By any objective standard, the southern coast of San Diego County is enduring a long-running environmental nightmare. Decades of billions of gallons of untreated human waste flowing north from broken sewage infrastructure in Tijuana have sickened a vast number of surfers and swimmers and many Navy SEALs training at Coronado. Especially because of ailments reported by border agents, some doctors worry that the health threat goes far beyond active ocean users to include those who spend extended time in coastal areas and breathe air that often smells like a filthy portable toilet. Now there is fresh confirmation of how uniquely awful this problem is. The Surfrider Foundation has released a report on 567 sites in which it tested water for unsafe bacteria levels and found Imperial Beach — which has been closed for more than two years — had far and away the dirtiest water in the United States.