Between Tijuana, Mexico, and San Diego, Calif., where the border meets the ocean, the Tia Juana River carries raw sewage north out of the overpopulated hills of Tijuana onto American flatlands. The mucky river meanders through eight-foot reeds, past some horse ranches, and out into the surf on an otherwise pristine southern California beach.
To Americans, this is a dangerous environmental crisis.
To Mexicans, San Diego anthropologist Joseph Nalven says, it is a problem for California surfers. Tijuaneros are more concerned with the raw sewage flowing in open ditches through crowded neighborhoods, many of which do not even have drinking water.
The Tijuana sewage problem has come a step closer to resolution in the past year through some down-to-earth diplomacy. Following an accord in August 1983 between President Reagan and de la Madrid to resolve the sewage problem, engineers on both sides of the border have come to an agreement.
They are planning a 40-million-gallon-a-day treatment plant on the American side that will pipe the treated sewage into the surf several miles south of the border.
Now, according to Richard Reavis, the Environmental Protection Agency official who headed up the negotiations on the United States side, the tough part begins: the bargaining over what each country will pay for the plant.
This is the biggest pollution problem in the border region for Americans. There are others: air pollution in southern Arizona from Mexican copper smelters; Mexican sewage in California's Imperial Valley and in Nogales, Ariz.; toxic chemical dumping and pesticides at points all along the border.
But Mexicans have problems with Americans, too. Many US industries dump their chemical wastes in Mexico, and the first permanent US storage site for radioactive waste is planned for Carlsbad, N.M., where underground water flow is south into Mexico.
Overall, more pollution flows south from the US than north from Mexico, says Fernando Ortiz de Monasterio, an environmental engineer at El Colegio de Mexico.
The border negotiations are unbalanced in other ways, too.
Americans have a well-honed concern for the quality of the environment, technical expertise, and money to spend on monitoring air quality and treating sewage. Mexicans are responsive to American concerns, as well as their own, but their government has a mere pittance to spend, since Mexico faces 30 to 50 percent unemployment, and many basic needs are unmet.
Americans can afford more stringent environmental standards, says Mr. Nalven, a consultant on border issues, but Mexico is a developing country where establishing a sufficient supply of drinking water takes precedence over treating sewage.
''They will agree to anything, and even sign official agreements,'' says Howard Applegate, a professor of civil engineering at the University of Texas at El Paso, of Mexican officials. ''But when it comes time to do something, they say they have other priorities. And I think that if I were Mexican, I would have other priorities, too.''
On a local - unofficial - level, however, Dr. Applegate sees more concern over pollution and more cooperation, especially between Mexican and American researchers.
''I think every (pesticide) monitoring station in Juarez (Mexico) uses my equipment,'' he adds.
Inevitably, Americans pay more than Mexicans for joint projects like sewage-treatment plants.
But Mexicans have ambivalent feelings about this, Nalven says. They can't afford what the US can spend, but they are very sensitive to the stigma of dependency on Americans and cautious of handing the US any added clout over Mexican affairs.