Librado Romero/The New York Times

It’s hard to fathom how one sentence has sparked nearly 40 years of heated controversy and relentless litigation. But when the sentence in question is part of the Clean Water Act and its application in practice has a potential price tag of $100 billion to some of the largest power plants in the country, it’s even harder to imagine how the issue will actually ever be resolved.

The troublesome sentence concerns the regulation of cooling water intake structures, and is fondly referred to as CWA 316(b).

Cooling-water intake structures are found in most large industrial facilities, including in chemical manufacturing plants and in petroleum refineries and thermal electric power plants whose turbines are turned by steam. The heat needed to generate the steam can rise to dangerous levels if the machinery itself is not cooled down. Such facilities usually use nearby bodies of water to do that.

Fish and other marine organisms are affected by the process. As the plants draw millions of gallons of water from lakes, streams and oceans, they suck fish and shellfish and their eggs into the plant or factory’s cooling system, where the marine organisms are usually killed by heat, physical stress or chemical exposure. This is called entrainment.

Larger marine organisms are killed or injured when they become pinned against an intake screen — trapped by the force of the water flowing past them. This is known as impingement.

The Clean Water Act requires that “any standard for cooling water established shall require that the location, design, construction and capacity of cooling water intake structures reflects the best technology available for minimizing adverse environmental impact.”

The tricky part of this sentence is the phrase “best technology available.” The Environmental Protection Agency has been sued several times over the years over the permissible interpretation of this language. In 2009, the Supreme Court ruled that it was permissible for the E.P.A. to weigh costs against benefits when deciding what technology is best — in essence, choosing the best technology available and affordable.

In accordance with this decision, the E.P.A. published its latest draft rule in April 2011. In a comment period that ended on August 18, the Electric Power Research Institute (E.P.R.I.) a nonprofit company that conducts research and development relating to the electric power industry and receives 63% of its funding from investor-owned utilities, submitted thousands of pages of comments.

There is a chance that the final version of CWA 316(b) would mandate all thermal electric generating plants nationwide to switch over to so-called closed-cycle cooling systems.

While these cooling systems would essentially eliminate both impingement and entrainment, according to E.P.R.I. such upgrades would cost industry an estimated $100 billion. and also result in a loss of 42,000 megawatts of power from plants which would not be able to afford to retrofit or don’t have enough land to comply. E.P.R.I. sees “technical merit” in a more flexible, site specific approach.

EPA’s final rule is due out on July 27, 2012. The next round of implementation or litigation can then begin. Forty years and counting.