A few years ago 'south of the border, down Mexico way' was just a lilting phrase in a popular American song. But nowadays it can mean, among other things, environmental pollution drifting north to US communities over the border.
For the US-Mexico boundary has joined the US-Canada border as an area of major pollution problems.
After years of stalemate, motion toward a solution now is evident in the US-Canada case. Progress also is being made along the southern border, but that is not as advanced.
At issue are several points: pollution control and cost, international diplomacy, and differing national priorities.
Ironically, the US may be the transgressor in the Canadian case, and the transgressed in the Mexican.
A joint US-Canadian study this fall is to examine the wind currents which carry air eastward from two northern industrial areas: Dayton, Ohio, and Sudbury , Ontario. The aim is to find out once and for all how much acid rain in northeastern North America comes from each area. And to what extent US pollution may be causing acid rain in Canada. Once this is determined cleanup efforts can be more effectively targeted.
Canadian officials are pleased by the concern now expressed by the US Environmental Protection Agency under William Ruckelshaus. Previously Ottawa had felt it was far more concerned than was Washington about the prospect that Canadian acid rain is caused by pollution blown north from the US.
Pollution on the southern US border is more varied. There's smog over El Paso , much of which city officials believe comes from the larger nearby Mexican city of Juarez; water-borne sewage near San Diego, which may come from Tijuana; and pesticide and dust problems near Brownsville, Texas, which in part may have Mexican roots.
For these and other border communities getting Mexico to stop exporting pollution, if in fact it is, is a high priority. But to a struggling Mexico, putting its energy and resources into reinvigorating its economy is the number one issue. Finding the money to solve cross-border pollution will be difficult for the Mexican government. Besides, as some US officials admit, Mexico has even more serious environmental problems in its own cities.
Last month President Reagan took one step toward meeting this challenge. He and Mexican President de la Madrid signed an agreement that forms a basis ''for cooperation'' toward cleaning up the border's environment. Needed now is for the two nations to take the next step: decide what should be done and how, and come up with the necessary funding.
On both borders the challenge is major, the expense considerable, the politics thorny. But solutions are there, and progress now is being made. It needs to be both supported and nudged forward.