Seems like you have to purge them for a while before you eat them.
They eat a lot of garbage.
I know a fellow who eats a lot of fresh water clams and mussels. He catches them and tosses them in a bucket of clean water. Throws in a handful of rolled oats.
After a few cycles of clean water and rolled oats he deems them okay to eat.
I think snails are the same type of deal. Maybe pesticide and herbicide free vegetables or something for a while.