The search for contaminated water has expanded north of Fairchild Air Force Base after four wells in the area showed elevated levels of toxic firefighting chemicals.

The perfluorinated compounds, known by the acronyms PFOS and PFOA, were previously reported in groundwater in the area south and east of the base. That’s near a training site where for decades base firefighters practiced dousing flames with a foam in which the chemicals were key ingredients.

In Airway Heights, the chemicals have been found in drinking water at levels far exceeding U.S. Environmental Protection recommendations. For three weeks beginning May 16, residents were advised not to drink or cook with water from city pipes, and the municipal water system had to be flushed of the contaminants.

In a news release Monday, base officials said they would sample roughly 50 more off-base residential wells through next week. The first round of tests found elevated levels of the chemicals in 17 private wells. The second phase found elevated levels in three private wells and two city wells, which is how the chemicals got into the tap water.

Many residents have expressed concerns that their private wells might be contaminated and complained that the Air Force would not test them.

“We make our decisions about areas to sample based upon three criteria: probability the Air Force mission was a source of contamination, our proximity to drinking water sources, and if there is a pathway between our installation and the water supply,” Marc Connally, a remedial project manager from Fairchild’s Civil Engineer Center, said in the news release.

The samples collected by Fairchild officials will take about 45 days to test in a private lab, according to the news release. Residents will be notified of the results, but the base does not publicly release results for each well, or indicate precisely which wells have been tested.

Editor’s note: This story was changed on June 11, 2017. A previous version included a recommendation from Fairchild directing residents to contact the Spokane Regional Health District with questions about private well sampling. Health district spokeswoman Kim Papich said the phone number was provided in error and directs callers to an employee in the district’s IT department.