MIAMI — When Florida’s largest power company added two nuclear reactors to an existing plant that sat between two national parks — Biscayne Bay and the Everglades — the decision raised the concerns of environmentalists and some government officials about the possible effects on water quality and marine life.

Now more than four decades later, Florida Power & Light’s reactors at Turkey Point, built to satisfy the power needs of a booming Miami, are facing their greatest crisis. A recent study commissioned by the county concluded that Turkey Point’s old cooling canal system was leaking polluted water into Biscayne Bay.

This has raised alarm among county officials and environmentalists that the plant, which sits on the coastline, is polluting the bay’s surface waters and its fragile ecosystem. In the past two years, bay waters near the plant have had a large saltwater plume that is slowly moving toward wells several miles away that supply drinking water to millions of residents in Miami and the Florida Keys.

Samples of the water at various depths and sites around the power plant showed elevated levels of salt, ammonia, phosphorous and tritium, a radioactive isotope that is found in nature but also frequently associated with nuclear power plants. The tritium, which was found in doses far too low to harm people, serves as a marker for scientists, enabling them to track the flow of canal water out from under the plant and into the bay. The tritium levels in December and January were much higher than they should be in ocean water.