White sand stretches for miles where Pacific Ocean waves crash into the shore. Nearby, bicycles lean against seaside cottages that are accented by banana and palm trees out front. A rickety wooden pier offers spectacular views of sherbet-hued sunsets over the water.

To the eye, Imperial Beach, Calif., is an idyllic beach town, a playground for tourists and Southern California residents alike at the southern border with Mexico.

But lately, the view has been ruined by the sea breeze, which reeks of rotten eggs. The surfers who once prepared for big-wave competitions are gone. So are the tourists who built intricate sand castles and licked ice cream cones on the pier.

Imperial Beach is now the center of one of the nation’s worst environmental disasters: Every day, 50 million gallons of untreated sewage, industrial chemicals and trash flow from Tijuana, Mexico, into southern San Diego County.

The cross-national problem traces back at least a century. But it has significantly worsened in recent years as the population of Tijuana has exploded and sewage treatment plants in both countries have fallen into disrepair.

“It’s a public health ticking time bomb that isn’t being taken seriously,” said Paloma Aguirre, the mayor of Imperial Beach. “We need help.”