fix it.”In the summer of 2020, Dedina sparked a minor diplomatic spat by saying, in an interview with a Mexican television station, “Tijuana’s sewage is killing us.” Jaime Bonilla, then governor of Baja, shot back, blaming the problem on foreign investors: “The vast amount of this contamination comes from American companies operating in Tijuana discharging their waste into the river; that’s where he needs to focus his attention.” As Dedina sees it, the pandemic broke down key bilateral relationships at all levels of government just as U.S. ties with Mexico were strained by other concerns. “All of a sudden, things just fell apart,” Dedina told me. “Fentanyl, migrants, the whole crisis in the relationship between the U.S. and Mexico—that’s taking all of Ken Salazar’s time,” he said, referring to the U.S. ambassador to Mexico. “This is not something the U.S. is willing to push on.”The modern Tijuana River is a hybrid, part natural waterway and part man-made infrastructure, whose flow is what ecologists call “urban drool.” What was once an intermittent, seasonal stream has been replaced, since the 1970s, by a steady flow of used tap water imported across 90 miles of open desert. The concrete flood channel that sheaths the river all the way through Tijuana ends just past the border. On the southern bank, the rusty bollards of the border fence climb a steep hillside at the city’s edge, flanked by shops and apartments all the way to the sea. On the north side, the enclosed tunnel of the pedestrian crossing follows the river right up to California, then turns 90 degrees and descends to U.S. Customs and Immigration Enforcement, a filter separating the people heading north from everything else.As the water slows down and spreads out across the floodplain, solids drop out of the current. Plovers and godwits peck at the mud among empty water jugs and motor oil containers, toys and soccer balls, sneakers, couch cushions, spaghetti knots of hardened caulking. Coke bottles urge “Recíclame.”Standing on an embankment 200 yards downstream, Chris Helmer, the director of environmental and natural resources for Imperial Beach, gazed out at a profusion of wild mustard and garland daisies sprouting from deposits that accumulate in the riverbed like layers of rock each spring.A few weeks earlier, Helmer explained, and the view would not have been obstructed by so much vegetation. “It’s highly nutrient rich water: What do you think is going to grow in here?” he said. Where water or bulldozers had cut into the banks, the structure that remained looked like a tall layer cake. “It’s almost like tree rings. Every single season you can see a new layer of sediment and trash, sediment, trash.”Clearing the debris is a Sisyphean undertaking, with each season’s work reset by the next rains, and in recent years the U.S. government hasn’t come close to keeping up. There are now something like 100,000 truckloads of material that will need to be moved to prevent flooding in the adjoining neighborhoods in San Ysidro and Tijuana.But Customs and Border Protection is also in the process of making the work much harder. Just upstream, construction had begun on a project announced abruptly in 2020: a bridge for Border Patrol agents to cross the flood channel as it enters the United States, combined with a fence, built along the upstream side, consisting of dozens of moving panels, or liftgates, that will have to be raised during heavy rains to allow the river’s flow to continue downstream. As a border security measure, the project is exempt from federal environmental review, but other agencies met the CBP’s proposal with pointed skepticism. California’s Environmental Protection Agency, CalEPA, warned that fortifying this area might simply create security issues near some of the other places where tributaries crossed the border, like Los Laureles. But the larger worry is that the liftgates will fail, or that CBP may not respond in time to raise them before a rainstorm, or debris will accumulate in back-to-back storms, and the fence, which is supposed to let water through, will act as a dam instead, leading to catastrophic flooding in a densely populated part of Tijuana. (CBP did not respond to interview requests for this story.)“All the debris and trash is going to back up in Mexico, so you rely on Mexico to maintain and clean this,” Helmer said—maintenance and cleaning the United States already fails to do. He called the project “utterly insane.”It’s expected to be completed by the end of the year.Residents of Imperial Beach sometimes seemed at a loss about where political pressure can be usefully applied. “When you’re in Washington, what is the federal government even saying about this happening to us?” one man asked at a recent city workshop on the sewage problem. After 16 years of going to public meetings about sewage, he wanted to know if there was a time frame for a solution. Other residents have compared spills to a “dirty bomb” dropped on the city. There were suggestions that the United States close the border to all travel north during sewage flows or cut off Tijuana’s access to Colorado River water.If residents don’t know how to pressure the feds, the feds often don’t seem to know how to pressure Mexico. The commissioner of the U.S. arm of the International Boundary and Water Commission, Maria-Elena Giner, has been frank about the challenges of treating sewage that originates in another country. “We cannot fine them for not treating their wastewater; we can’t fine them for discharging water,” Giner told me. What the IBWC can do is conduct meetings, collect evidence, write stern letters, appeal for more funding from Congress. The sewage that reaches the IBWC plant would be anomalous anywhere else in California. “You get rags, you get a lot of sediment, and it tears up the pumps, it tears up the concrete,” Giner said.Rags and sediment do a number on the sewer infrastructure in Tijuana, too. But the root of the problem isn’t technical so much as financial: The Comisión Estatal de Servicios Públicos de Tijuana, or CESPT, which provides water and sewer services to the city, gets most of its revenue from ratepayers, but loses money on nearly 80 percent of the water that flows through its pipes.In the spring, I met with CESPT’s director, Jesús García Castro, and deputies responsible for finance and operations, around a coffee table spread with chocolate-covered nuts and cut fruit. García Castro had been on the job only a few months, but it turned out to be an auspicious moment to take over: A few weeks after he began, Mexican President Andrés Manuel López Obrador, or AMLO, as he’s known, announced that the city’s largest treatment plant, which hasn’t operated effectively in five years, would be rebuilt by a unit of the Secretaría de Defensa Nacional, or SEDENA, comparable to the Army Corps of Engineers. Salazar, the U.S. ambassador to Mexico, traveled to Tijuana to attend the groundbreaking: Finally, it seemed, sewage had risen up the list of issues competing for political oxygen in the U.S.