The first barrel was found Thursday on a ranch in the town of Penitente Abajo, about 40 miles (62 kilometers) from the capital.
After three days counting the bundles of $100, $50 and $20 bills, authorities announced that it contained about $9 million in U.S. dollars, which the Central American nation uses as its currency.
Another plastic drum was uncovered Saturday about 5 yards (5 meters) away, also crammed with money. A count of the second stash was under way.
"It is possible that there are more, and we are obliged to conduct a detailed investigation," said Carlos Ascencio, director of the National Civil Police.
The Attorney General's Office said residents reported "frequent, strange movements" in the area.
The money was found with the help of the Drug Enforcement Administration, according to the U.S. Embassy in San Salvador.