Today we’ll unpack the science behind a brand-new technology to measure water quality. San Diego is first in the world to use it, and it’s already sparked controversy.

The more sensitive test shows there’s more poo plaguing San Diego’s southernmost beaches than we could ever tell before — especially in summer when coastal cities like Coronado virtually never failed water quality tests using the old tests. In the case of South Bay, there is an obvious source of human sewage that’s plagued the coastline for decades: Tijuana.