-Mexico relationship. The new treatment plant “will reduce the flows of untreated sewage to the Pacific Ocean by 90 percent,” García Castro said confidently. “So that’s a big part of the solution.”Under AMLO, the military has become something like Mexico’s contractor of first resort, with the ability to bypass environmental review and typical procurement processes. Keeping with SEDENA’s style, the construction schedule announced at the groundbreaking was ambitious, condensing what would ordinarily be a multiyear project into nine months. U.S. officials received the news with wary optimism. “SEDENA works fast,” one federal official told me in a text message. “Not necessarily a good thing as they tend to build BEFORE design.” Still, the Mexican government had ultimately scuttled a series of earlier initiatives to rebuild the plant with private funding, as far back as 2011, and there was a sense that if it wasn’t built by SEDENA, it might not be built at all.Before the existing plant at San Antonio de los Buenos went offline, it limped along well past its useful life, hobbled by a lack of maintenance. Sludge accumulated in treatment lagoons that were rarely dredged. Eventually, they stopped functioning altogether. A 2019 review by an independent consultant found that there was no backup power system and “no preventive maintenance program,” noting that CESPT typically received just a third of the operating budget it asked for.After years of delay, García Castro was adamant that the utility was making up for lost time. “We’ll have results this year,” he said. “Next year, already, we’ll be able to have clean beaches.” The reality is that most directors don’t stick around at CESPT long enough to see such promises through. When I asked how many people had held García Castro’s job before he got there, his deputies, both longtime employees, began counting on their fingers, seeming to flip through a mental catalog of past bosses like baseball fans trying to name bench players on favorite childhood teams. Eventually, they came up with a figure. “Thirteen in 10 years,” García Castro said.The IBWC’s South Bay International Wastewater Treatment Plant may be the easiest place on the entire border to cross legally between the United States and Mexico. Here, flatbed trucks bearing CESPT’s blue logo make daily return trips to a loading bay beneath an elevated conveyor belt carrying trash extracted from Tijuana sewage.In the spring, Morgan Rogers, who oversees the plant’s operations, watched through wraparound shades as a stream of refuse fell from a chute in the ceiling into a waiting dumpster. “Mexico hauls that off when it gets full,” he explained. “They own the trash, they own the sludge, they own the sediment”—he paused—“and they actually own the water. But we throw the water off the coast because they can’t do anything with it.”“But they can do stuff with the trash and sludge and sediment?” I asked.“Well, we make them take that.”Rogers nodded in the direction of the border wall, just on the other side of the building, its tall, rusted slats climbing to the horizon in either direction. “We have a gate out back here,” he said. The IBWC plant was built in the years after NAFTA was passed, partly out of a conviction that the Mexican government couldn’t be relied on to treat sewage to standards that would keep California beaches swimmable. It’s meant to handle about a third of Tijuana’s wastewater, along with dry weather flows from the Tijuana River and runoff from neighborhoods like Los Laureles. But as Tijuana has continued to grow, the plant has been broken down by the combined effects of storm events and infrastructure failures.In July 2022, Rogers explained, two critical pipelines, serving a pump station that allowed CESPT to divert sewage flows several miles south of the border, failed one after the other. That December, a series of atmospheric rivers also worsened damage to a valve controlling how much sewage the IBWC plant lets in. As a result, Rogers said, “Whatever flows come from Mexico, we take.”The consequences have been disastrous. During rainstorms, as much as 80 million gallons a day poured into a plant designed to handle 25 million, carrying trash and sediment that clogged critical equipment. By the end of 2023, all five of the plant’s primary treatment tanks were inoperable, filled to the brim with sludge.Tropical Storm Hilary made things even worse, destroying all but one of the pumps that moved sewage into the treatment tanks. Rogers leaned over a concrete wall where backed-up trash formed a dam during the storm, and he peered down at the pumps below. “There was eight feet of water down there,” he said. “We were on the edge: If you lost that pump, we’d have been out of business.”As it was, the plant was still recovering, with new pumps waiting on the grass to be installed, and waist-high weeds sprouting from treatment tanks, which were still being emptied and overhauled one at a time. Rogers credited Tropical Storm Hilary with spurring a new sense of urgency somewhere above his pay grade. “Hilary, really as much damage [as] it did, it kind of woke us up”—he paused, looking at his counterpart from IBWC’s political side, Sally Spener, following along in a pink button-down and maroon cowboy boots. “Not us, but it woke—Who would you say it woke up?” Spener didn’t answer.Throughout the tour, Spener countered Rogers’s blunt assessments of the plant’s condition with steadfast diplomacy: the broken valve (“But the contract has been awarded to fix it, right?”); the failing pump stations (“That’s all part of that rehab that’s envisioned.”); the oversize pipeline that sent the plant’s treated water three miles off the coast (“the award-winning ocean outfall!”). The dynamic captured the unwieldiness of the agency’s mission: to operate a utility whose “customers” live in another country, and to manage a spiraling set of technical problems enmeshed in a much larger diplomatic relationship.In June, Mexico elected its first woman president, Claudia Sheinbaum, a former professor of engineering from AMLO’s Morena party. The alignment between the levers of power in Baja state and Mexico