They can be 'transparent' (opacity), but they can't be 'refractive' in true sense.
That's because Decal is just automatic LayeredMaterial, which isn't physically "layering" anything at all (like real water on top of dry ground), it's just mixing two materials in different ratio, like Photoshop layers..
So certain hacks which look similar must be employed but it will never simulate actual multi-layered mesh behavior.