They said 26 people were treated at hospitals, including a 3-year-old who was "under severe condition," after the incident at the children's pool at Six Flags Harbor Splashdown in Spring, Texas.
The Harris County Fire Marshal's Office tweeted that it was investigating the source of the leak while monitoring air quality at the water park outside Houston.
"People affected are experiencing minor skin and/or inhalation irritation," the office said. It did not elaborate on what the decontamination procedure entailed, NBC News reported.
The water park was closed for the remainder of the day as a precaution, said Six Flags spokeswoman Sandra Daniels in a statement.
"A small number of guests in a section of the park reported feeling ill with respiratory irritation," she said. "The safety of our guests and team members is always our highest priority and the park was immediately cleared as we try to determine a cause."
The park fully reopened July 3 after a pandemic-related closure.