LOS ANGELES (AP) -- The head of the U.S. Environmental Protection Agency said Wednesday it would likely tighten drinking water standards to address potential health risks of a carcinogen recently detected in the tap water of 31 cities across the country.

EPA Administrator Lisa P. Jackson said she was troubled by the prevalence of the chemical hexavalent chromium, also known as chromium 6.

Jackson detailed a series of actions to be taken by the EPA, including working with state and local officials to determine how widespread the contaminant is and issuing guidance to all water systems on how to test for the carcinogen.

Jackson said the agency will likely tighten standards based on a draft chromium 6 risk assessment the agency released in September that is expected to be finalized next year.

"EPA has already been working to review and incorporate the groundbreaking science referenced in this report," her statement read. "However, as a mother and the head of EPA, I am still concerned about the prevalence of chromium 6 in our drinking water."

The Environmental Working Group released a study Monday that analyzed drinking water across the country and found the five cities with the highest levels of chromium 6 were Norman, Okla.; Honolulu, Hawaii; Riverside, Calif.; Madison, Wis.; and San Jose, Calif.

Following the report, Jackson met with 10 U.S. senators to discuss the issue. That same day U.S. Sens. Barbara Boxer and Dianne Feinstein sent a letter to the EPA urging the agency to move quickly to set drinking water standards for the carcinogen.

The senators called the agency's current chromium standard outdated because it was set nearly two decades ago. The agency currently requires tests for total chromium levels in drinking water, but the results don't show precise amounts of chromium 6.

Boxer, a California Democrat who chairs the Senate environment and public works committee, welcomed the agency's new push.

"The EPA is being vigilant and acting properly to address the issue of chromium 6," Boxer said in a statement to The Associated Press.

Following the tap water survey, Boxer and Feinstein, another California Democrat, called on the EPA to move quickly to protect the public from hexavalent chromium.

The senators said they planned to introduce legislation that would set a deadline for the EPA to establish an enforceable standard.

Studies show that chromium 6 can cause cancer in people and has also been found to cause damage to the gastrointestinal tract, lymph nodes and liver of animals.

The federal government's current total chromium standard is 100 parts per billion. California has proposed a goal for safe limits for chromium 6 at 0.06 parts per billion.

Ken Cook, president of the Environmental Working Group, said he was surprised federal officials prioritized the issue during the busy end of the congressional session and the holidays.

"That signals that they're taking it quite seriously," he said.