why would they keep 1.2 million gallons of this sitting around in liquid form? (22k x 55gals)
It’s probably the content of the spent fuel pool. Spent fuel that is withdrawn from the reactor is screaming hot - both thermally and radiation-wise. It is kept underwater in the reactor containment until the heat and the radioactivity die down and it can be put into dry storage.
The spent fuel pool is like an olympic-sized pool, only deeper.
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.