EL PASO

ON a hot August day in 1873, El Paso's 800 citizens elected saloon operator Ben Dowell to be their first mayor. Ordinance No. 1, his first official act, outlawed bathing in the irrigation ditch that supplied the desert town's drinking water.

Fast forward 120 years to a slide show narrated by Laurance Nickey, director of the El Paso City-County Health District.

"This is an outhouse. They're not legal, but we have hundreds and hundreds of them," Dr. Nickey says.

Click. "This gentleman is getting his breakfast water out of two old sulfuric-acid vats."

Click. "This is a septic tank. First of all, it's not covered. Secondly, it's overflowing into the Rio Grande River." Both conditions are illegal.

Click. This slide reveals the scant distance separating a man's water well and the cesspool into which his household plumbing directly gushes. "He's drinking, bathing, washing his clothes in his own waste," Nickey says.

Always a concern, sanitation and water quality preoccupy officials in El Paso and its Mexican neighbor, Ciudad Juarez, as never before. Rather than receding with time, these problems have been compounded by geology, climate, poverty, rapid growth, industrialization, and a confluence of competing local, state, and national jurisdictions.

But the current overload is nothing compared with what the North America Free Trade Agreement (NAFTA), signed in December, might bring along the 2,000-mile border, say businessmen, health officials, and environmental regulators here. Under the Integrated Border Environmental Plan, the United States and Mexico are spending hundreds of millions of dollars to identify and deal with such problems.

El Paso and Juarez, at the midpoint of the border, are also the epicenter of the problem. With 600,000 and 1.2 million residents, respectively, the cities combine to form the world's largest international metroplex. Juarez is also the fifth-largest city in Mexico, while the El Paso metropolitan area is the 22nd-largest in the US and the fourth-largest in Texas.

The cities achieved their sizes through staggering growth: 32 percent from 1975 to 1985 for El Paso, and 134 percent for Juarez. Those trends mirror trade patterns. Mexico is on its way to replacing Japan as the No. 2 US trading partner (Canada is No. 1). US exports to Mexico are growing at 22 percent a year, more rapidly than to anywhere else. That gave the US a $2 billion trade surplus with Mexico in 1991, a number that may have doubled in 1992.

One component of the trade explosion has been the maquila (also called maquiladora) program. Initiated by Mexico in 1965, the program for the first time allowed foreign companies to fully own factories (maquilas) that make products for export only.

US companies paid little attention to the maquila program until the late 1970s, when rising costs of labor and regulations at home made them look offshore for new plant locations. Beginning in 1983, the devaluation of the peso - then, eight per dollar; now, more than 3,000 - made Mexican labor a bargain.

Today 1,700 maquilas lie just across the Texas border, where the total cost of an employee is one-sixth of the US average. More than 300 maquilas in Juarez employ 125,000 workers. Half of them work in Bermudas Park, the world's largest industrial park. About 18,000 are middle-managers, contributing to the rise of a middle class in that city.

The maquilas have united El Paso and Juarez in a bustling economic symbiosis. The three bridges connecting the cities recorded 42 million northbound crossings in 1991.

Every day, 1,500 US engineers and managers drive from upscale El Paso neighborhoods to work at Juarez maquilas. Another 10,000 direct jobs and 30,000 indirect jobs in El Paso owe their existence to those factories, says Robert Cook, a spokesman for the Greater El Paso Chamber of Commerce.

Meanwhile, Mexican shoppers support 8,500 of El Paso's retail jobs, or 1 in 4, Mr. Cook says. Payless ShoeSource, Wal-Mart, and other national chains have their highest-volume-per-square-foot stores here, thanks to Mexicans bringing their wages north. Less-prosperous Mexicans come over to buy used clothing by the pound at open-air markets.

El Paso's job base is growing at a 3.9 percent annual rate, compared with 2.7 for the rest of Texas. "Without the maquila program," Cook summarizes, "we wouldn't have the economy that we have today."

Yet El Paso is still the eighth-poorest US city per capita, the 1990 census found. "Thirty percent of our population at any one given time is either at or below federal poverty guidelines," Nickey says.