City increases the chances that the leadership team at CESPT will keep their jobs long enough to make a dent in the priorities they’ve outlined.But the shift that may ultimately force the city into a different relationship with wastewater is climate change. As in urban areas across the U.S. Southwest, Tijuana’s water supply is dominated by the dwindling Colorado River. In 2023, CESPT was forced to shut off supply to nearly half the city and get emergency allocation from a cross-border connection with California to avoid prolonged water outages. Water, then, is the limiting factor on Tijuana’s growth: CESPT is now pursuing a long-postponed plan to reuse a portion of Tijuana’s treated water for agriculture, a signal that sewage is finally seen as a commodity worth capturing. “Mexico owns the water rights to this,” Chris Helmer told me as we watched Tijuana’s stream of urban drool meander past us into the estuary. “It’s written in the treaties. At some point, Mexico is going to want to use this water.”Straddling one of the busiest land crossings in the world, the Tijuana River reminds us that both sides of the border constitute a single place. Once the poop is in the water, no amount of barbed wire can get it out.When plans for a U.S. plant to treat Mexican sewage were first proposed, in the 1970s, they called for a facility big enough to handle 100 percent of Tijuana’s wastewater. Gradually, the plant-to-be was whittled down to a quarter of that size and simplified so that it could be built more cheaply, with the idea that upgrades would be made over time. David Gibson, an executive officer of the San Diego Regional Water Quality Control Board, said the IBWC plant was already outdated by the time it was completed, in 1997. “Design decisions that were made in the 1990s for that treatment plant, we’re paying for even now,” Gibson said. Without ratepayers to cover maintenance costs, the plant has also fallen victim to the Darwinian logic of the federal budget, receiving just $4 million for maintenance, cumulatively, from 2010 to 2020, a period when billions of additional dollars were allocated to border security. “This is like buying a nice Corolla in 1997 or a nice Ford, but you never change the tires, you never change the oil,” Gibson said. The plan now is to make overdue repairs and double its capacity. But the current funding, Gibson said, is “barely half” what’s needed “for the economy model.” He worries the region is on track to reprise nearly 100 years of sewage history, “outgrowing the infrastructure only a decade or so after it’s installed.”Still, Gibson echoed the point of view I heard from nearly every American official I spoke to—that the only reliable solution to Tijuana’s sewage problem is building the infrastructure on the U.S. side. In this, sewage treatment for Tijuana seems destined to operate as something like an extension of the border wall, a constant, churning intervention made at the river’s mouth, rather than its source, whatever the price. “I don’t think Mexico in general has sufficient resources to attend to their problems,” Giner, the IBWC commissioner, told me. “How are we going to ensure this moves forward with sufficient resources after all of this is built?” she asked, referring to upgrades on the U.S. side. “Let’s say we’ve caught up. Once we catch up, we will have to answer that question.”Nearly wherever you look, border politics in the United States is animated by a persistent myth: that with enough money and willpower, you could eventually seal off the countries from one another, like apartments that share a 1,954-mile wall. One way to describe decades of militarization on the border is that it serves to make Mexico invisible to residents of the United States. The same might be said of cross-border industrial development: porous to money and airplane parts, hardened to everything else. Straddling one of the busiest land crossings in the world, the Tijuana River offers a stubborn rebuttal, a reminder that both sides of the border constitute a single place. Once the poop is in the water, no amount of barbed wire can get it out.

One day in March 2017, Mitch McKay and his wife, Suzanne, took a walk on the sand near Imperial Beach, a small surf town south of San Diego where they’d raised their children. Suzanne liked to collect sea glass, and they often brought a spare grocery bag to pick up any trash they found amid the seaweed and driftwood. “It was our ritual,” Mitch said. Back home, Suzanne started to suffer from splitting headaches that seemed to emanate from the back of her neck, near the base of her skull. The headaches soon got bad enough that she went to the emergency room, where doctors performed a spinal tap. She had, they determined, spinal meningitis.Suzanne spent 12 days in the hospital, taking antibiotics and slowly regaining strength as doctors tried to deduce how she’d gotten sick. The U.S. Centers for Disease Control and Prevention sent a representative from Los Angeles to review her case. Ultimately, only one coherent explanation materialized: Bacteria living in fecal matter in seawater had entered her body through a small open blister on her foot. “That was my first slap in the face in terms of what’s going on down here,” Mitch recalled. “People can die from this.”The McKays’ fateful walk came at the end of a wet winter. That January, just over the U.S.-Mexico border, workers from Tijuana’s water utility had been called to an industrial stretch of the city, where a rapidly growing sinkhole claimed a bus shelter, then the sidewalk beside it, and soon threatened traffic along a major thoroughfare. The cause, foretold by the smell, was a break in an even more important artery: a sewage pipeline five feet in diameter carrying the feces and dishwater of hundreds of thousands of people.Residents of imperial beach smelled the change within days, as a plume of turgid, foamy sewage pushed out to sea. For many, the spill was a signal event, dividing life into “before” and “after.”This flow ordinarily made its way across the border and into California underground, to a treatment plant owned by the U.S. arm of the International Boundary and Water Commission, or IBWC, an agency that administers bilateral agreements for watersheds shared by both countries. After the pipeline broke, workers used an inflatable plug to stop the sewage and redirect it. But when repairs got underway, the pumps weren’t capable of sending the backed-up sewage to the plant by another route. Instead, the waste began to empty into the Tijuana River, which heads north through a concrete flood channel and