WASHINGTON — Nearly 300 drinking water wells and other water sources in California have traces of toxic chemicals linked to cancer, new state testing has found.

Testing conducted this year of more than 600 wells across the state revealed pockets of contamination, where chemicals widely used for decades in manufacturing and household goods have seeped into the public’s water supply. An analysis by the Los Angeles Times found that within this class of chemicals, called perfluoroalkyl and polyfluoroalkyl substances, the two most common compounds were detected in 86 water systems that serve up to 9 million Californians.

State officials released the water quality results Monday, the first step in what’s likely to be a yearslong effort to track the scale of the contamination and pinpoint its sources. Only a small fraction of California’s thousands of drinking water wells were tested in this initial study. Officials said they planned to examine many more, but have not committed to future statewide testing.

The results offered the clearest picture yet of California’s exposure to a public health crisis that is playing out nationally.

“This has the potential of being an enormously costly issue both on the health side as well as on the mitigation and regulatory side,” said Kurt Schwabe, an environmental policy professor at the University of California, Riverside. “It’s going to be one of the defining issues in California, environmentally, for decades.”