You might have found it by now, but there's a key to be looted nearby

Yes I went jumping around the nearby lake and found that ... after picking the lock :x
Theres no way to disarm the trap though, so I save scummed to get the staff inside as I like to play with two wizards.
I gave the staff from the chest to Gale, meanwhile I've now dumped Con on my main wizard to 10, and he needs to use the resistance on hit staff from the old lady in the druid camp.