crosses into California six miles from the Pacific Ocean. Residents of Imperial Beach smelled the change within days, as a plume of turgid, foamy sewage pushed out to sea. By the time the spill stopped, at the end of February, up to 256 million gallons had flowed through a protected estuary and out to the ocean, leaving a dark residue in the sand that technical reports refer to vaguely as “organic material.”For the McKays and many of their neighbors in Imperial Beach, including Serge Dedina, the mayor at the time, the spill was a signal event, dividing life in the town into “before” and “after.” For more than a week, Dedina tried to reach federal officials in the United States and Mexico to learn what was going on. Nobody answered his calls. “Like, literally, there was no response,” he said.Sewage overflow and beach closures are a long-standing problem on this part of the border—U.S. officials barred the sale of vegetables grown in the Tijuana River Valley as far back as the 1930s, fearing sewage contamination in the water there—but the 2017 spill heralded an era of cascading failures. Repairs to one section of the pipeline revealed more damage elsewhere. Pumps and valves failed. More pipes broke. Tijuana’s largest sewage treatment plant, five miles south of the border, was eventually degraded beyond repair, and soon began sending 40 million gallons a day of essentially untreated sewage straight out to sea. From there, it was carried north on summer swells to Imperial Beach and Coronado, one of the wealthiest communities in California—perhaps best-known for the iconic Hotel del Coronado, made famous by Marilyn Monroe in the film Some Like It Hot.Though a few bold surfers disregard the warnings, sections of Imperial Beach have been closed to swimmers for more than 900 consecutive days. Bars and restaurants have seen business dry up; lifeguards keep leaving for towns where they don’t have to tell people to get out of the water. A recent City Council meeting featured a debate on whether to cancel a popular summer sand-castle competition. Citing sales data on comparable housing elsewhere in coastal San Diego County, Norm Miller, an emeritus professor of real estate at the University of San Diego, estimated that homes in Imperial Beach are discounted by as much as 50 percent.For Dedina, the whole thing can feel like an exercise in futility. A lifelong surfer and geographer by training, Dedina runs the nonprofit Wildcoast, which works on coastal conservation on both sides of the border. He ran for mayor to put in sidewalks and pave alleyways, only to find himself suing the federal government under the Clean Water Act and “leading an international coalition to fix a sewer system,” he said. Why, he wondered, did it fall to “a small city with no money” to press for change?By some measures, Dedina’s lawsuit was a success, providing leverage that helped members of Congress secure $300 million in federal funding to address sewage pollution as part of the United States-Mexico-Canada Agreement, the successor to the North American Free Trade Agreement. It also helped spur a review by the Environmental Protection Agency, outlining a range of projects on both sides of the border needed to provide a more durable solution. Unfortunately, no one thinks $300 million, or even the additional $156 million secured earlier this year, will come close to resolving the issue.Earth is home to hundreds of border-spanning watersheds, and versions of this struggle exist all over the planet: where the Ganges carries untreated effluent and industrial runoff from India into the lowland farms and coastal swamps of Bangladesh; along the Zambezi, the Mekong, the Danube. One merciful quality of the Tijuana River is that it’s not longer, limiting the scope of the conflict to two metro areas in two countries, as opposed to, say, the 11 nations whose disputes span the 4,000-mile course of the Nile.At the heart of the sewage crisis in Tijuana is the question of who bears responsibility for keeping up with the city’s growth. As Carlos de la Parra, an environmental planner from the city, put it, “We have no business being this large, except for the fact that we border California, and that Mexico and the U.S. signed the North American Free Trade Agreement 30 years ago.”Parts of NAFTA anticipated this dynamic; the agreement included a provision to set aside $100 million a year for environmental infrastructure along the border. As time went on, though, Congress lost its appetite for funding public health upgrades in Mexican cities. How about building a wall and making Mexico pay for it?That’s the kind of solution that appeals to the American political psyche, but it suffers from a basic misunderstanding. You can draw the border as a line on a map, but you still have to deal with the world on the other side. A sewage crisis in Mexico can’t be solved with pipes in California any more than a migration crisis that spans the hemisphere can be solved with a wall across Texas and Arizona.Extreme cases like Suzanne McKay’s haunt Imperial Beach: the surfer with the lung abscess, the Border Patrol agent with a flesh-eating bacterial infection. But no one quite knows how many people get sick from the water south of San Diego.Kimberly Dickson and her husband, Matt, both doctors, moved to town in 2011 to open an urgent care clinic. Over the years, they estimated that ailments associated with sewage made up 10 to 15 percent of their business, the way a clinic in a popular hiking area might see more than its share of scrapes and sprains. But they hadn’t tracked patterns in the data until August 2023, when Tropical Storm Hilary lashed the Pacific Coast with torrential rains from the tip of the Baja Peninsula as far north as Los Angeles.Suddenly, instead of seeing five or six cases of diarrhea in a week, they counted 34. “We just started noticing, ‘Gosh, we have just a full clinic in the middle of summer with people with vomiting and diarrhea and abdominal cramping.’ And the thing is, none of these people were going in the water,” Matt recalled. “That was the really startling thing for us … where are they getting this?”One answer lay at the south end of town, where sewage overflowed along Hollister Street, leaving a layer of foul-smelling mud to dry into dust on the roadway. “Kids go to school on that sidewalk,” Matt explained. “They walk into class, maybe they touch their feet, and then they eat lunch. Now they’re sick. Or, you know, you drive through it and you drive into your garage. Well, now you’ve tracked sewage into your garage.”It wasn’t only diarrhea. People complained of skin infections, sinusitis, sore throats, headaches, asthma flare-ups, and general cloudiness, all of which the Dicksons associated with what Kimberly called the “whiff test”—as in, “If you open your window and you smell, there’s your whiff test.” Many ailments seemed to track the San Diego County Air Pollution Control District’s measurements of hydrogen sulfide, the rotten-egg smell released when organic material breaks down with insufficient oxygen—say, during a sewage spill. The Dicksons are now collaborating with the epidemiology unit of the county health department to evaluate both routes of infection: water and air.In 2021, Falk Feddersen, an oceanographer at the Scripps Institution of Oceanography, led a study funded by the EPA to model the path of sewage plumes along the coast in order to evaluate which infrastructure upgrades would deliver the most benefit. Nestled among the findings was a startling estimate: Based on the prevalence of norovirus in the waters off Imperial Beach, nearly one out of every 25 swimmers could be getting sick—potentially thousands of people a year. Heather Buonomo, who leads the unit responsible for water testing at the County Department of Environmental Health and Quality, declined to comment directly on that projection, because, she said, the county was not involved in the research. But she suggested that the system of closures and health advisories triggered by evidence of sewage spills has been an effective deterrent: “People aren’t going in the water,” she said. “So the work that we’re doing to protect public health is working.”The Dicksons aren’t so sure. “We’re only seeing the tip of the iceberg,” Kimberly said. “There’s probably more out there, and it’s flying under the radar because it’s not reportable.” The real worry, Matt said, comes if Tijuana experiences a more virulent disease that sheds into sewage flows that cross the border: cholera, for instance, or shigella, a leading cause of diarrhea globally. (Though both pathogens are rare in Mexico, they are often spread through untreated water and can cause fatal illness.) “That’s where we’re gonna have a big problem,” he said. “It all depends on what’s coming across in that water.”One afternoon in April, I accompanied Rosario Norzagaray, who works with Dedina at Wildcoast, to visit Los Laureles, a neighborhood where a small share of the sewage that makes its way to the Tijuana River—and, ultimately, Imperial Beach—begins its journey. Norzagaray took a circuitous route through steep ravines along Tijuana’s western flank, bringing us to a ramshackle neighborhood in the headlands of a canyon whose waters drain into the United States above the river’s floodplain. She waved her arm at the sweep of pastel-colored homes clinging to the eroded slopes above us. “All this is invasion,” she said, explaining that the so-called colonias were settled by people who built homes without title to the land. Then again, she added, chuckling, “half of Tijuana is an invasion.”Tijuana’s population has followed a path of near exponential growth over the last century, ballooning from a community of 21,000 in 1940 to nearly two million at last count. The city’s first sewer infrastructure, a septic tank for 500 people, was built in 1928; within a decade, it served 10 times that number. U.S. officials made their first effort to stop untreated sewage fouling the coast in the 1930s, with an underwater pipe, or “outfall,” that discharged around 140 feet offshore. By then, Tijuana had built another, larger tank, to serve 5,000, but it was quickly oversubscribed to the point of obsolescence. It wasn’t until 1983 that another tank was built; at that time, Tijuana’s population was passing half a million, with dozens of maquiladoras, or foreign-owned manufacturing plants, attracting new transplants each year. By the late 1980s, Tijuana had become the world capital of television manufacturing, producing 30 million TVs a year. The passage of NAFTA in 1994 only accelerated the city’s growth.“Tijuana is a stop, not a destination,” Norzagaray said. “People don’t come to Tijuana thinking they’ll stay; they come thinking they’ll go find their American dream, but when they can’t get there, they come back here.”Though some houses in Los Laureles were built half a century ago, the neighborhood today reminded me of a packed open-air concert, where each group of new arrivals crowding onto the grass forces those who came earlier to rearrange their picnic blankets. Unpermitted homes, built and expanded in stages, jam the hillsides. We got out of the truck at the entrance to a concrete flood control structure. A trash boom, installed in 2021, stretched across the ravine like an oversize necklace made of corrugated plastic piping and steel. When it rains, the boom floats up with the floodwaters, skimming off piles of plastic bags, milk bottles, and Styrofoam as the runoff continues downstream. This is the linchpin of Wildcoast’s work in the area, a community recycling program that has removed more than 100,000 pounds of plastic waste in the last four years. But plastic is only the most tractable part of the equation. The other major components of runoff—sediment and sewage—require more than a trash boom.All around us were signs of development that had outpaced the infrastructure to support it. A garden hose snaked along 50 yards of concrete wall, splitting a single paid water connection among several houses. Raw sewage trickled into the street from exposed, broken drainage pipes that zigzagged down from homes high above us. Narrow stairways and retaining walls made of used tires ran up the slopes. “They’re trying to control all this with tires; but the water takes it,” Norzagaray explained about the eroding hillside. “And this situation is replicated in every canyon in Tijuana. Wherever there’s not supposed to be construction and there is—there are problems with sewage.” She pointed out an empty expanse on the slope above us where 20 homes had stood until 2015, when they were damaged during a landslide brought on by heavy rains.Over the years, the government has extended services to Tijuana’s colonias piecemeal. But many residents have no choice but to make do with latrines and DIY septic tanks: Though they’d be willing to pay the connection fee, they aren’t eligible for a new sewer line without title to the land. Maria del Pilar Márquez Gómez and her husband, Manuel López Paz, live in a modest white and blue cement house on a shared lot that backs up against the flood control channel. Each moved to the city during the boom years of the 1990s. Pilar