Enviros train drone pilots to find and pursue pollution

Brent Walls, the Upper Potomac Riverkeeper with Potomac Riverkeeper Network, shows how to catch a drone with his hand during a training session on June 7 in Poolesville, Md. Walls and other people who work to protect rivers and waterways have begun using drones to catch polluters in places where wrongdoing is difficult to see or expensive to find. The images they capture have already been used as evidence to formally accuse companies of wrongdoing. (AP Photo/Julio Cortez)

Quincey Johnson, left, outreach director Upper Missouri Waterkeeper in Bozeman, Mont., and Martin Lizely, Grand Riverkeeper with LEAD Agency in Miami, Okla., fly their drones during a training session on June 7 in Poolesville, Md. (AP Photo/Julio Cortez)

A drone operated by Martin Lizely, Grand Riverkeeper with LEAD Agency in Miami, Okla., flies with the Potomac River at a distance during a training session, Tuesday, June 7, 2022, in Poolesville, Md. People who work to protect rivers and waterways have begun using drones to catch polluters in places where wrongdoing is difficult to see or expensive to find. The images they capture have already been used as evidence to formally accuse companies of wrongdoing. (AP Photo/Julio Cortez)

Robby Lewis-Nash, a staff writer with Friends with Casco Bay in Portland, Maine, inspects a participant’s drone during a training session Tuesday, June 7, 2022, in Poolesville, Md. People who work to protect rivers and waterways have begun using drones to catch polluters in places where wrongdoing is difficult to see or expensive to find. The images they capture have already been used as evidence to formally accuse companies of wrongdoing. (AP Photo/Julio Cortez)

POOLESVILLE, Md. — When environmentalist Brent Walls saw a milky-white substance in a stream flowing through a rural stretch of central Pennsylvania, he suspected the nearby rock mine was violating the law.