San Diego, Calif. ? Mexico ships televisions, cars, sugar and medical equipment to the United States. Soon, it might be sending water north.

Western states are looking south of the border for water to fill drinking glasses, flush toilets and sprinkle lawns, as four major U.S. water districts help plan one of two huge desalination plant proposals in Playas de Rosarito, about 15 miles south of San Diego. Combined, they would produce 150 million gallons a day, enough to supply more than 300,000 homes on both sides of the border.

The plants are one strategy by both countries to wean themselves from the drought-prone Colorado River. Decades of friction over the Colorado, in fact, are said to be a hurdle to current desalination negotiations.

The proposed plants have also sparked concerns that American water interests looking to Mexico are simply trying to dodge U.S. environmental reviews and legal challenges.

Desalination plants can blight coastal landscapes, sucking in and killing fish eggs and larvae. They require massive amounts of electricity and dump millions of gallons of brine back into the ocean that can, if not properly disposed, also be harmful to fish.

But desalination has helped quench demand in Australia, Saudi Arabia and other countries lacking fresh water.

Dozens of proposals are on the drawing board in the United States to address water scarcity but the only big project to recently win regulators’ blessings would produce 50 million gallons a day in Carlsbad, near San Diego. A smaller plant was approved last year in Monterey, some 110 miles south of San Francisco.

Mexico is a relative newcomer to desalination. Its largest plant supplies 5 million gallons a day in the Baja California resort town of Cabo San Lucas, with a smattering of tiny ones on the Baja peninsula. Skeptics already question the two proposed plants in Playas de Rosarito — known as Rosarito Beach to American expatriates and visiting college spring-breakers.

“It raises all kinds of red flags,” said Joe Geever, California policy coordinator for the Surfrider Foundation, an environmentalist group that has fought the Carlsbad plant for years in court, saying it will kill marine life and require too much electricity.

Water agencies that supply much of Southern California, Phoenix, Las Vegas and Tijuana, Mexico, are pursuing the plant that would produce 50 million gallons a day in Rosarito near an existing electricity plant. They commissioned a study last year that found no fatal flaws and ordered another one that will include a cost estimate.