Márquez Gómez came to Tijuana on a lark, and stayed when she found steady work cleaning beachfront apartments owned by wealthy Mexicans and American expats. A mason, López Paz recalled construction foremen driving trucks around the city and calling out for workers from their windows. His brother was the first to settle in Los Laureles, and he gradually brought in new families to share a 5,000-square-meter lot. “When I got here, all this didn’t exist,” López Paz said, looking up at the homes around us. “It was only farms.”Though none of the occupants had formal title, the arrangement came to feel settled, even sanctioned by the city. When the government announced plans to channelize the arroyo out back, the family lost most of their backyard, but the authorities didn’t touch the houses. Not long afterward, they returned from an extended trip to visit family in Guanajuato to see that their street was being paved, and their neighbors’ homes had been connected to water and sewer service as construction proceeded up the canyon. It turned out to be something like a onetime amnesty. Thirteen years later, they are still trying to get the right paperwork through City Hall to acquire title, and still using their septic tank. “We missed our chance,” Pilar Márquez Gómez said.In this instance, however, it’s not clear whether a sewer connection would make much difference. A mile uphill, the pavement stopped and the concrete channel gave way to an overgrown ditch lined with trash, a canyon in miniature etched by a small stream that ran downhill. Two children in pigtails skipped home across a makeshift bridge made of shipping pallets. Nearby, the stream’s “headwaters” spouted from a manhole cover atop a sewer main, where a persistent blockage sent raw sewage bubbling over in a man-made waterfall, destined for the beaches of California.Even as Dedina saw his view of the sewage crisis vindicated through his lawsuit—the federal judge in the case went so far as to visit the Tijuana River estuary to smell the stench in person—the underlying conditions were growing more dire. In late 2019, another major pipeline broke in Tijuana; nine million gallons of sewage crossed into California in two days. As the pandemic set in and the border was closed to nonessential travel, the combination of wet weather and failing infrastructure, Dedina said, seemed to create a new baseline. “It’s just polluted every day. The river’s going to flow, and there doesn’t have to be a response or timeline to fix it.”In the summer of 2020, Dedina sparked a minor diplomatic spat by saying, in an interview with a Mexican television station, “Tijuana’s sewage is killing us.” Jaime Bonilla, then governor of Baja, shot back, blaming the problem on foreign investors: “The vast amount of this contamination comes from American companies operating in Tijuana discharging their waste into the river; that’s where he needs to focus his attention.” As Dedina sees it, the pandemic broke down key bilateral relationships at all levels of government just as U.S. ties with Mexico were strained by other concerns. “All of a sudden, things just fell apart,” Dedina told me. “Fentanyl, migrants, the whole crisis in the relationship between the U.S. and Mexico—that’s taking all of Ken Salazar’s time,” he said, referring to the U.S. ambassador to Mexico. “This is not something the U.S. is willing to push on.”The modern Tijuana River is a hybrid, part natural waterway and part man-made infrastructure, whose flow is what ecologists call “urban drool.” What was once an intermittent, seasonal stream has been replaced, since the 1970s, by a steady flow of used tap water imported across 90 miles of open desert. The concrete flood channel that sheaths the river all the way through Tijuana ends just past the border. On the southern bank, the rusty bollards of the border fence climb a steep hillside at the city’s edge, flanked by shops and apartments all the way to the sea. On the north side, the enclosed tunnel of the pedestrian crossing follows the river right up to California, then turns 90 degrees and descends to U.S. Customs and Immigration Enforcement, a filter separating the people heading north from everything else.As the water slows down and spreads out across the floodplain, solids drop out of the current. Plovers and godwits peck at the mud among empty water jugs and motor oil containers, toys and soccer balls, sneakers, couch cushions, spaghetti knots of hardened caulking. Coke bottles urge “Recíclame.”Standing on an embankment 200 yards downstream, Chris Helmer, the director of environmental and natural resources for Imperial Beach, gazed out at a profusion of wild mustard and garland daisies sprouting from deposits that accumulate in the riverbed like layers of rock each spring.A few weeks earlier, Helmer explained, and the view would not have been obstructed by so much vegetation. “It’s highly nutrient rich water: What do you think is going to grow in here?” he said. Where water or bulldozers had cut into the banks, the structure that remained looked like a tall layer cake. “It’s almost like tree rings. Every single season you can see a new layer of sediment and trash, sediment, trash.”Clearing the debris is a Sisyphean undertaking, with each season’s work reset by the next rains, and in recent years the U.S. government hasn’t come close to keeping up. There are now something like 100,000 truckloads of material that will need to be moved to prevent flooding in the adjoining neighborhoods in San Ysidro and Tijuana.But Customs and Border Protection is also in the process of making the work much harder. Just upstream, construction had begun on a project announced abruptly in 2020: a bridge for Border Patrol agents to cross the flood channel as it enters the United States, combined with a fence, built along the upstream side, consisting of dozens of moving panels, or liftgates, that will have to be raised during heavy rains to allow the river’s flow to continue downstream. As a border security measure, the project is exempt from federal environmental review, but other agencies met the CBP’s proposal with pointed skepticism. California’s Environmental Protection Agency, CalEPA, warned that fortifying this area might simply create security issues near some of the other places where tributaries crossed the border, like Los Laureles. But the larger worry is that the liftgates will fail, or that CBP may not respond in time to raise them before a rainstorm, or debris will accumulate in back-to-back storms, and the fence, which is supposed to let water through, will act as a dam instead, leading to catastrophic flooding in a densely populated part of Tijuana. (CBP did not respond to interview requests for this story.)