SAN CRISTÓBAL DE LAS CASAS, Mexico — Maria del Carmen Abadía lives in one of Mexico’s rainiest regions, but she has running water only once every two days. When it does trickle from her tap, the water is so heavily chlorinated, she said, it’s undrinkable.

Potable water is increasingly scarce in San Cristóbal de las Casas, a picturesque mountain town in the southeastern state of Chiapas where some neighborhoods have running water just a few times a week, and many households are forced to buy extra water from tanker trucks.

So, many residents drink Coca-Cola, which is produced by a local bottling plant, can be easier to find than bottled water and is almost as cheap.

In a country that is among the world’s top consumers of sugary drinks, Chiapas is a champion: Residents of San Cristóbal and the lush highlands that envelop the city drink on average more than two liters, or more than half a gallon, of soda a day.