Mercury flows through industrial wastewater into the Ohio River, and damages young brains. But the multi-state agency tasked with keeping the waterway clean hasn’t tightened controls on this pollution because it doesn’t have the authority to do so. While coal-fired power plants, chemical manufacturers and other facilities along the Ohio River are piping mercury directly into the river and there’s a permitting process to regulate that, the more significant source appears to be mercury blown into the atmosphere from smokestacks — both locally and across the globe from mining, energy and other industries. The mercury eventually settles on land and flows into water. Figuring out how much of the toxin is coming from local industries or wind currents remains a challenge.