“All the debris and trash is going to back up in Mexico, so you rely on Mexico to maintain and clean this,” Helmer said—maintenance and cleaning the United States already fails to do. He called the project “utterly insane.”It’s expected to be completed by the end of the year.Residents of Imperial Beach sometimes seemed at a loss about where political pressure can be usefully applied. “When you’re in Washington, what is the federal government even saying about this happening to us?” one man asked at a recent city workshop on the sewage problem. After 16 years of going to public meetings about sewage, he wanted to know if there was a time frame for a solution. Other residents have compared spills to a “dirty bomb” dropped on the city. There were suggestions that the United States close the border to all travel north during sewage flows or cut off Tijuana’s access to Colorado River water.If residents don’t know how to pressure the feds, the feds often don’t seem to know how to pressure Mexico. The commissioner of the U.S. arm of the International Boundary and Water Commission, Maria-Elena Giner, has been frank about the challenges of treating sewage that originates in another country. “We cannot fine them for not treating their wastewater; we can’t fine them for discharging water,” Giner told me. What the IBWC can do is conduct meetings, collect evidence, write stern letters, appeal for more funding from Congress. The sewage that reaches the IBWC plant would be anomalous anywhere else in California. “You get rags, you get a lot of sediment, and it tears up the pumps, it tears up the concrete,” Giner said.Rags and sediment do a number on the sewer infrastructure in Tijuana, too. But the root of the problem isn’t technical so much as financial: The Comisión Estatal de Servicios Públicos de Tijuana, or CESPT, which provides water and sewer services to the city, gets most of its revenue from ratepayers, but loses money on nearly 80 percent of the water that flows through its pipes.In the spring, I met with CESPT’s director, Jesús García Castro, and deputies responsible for finance and operations, around a coffee table spread with chocolate-covered nuts and cut fruit. García Castro had been on the job only a few months, but it turned out to be an auspicious moment to take over: A few weeks after he began, Mexican President Andrés Manuel López Obrador, or AMLO, as he’s known, announced that the city’s largest treatment plant, which hasn’t operated effectively in five years, would be rebuilt by a unit of the Secretaría de Defensa Nacional, or SEDENA, comparable to the Army Corps of Engineers. Salazar, the U.S. ambassador to Mexico, traveled to Tijuana to attend the groundbreaking: Finally, it seemed, sewage had risen up the list of issues competing for political oxygen in the U.S.-Mexico relationship. The new treatment plant “will reduce the flows of untreated sewage to the Pacific Ocean by 90 percent,” García Castro said confidently. “So that’s a big part of the solution.”Under AMLO, the military has become something like Mexico’s contractor of first resort, with the ability to bypass environmental review and typical procurement processes. Keeping with SEDENA’s style, the construction schedule announced at the groundbreaking was ambitious, condensing what would ordinarily be a multiyear project into nine months. U.S. officials received the news with wary optimism. “SEDENA works fast,” one federal official told me in a text message. “Not necessarily a good thing as they tend to build BEFORE design.” Still, the Mexican government had ultimately scuttled a series of earlier initiatives to rebuild the plant with private funding, as far back as 2011, and there was a sense that if it wasn’t built by SEDENA, it might not be built at all.Before the existing plant at San Antonio de los Buenos went offline, it limped along well past its useful life, hobbled by a lack of maintenance. Sludge accumulated in treatment lagoons that were rarely dredged. Eventually, they stopped functioning altogether. A 2019 review by an independent consultant found that there was no backup power system and “no preventive maintenance program,” noting that CESPT typically received just a third of the operating budget it asked for.After years of delay, García Castro was adamant that the utility was making up for lost time. “We’ll have results this year,” he said. “Next year, already, we’ll be able to have clean beaches.” The reality is that most directors don’t stick around at CESPT long enough to see such promises through. When I asked how many people had held García Castro’s job before he got there, his deputies, both longtime employees, began counting on their fingers, seeming to flip through a mental catalog of past bosses like baseball fans trying to name bench players on favorite childhood teams. Eventually, they came up with a figure. “Thirteen in 10 years,” García Castro said.The IBWC’s South Bay International Wastewater Treatment Plant may be the easiest place on the entire border to cross legally between the United States and Mexico. Here, flatbed trucks bearing CESPT’s blue logo make daily return trips to a loading bay beneath an elevated conveyor belt carrying trash extracted from Tijuana sewage.In the spring, Morgan Rogers, who oversees the plant’s operations, watched through wraparound shades as a stream of refuse fell from a chute in the ceiling into a waiting dumpster. “Mexico hauls that off when it gets full,” he explained. “They own the trash, they own the sludge, they own the sediment”—he paused—“and they actually own the water. But we throw the water off the coast because they can’t do anything with it.”“But they can do stuff with the trash and sludge and sediment?” I asked.