Alvin Young, a retired Army colonel now employed as a consultant by the Defense Department, told Stars and Stripes the barrels probably contained discarded solvents and waste from mess halls and hospitals.
The soccer field is on land formerly used by the U.S. military as a landfill that was returned to Okinawa in 1987.
Young said Agent Orange, the defoliant used as a weapon during the Vietnam War, was shipped there under tightly controlled conditions. He said none was stored on Okinawa.
Japanese news media have reported stories by U.S. veterans of burying Agent Orange on Okinawa decades ago. Tests appeared to show the presence of dioxin but Young said the tests probably detected plastic residue from burning waste at low temperatures.