I agree with JoeM.
Something to think about is that when you decide where to get your grapes it's a good idea to take a look at the condition of the vineyard and the winery/cellar. A good healthy vineyard and well maintained winery inspire more confidence than those that are less well maintained. During the crush wineries are pretty busy and there'a lot of fruit laying around in crates and such so pick a grape or two and taste them. If you like them and they're not nasty looking go for it.
Also, consider the surrounding area of the vinyards and winery. If they're close to big highways, factories, nasty green looking ponds etc. I'd move on to the ones that are in areas further off from civilization. This in no way guarantees anything, but, the less nasty stuff blowing around the vineyards the better in my book since it will ultimately settle on the grape, soil, etc.
Hope that helps,
Oskaar