IMPERIAL BEACH, California — Raw sewage teeming with toxic chemicals dumped in Mexico’s northern border city of Tijuana has been flowing for decades into the United States while local, state, and federal authorities in both countries have failed to prevent it. Now, those on the front lines of this crisis are saying enough is enough.

Unrestrained run-offs of ugly green and brown liquids have caused both figurative and literal headaches for U.S. Border Patrol agents assigned to the Tijuana River Valley, a stretch of land south of San Diego from where the Tijuana River flows north from downtown Tijuana into California and then west to the Pacific Ocean. Some of the sewage is intentionally dumped in Mexico with the full knowledge that it will end up as a gruesome export to the United States.