I don’t think OP was saying we should store it that way, just that it’s not as physically as big a problem as people imagine. It’s a question of scale.

We could just leave them all in the storage containers that the plants use and put them in one place and the overall facility wouldn’t be that big.

As for groundwater contamination, you are right. No leaks allowed. This is a consideration for any waste management really (superfund sites, anyone?). Transportation is another issue for consideration. If you move the waste on a roadway, and there’s an accident, you’re going to have to clean up the mess (assuming there is any. We have really amazing containers for shipping) or at least check for any issues, which means every fire department with a major roadway needs to both be trained and have the equipment to handle a radioactive car crash.