Environmental Policy![Figure][1] Wastewater treatment conducted under the mandates of the 1972 Clean Water Act has lead to demonstrably cleaner rivers in the United States. PHOTO: DANITA DELIMONT/GETTY IMAGES Though the U.S. Clean Water Act has been in effect since 1972, evidence of its effectiveness has been unclear. Keiser and Shapiro compile comprehensive data on water pollution and its drivers, including 50 million pollution readings from 240,000 monitoring sites. They find that $650 billion in federal grants to wastewater treatment plants made rivers significantly cleaner for 25 miles downstream for 30 years ($1.5 million per mile per year). But the economic benefit in terms of home values within 25 miles of improved waters increased by only 25% of the cost of the grants. Quart. J. Econ. 10.1093/qje/qjy019 (2018). [1]: pending:yes