The proposed stadium site for David Beckham's Inter Miami franchise in the MLS has arsenic contamination levels more than twice the legal limit, says an environmental report.
Melreese golf course has been closed until further notice by Miami's city council after the findings.
Plans are in place to build a stadium, shopping mall, hotel and public park on the site as part of a $1bn development.
"This obviously causes great concerns," Miami Mayor Francis Suarez said. "Basically, the site has significantly more contamination than is commercially reasonable.”
Long-term exposure to the chemical arsenic can lead to a variety of chronic health conditions, including skin disorders, cancers of the lung and bladder and cardiovascular issues.
Environmental firm EE&G, commissioned by Inter Miami, found barium and lead levels also exceeded legal limits. The site used to be a waste site for incinerator ash.
A deal to build a stadium for Inter Miami on the proposed Miami Freedom Park has not yet been finalised.
A Inter Miami spokesperson said: "Miami Freedom Park has continued to move forward with the various studies needed for the redevelopment of the site. Our legal team and engineers have been meeting with city commissioners and staff about the environmental report our consultants conducted on the city's land. We will be working with the city to implement a remediation plan that is thorough and provides residents and visitors a destination that is safe for all to enjoy."
Inter Miami are set to join the MLS in 2020 as a new franchise and will initially play in an 18,000-seat arena in Fort Lauderdale.