not sure about the safe part, i guess it depends on the water quality of the stream youre at.
my son and i sein crawdads to eat every summer.
large pot of boiling water with a bag of Zatarains Crab Boil in the water.
drop the crawdads in the boiling water for between 1 and 2 minutes. remove from the boiling water and put in a strainer and run cold water over them.
put in a closed container and chill in the fridge for a couple of hours.
twist and peel the tail and dip in clarified butter...wow i gotta go get the sein.
p.s. they sure are good in a pot with fresh corn on the cob, red potatoes with a 1/2 pound of real butter and a bag of crab boil