A report on the inspection by the Okinawa Defense Bureau was made public Saturday, Kyodo News reported.
The report said contamination by what appeared to be jet fuel and gasoline was found at Camp Kuwae, part of which was handed back to Japan in 2003.
Groundwater likely caused the contamination to spread over a "wide area," the report said.
The United States had no obligation under the Japan-U.S. Status of Forces Agreement to clean the contamination or pay any compensation when it returned the 94-acres piece of land, the report said.
The Okinawa Defense Bureau said it would carry out further tests on the land.