“Well, we make them take that.”Rogers nodded in the direction of the border wall, just on the other side of the building, its tall, rusted slats climbing to the horizon in either direction. “We have a gate out back here,” he said. The IBWC plant was built in the years after NAFTA was passed, partly out of a conviction that the Mexican government couldn’t be relied on to treat sewage to standards that would keep California beaches swimmable. It’s meant to handle about a third of Tijuana’s wastewater, along with dry weather flows from the Tijuana River and runoff from neighborhoods like Los Laureles. But as Tijuana has continued to grow, the plant has been broken down by the combined effects of storm events and infrastructure failures.In July 2022, Rogers explained, two critical pipelines, serving a pump station that allowed CESPT to divert sewage flows several miles south of the border, failed one after the other. That December, a series of atmospheric rivers also worsened damage to a valve controlling how much sewage the IBWC plant lets in. As a result, Rogers said, “Whatever flows come from Mexico, we take.”The consequences have been disastrous. During rainstorms, as much as 80 million gallons a day poured into a plant designed to handle 25 million, carrying trash and sediment that clogged critical equipment. By the end of 2023, all five of the plant’s primary treatment tanks were inoperable, filled to the brim with sludge.Tropical Storm Hilary made things even worse, destroying all but one of the pumps that moved sewage into the treatment tanks. Rogers leaned over a concrete wall where backed-up trash formed a dam during the storm, and he peered down at the pumps below. “There was eight feet of water down there,” he said. “We were on the edge: If you lost that pump, we’d have been out of business.”As it was, the plant was still recovering, with new pumps waiting on the grass to be installed, and waist-high weeds sprouting from treatment tanks, which were still being emptied and overhauled one at a time. Rogers credited Tropical Storm Hilary with spurring a new sense of urgency somewhere above his pay grade. “Hilary, really as much damage [as] it did, it kind of woke us up”—he paused, looking at his counterpart from IBWC’s political side, Sally Spener, following along in a pink button-down and maroon cowboy boots. “Not us, but it woke—Who would you say it woke up?” Spener didn’t answer.Throughout the tour, Spener countered Rogers’s blunt assessments of the plant’s condition with steadfast diplomacy: the broken valve (“But the contract has been awarded to fix it, right?”); the failing pump stations (“That’s all part of that rehab that’s envisioned.”); the oversize pipeline that sent the plant’s treated water three miles off the coast (“the award-winning ocean outfall!”). The dynamic captured the unwieldiness of the agency’s mission: to operate a utility whose “customers” live in another country, and to manage a spiraling set of technical problems enmeshed in a much larger diplomatic relationship.In June, Mexico elected its first woman president, Claudia Sheinbaum, a former professor of engineering from AMLO’s Morena party. The alignment between the levers of power in Baja state and Mexico City increases the chances that the leadership team at CESPT will keep their jobs long enough to make a dent in the priorities they’ve outlined.But the shift that may ultimately force the city into a different relationship with wastewater is climate change. As in urban areas across the U.S. Southwest, Tijuana’s water supply is dominated by the dwindling Colorado River. In 2023, CESPT was forced to shut off supply to nearly half the city and get emergency allocation from a cross-border connection with California to avoid prolonged water outages. Water, then, is the limiting factor on Tijuana’s growth: CESPT is now pursuing a long-postponed plan to reuse a portion of Tijuana’s treated water for agriculture, a signal that sewage is finally seen as a commodity worth capturing. “Mexico owns the water rights to this,” Chris Helmer told me as we watched Tijuana’s stream of urban drool meander past us into the estuary. “It’s written in the treaties. At some point, Mexico is going to want to use this water.”Straddling one of the busiest land crossings in the world, the Tijuana River reminds us that both sides of the border constitute a single place. Once the poop is in the water, no amount of barbed wire can get it out.When plans for a U.S. plant to treat Mexican sewage were first proposed, in the 1970s, they called for a facility big enough to handle 100 percent of Tijuana’s wastewater. Gradually, the plant-to-be was whittled down to a quarter of that size and simplified so that it could be built more cheaply, with the idea that upgrades would be made over time. David Gibson, an executive officer of the San Diego Regional Water Quality Control Board, said the IBWC plant was already outdated by the time it was completed, in 1997. “Design decisions that were made in the 1990s for that treatment plant, we’re paying for even now,” Gibson said. Without ratepayers to cover maintenance costs, the plant has also fallen victim to the Darwinian logic of the federal budget, receiving just $4 million for maintenance, cumulatively, from 2010 to 2020, a period when billions of additional dollars were allocated to border security. “This is like buying a nice Corolla in 1997 or a nice Ford, but you never change the tires, you never change the oil,” Gibson said. The plan now is to make overdue repairs and double its capacity. But the current funding, Gibson said, is “barely half” what’s needed “for the economy model.” He worries the region is on track to reprise nearly 100 years of sewage history, “outgrowing the infrastructure only a decade or so after it’s installed.”Still, Gibson echoed the point of view I heard from nearly every American official I spoke to—that the only reliable solution to Tijuana’s sewage problem is building the infrastructure on the U.S. side. In this, sewage treatment for Tijuana seems destined to operate as something like an extension of the border wall, a constant, churning intervention made at the river’s mouth, rather than its source, whatever the price. “I don’t think Mexico in general has sufficient resources to attend to their problems,” Giner, the IBWC commissioner, told me. “How are we going to ensure this moves forward with sufficient resources after all of this is built?” she asked, referring to upgrades on the U.S. side. “Let’s say we’ve caught up. Once we catch up, we will have to answer that question.”Nearly wherever you look, border politics in the United States is animated by a persistent myth: that with enough money and willpower, you could eventually seal off the countries from one another, like apartments that share a 1,954-mile wall. One way to describe decades of militarization on the border is that it serves to make Mexico invisible to residents of the United States. The same might be said of cross-border industrial development: porous to money and airplane parts, hardened to everything else. Straddling one of the busiest land crossings in the world, the Tijuana River offers a stubborn rebuttal, a reminder that both sides of the border constitute a single place. Once the poop is in the water, no amount of barbed wire can get it out.