Tritium occurs naturally and as a byproduct of nuclear plants. It emits a weak form of radiation, but people exposed to it may face increased risks of cancer or pass on genetic abnormalities.
One well at the Catawba plant had a tritium concentration twice as high as the federal government says is safe in drinking water.
Duke says the contamination poses no threat to the public because it is confined within the plant's boundaries.
The S.C. Department of Health and Environmental Control will sample water from about two dozen residential wells near the plant, spokesman Thom Berry said.
At least six other nuclear plants, none in the Carolinas, have reported tritium leaks in recent years. The Nuclear Regulatory Commission says the leaks posed no threats to public health, but revised inspection procedures to ferret out potential leaks.
Nuclear reactors produce tritium from the use of a chemical, boron, to help control the chain reaction that produces heat. Boron is also added to the water in which fuel cools after it has been used in a reactor.
Under an industry initiative, Duke spokesman Valerie Patterson said, Catawba installed 30 new wells to monitor groundwater at the plant.
One of those wells - not used for drinking water purposes - detected the concentration Duke reported.
"We have no reason to believe, based on other testing of other wells, that they have elevated levels," Patterson said.
Duke's McGuire nuclear plant on Lake Norman has installed 41 wells and will add nine. It was unclear whether reportable levels of tritium had been detected there.
The Oconee plant in northwestern South Carolina will install 28 wells later this year, Patterson said. Duke said it's investigating the source of the leak. Other nuclear plants have traced leaks to spent fuel pools and to valves.