Mosaic started to test wells at the request of homeowners after a sinkhole at the Mulberry plant leaked slightly radioactive water into the ground.
James Maxwell, a resident near the plant, called Mosaic to have the company test his well for free.
"I'm 75," Maxwell said. "I've lived my life, but I've got grandchildren. And I worry about what this (will) do to them."
Almost 30 other residents also opted to have Mosaic test their wells by a third party. The company is looking for sodium, sulfate, fluoride and radioactivity.
Mosaic executives said no contaminated water made its way from the plant to private wells, but Maxwell said he doesn't buy it.
"I just don't trust them," Maxwell said. He said he may have trusted Mosaic if the company revealed the sinkhole before three weeks had passed.
Mosaic told the U.S. Environmental Protection Agency and the Florida Department of Environmental Protection, but the company kept the public in the dark.
Each private well testing costs more than $1,000.
Mosaic said more than 200 million gallons of slightly radioactive water seeped into the ground when the sinkhole opened under a phosphogypsum stack on Aug. 27.