Imperial Beach, California, is a city with a dirty secret.

There’s a reason why nobody, beyond a determined bunch of early-morning surfers, ventures beyond its sandy beaches and into the sea. It’s the same reason local restaurants source their fish from further up the coast, and residents keep their windows shut at night, even during the sweltering West Coast summers.

The city, a short drive south of San Diego, is being polluted by billions of gallons of raw sewage flowing across the Mexican border every year.

Its beaches have been forced to close, its air is being contaminated by pollutants hundreds of times above levels deemed safe, and locals are falling violently unwell.

The issue is now a source of tension between the US and Mexico, and The Telegraph understands that Donald Trump has given a personal commitment to tackle it as the two countries attempt to negotiate a solution.