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. . .