It may cost up to $2 billion to clean up toxic firefighting chemicals that have leaked from more than 400 U.S. military installations, including Fairchild Air Force Base, a group of Democratic senators said Tuesday in a letter to the Senate Appropriations Committee.

The senators, including Patty Murray and Maria Cantwell of Washington, attributed that cost estimate to U.S. Department of Defense officials.

The senators requested a study of the chemicals known as PFOS and PFOA, which were key ingredients in a foam that was used for decades to douse aircraft fires at military bases and civilian airports.

In recent months, it was revealed that the chemicals contaminated groundwater at many of those sites. Dozens of residents who live near Fairchild have learned that the chemical concentrations in their drinking water exceed recommendations from the U.S. Environmental Protection Agency.

Other senators who signed the letter include Michael Bennet of Colorado, Jeanne Shaheen and Maggie Hassan of New Hampshire, Kirsten Gillibrand of New York and Bob Casey of Pennsylvania.

They asked that funds be included in the 2018 budget for the Centers for Disease Control, the EPA and the Department of Defense to study the spread of the chemicals, the health effects and viable alternatives for the toxic firefighting foam.

The chemicals have been linked to cancer, thyroid problems and immune system disorders, although scientists aren’t sure exactly how they interact in the human body.

“Residents of our states are concerned about exposure, and what this means for their health and safety,” the senators wrote. “We urge the Committee to include language directing DOD and the military services to budget robustly for assessment, investigation, and remediation activities in the upcoming fiscal years.”