Just because it’s in barrels doesn’t mean it’s in a liquid form. They could contain spent fuel rods, dirty rags, grease, old pipes, etc. Anything dangerous would also be inside another containment vessel within the barrel. A barrel alone would not be a very good storage device for radio active material, it just to protect the inner vessel from damage.
And don’t forget the martians sealed in drums since the 30’s.
As I understand it, a lot of the waste is ordinary PPE and contaminated tools.