I think that depends on the course. My track is built on sand and so the bunkers are full of whatever they happen to find when they dig a hole. No sand is brought it. So its all quite natural and is probably quite good for the greens I would think, although over time the lips will increase in height and retreat from the green but that is slow
Im not what is in imported sand, but Im guessing most is washed and so is likely to be pretty clean and pure, but I can certainly see the potential for contamination if not careful