I'm hoping you don't actually have a big pile of carfentail powder, that's a big risk to handle.
What I would do is glove up, goggles, dust mask, and use a milligram scale to weigh out whatever, then dilute it into a LARGE volume of saline. Afterwards wipe everything down with a bleach soaked rag. Discard PPE and soak it in bleach too.
Or, throw it down the toilet... carfentanyl is unreasonably toxic and has caused many overdoses, even from as little as 1mg of drug.
Anyway, the comment about carfent possibly breaking down in water: drugs that contain an ester functionality can react with water to degrade (c.f. heroin, cocaine). It happens at different rates depending on all sorts of things. Carfentanyl could, in theory, break down this way, but I would expect it to do so very slowly. That is, it's not something to be concerned about unless you are planning to store carfentanyl solutions for a long time.