The stormwater drains are pretty much always providing some amount of water, so pumping it dry would mean blocking the pipes, which I don't think I can do.
There is no plastic liner, so it must be clay. In the summer if there hasn't been rain for a while, it dries up quite a bit, so I could potentially excavate then. The reason why I wanted to go the vacuum route is that I live in the suburbs with several neighbors and that way is "cleaner." But if that isn't an option, I guess excavation is the way to go.