In El Paso County, 68,000 people live in 350 colonias - the shantytown subdivisions featured in Nickey's slide show. Up to 400,000 people live outside Juarez in similar squalor.

Astonishingly, Juarez itself has no wastewater treatment plant. None. Its 55,000 gallons a day of raw sewage flows into the aguas negras, a 30-foot-wide canal that for 18 miles parallels the Rio Grande at a distance of 200 yards.

Eventually the canal spills onto 40,000 acres of farmland, although some of the slime probably finds its way into the river, Nickey says.

Rising trade-related traffic and runaway growth boost the potential for sanitation-related illness to spread instantly from Juarez to El Paso and beyond. By the year 2000, Nickey says, 2.3 million people will share the same water, air, and pollution. "And they're going to share them instantly."

Nickey calls the situation a ticking time bomb that is ready to explode. "And when it does," he warns. "It's not going to only impact Juarez and El Paso. It will impact inland in the United States also." Richard Kiy, special assistant on US-Mexico border affairs at the Environmental Protection Agency, says the Integrated Border Environmental Plan (IBEP) calls for spending $50 million on sewage treatment and $25 million on drinking water facilities this year in Texas colonias alone.

Nickey calls that "a drop in the bucket." Mr. Kiy acknowledges that "this is just the beginning" of a multiyear effort to aid the shantytown subdivisions.

Another concern is dumping in the desert or in the Rio Grande. As her car heads east on highway I-10, Tracy Williams points out the tires, plastic, and cardboard that litter the mesquite brush and gravel landscape. Eyesores to be sure, but for Ms. Williams and her agency, the Texas Water Commission (TWC), a bigger worry is toxic waste.

Some US-based companies sneak toxics into Mexico for illegal disposal to avoid the cost of incineration. The TWC tries to intercept these shipments. However, Williams says, when the agency sets up a check station, truckers use citizens' band radios to warn each other. They circle until the TWC leaves the scene.

"That's why you have to do these spontaneously, when they least expect it," Williams says. "I'd like to do this at night. I'd like to hit smaller trucks they call hucksters [that] may have a barrel or two. We're going to try to be more creative."

The agency is also trying to calculate the amount of toxic waste that maquilas should be shipping back to the US for disposal. By Mexican law, all that they generate should be disposed of in the US, "but it's hard to tell whether or not they're bringing it all back. We suspect that they're not," Williams says.

Kiy says IBEP money is being used to develop a system that will monitor toxics from "cradle to grave." The TWC was recently given $352,000 in IBEP funds to conduct the first-ever El Paso-to-Brownsville test for toxics in the Rio Grande. Cook does not expect NAFTA to trigger a stampede by toxic-producing US factories to relocate to the Mexican bank of the Rio Grande. Free trade will allow US-owned factories to serve the Mexican market, so an interior location would make more sense than one at the border.

As for maquilas, NAFTA will eliminate duties that add 1 to 2 percent to the finished product's total cost. "I certainly don't see a whole lot of manufacturing rushing from the United States into Mexico for that kind of minimal saving," Cook says.

Further, Mexico's labor force is best-suited for simple assembly operations, he says. Despite the headlines, most maquilas are not engaged in the kind of manufacturing that has chemical waste as a byproduct. Other observers contend that maquilas are beginning to perform more technically sophisticated manufacturing.

Toxics aside, Cook points to one unambiguous challenge. Even before factoring in NAFTA, Juarez should grow 111 percent by 2010 and El Paso County 57 percent. What will those people drink? If current consumption continues, the aquifer used by both cities will be dry by 2025.

For 11 years El Paso tried through the courts to get access to subsurface water in New Mexico. It recently gave up and agreed with that state to evaluate jointly the feasibility of supplying additional surface water from New Mexico to El Paso.

Meanwhile, the city is reinjecting treated wastewater into its aquifer. An "aggressive" water conservation plan has created a new local landmark: a heap of junked toilets replaced by low-flow models. Cook says eastern El Paso County has a separate aquifer, but its brackish water would require desalination if tapped. Somehow "we'll have water here well past the year 2025," he says.