The 1972 Clean Water Act has driven significant improvements in US water quality, according to the first comprehensive study of water pollution over the past several decades by researchers at UC Berkeley and Iowa State University.

The team analyzed data from 50 million water-quality measurements collected at 240,000 monitoring sites throughout the US between 1962 and 2001. Most of 25 water-pollution measures showed improvement, including an increase in dissolved oxygen concentrations and a decrease in fecal coliform bacteria. The share of rivers safe for fishing increased by 12% between 1972 and 2001.

Despite clear improvements in water quality, almost all of 20 recent economic analyses estimate that the costs of the Clean Water Act consistently outweigh the benefits, the team found in work also coauthored with researchers from Cornell University. These numbers are at odds with other environmental regulations like the Clean Air Act, which show much higher benefits compared to costs.

“Water pollution has declined dramatically, and the Clean Water Act contributed substantially to these declines,” said Joseph Shapiro, an associate professor of agricultural and resource economics in the College of Natural Resources at UC Berkeley. “So we were shocked to find that the measured benefit numbers were so low compared to the costs.”

The researchers propose that these studies may be discounting certain benefits, including improvements to public health or a reduction in industrial chemicals not included in current water-quality testing.

The analyses appear in a pair of studies published in the Quarterly Journal of Economics and the Proceedings of the National Academy of Sciences.

Read the paper here.