CALEXICO, Calif. — For generations, residents of the Southern California border town of Calexico watched with trepidation as their river turned into a cesspool, contaminated by the booming human and industrial development on the other side of the border in Mexico.

Noxious sewage filled with feces, industrial chemicals and other raw waste regularly comes in through the New River, which flows from Mexico’s Mexicali Valley and through Calexico, leaving neighborhoods along the waterway engulfed in pungent fumes. And it’s not just the river: From above, smoke billows from Mexican factories, illicit medical burn sites and tire pits, fueling widespread asthma in the region.

As Washington debates spending billions to shore up barriers along the 2,000-mile southwest border, many residents in California’s Imperial Valley feel at least some of that money could be spent to address the region’s public health threats. Just feet away from Calexico, Mexico’s lax environmental rules and enforcement pose a regular menace.

“It’s a pit of infection,” said Arturo Santiago, 50, who lives in a neighborhood adjacent to the river in west Calexico, atop a steep overlook. “It smells like farts if you open your window.”