What's the existential risk? The absolute worst case that I can imagine is that all the waste somehow ends up in the ground water. That would be extremely bad, but localized to the area where the waste is stored.
Existential is as you mentioned: ground water pollution of the entire NE seaboard. Terrorist dirty bomb in NYC that affects 30 million people as the weather carries particles etc.
inverse square law and lack of any volume of any highly active source makes dirty bombs a nonissue. they are scarier than they are effective.
Again, waste storage is incredibly secure and safe. It wouldn't leech into the water. Nuclear subs have sank into the ocean, and, surprise, there wasn't a global disaster.