San Diego — The border region here has been mired for several years in a very down-to-earth problem in international relations. And Mexico, significantly, and literally, has the higher ground.
The problem is that Tijuana's sewage system hasn't been able to accommodate the city's rapid growth in population. And since Tijuana is cradled in hills that drop off on the American side, it is more San Diego's problem than Tijuana's.
But there are some promising signs these days of real action to resolve a long-rankling predicament.
A few miles inland, the Tia Juana River flows out of Mexico and meanders through a marshy plain to the sea. It is usually dry this time of year, but now it is flowing now with raw sewage.
A popular beach north of the border is currently posted with a ''keep out'' sign warning beachgoers to avoid all contact with the contaminated water.
The city of San Diego processes 13 million gallons of sewage from Mexico each day at an American plant. The city of Tijuana pays $200,000 a year for the service - a 10th of its actual cost - under a 1965 contract intended for emergencies.
To date, despite the best intentions on all sides, the problem has only worsened. Since Mexico's recent peso devaluations, Tijuana simply doesn't have the money. It faces more immediate concerns, such as repairing storm-damaged pipelines for drinking-water supplies.
Sewage disposal is a problem elsewhere along the border as well.
One hundred miles inland, the New River also carries raw sewage into the United States, and sometimes floods.
In Arizona, the little town of Nogales (population 15,000) runs a treatment plant jointly with Nogales, Mexico (population 150,000). Yet some 3 million gallons of sewage a day are running down a wash from Mexico right through the middle of the Arizona town.
On the American side, the political will to correct these problems has been hardening, especially after Imperial Beach, Calif., lost some of its best weeks this summer to a polluted shoreline. The town, which falls between San Diego and the border, relies heavily on the beach trade.
A heartening sign came when President Reagan and the Mexican president, Miguel de la Madrid Hurtado, took up the matter in an August meeting and signed an agreement to resolve the sewage problem.
Now a couple of things are happening:
* A consultant, Lowry and Associates, is completing a study on building new treatment plant on the American side of the border to treat both Mexican waste and the anticipated waste from new US land developments. Rough estimates are that such a plant will cost $600 million.
* An Environmental Protection Agency official, Richard Reavis, has just moved his office to San Diego to lead the negotiations. He hopes to come up with a proposal to which all the American agencies and government levels can agree, and then, to present it to Mexican officials.
''I don't see the Mexicans signing anything we propose and want to pay for,'' he cautions.
''I don't underestimate their national pride and desire to pay their share of whatever is built.''