We have 2 above ground pools within 200 ft of a very busy freeway. When I vacuum once a week there is a black powdery something spread very evenly along the bottom. The filter catches it just fine and the water is crystal clear. I 'm just wondering if the water is safe to swim in or not. I'm guessing it's rubber from the tires wearing on the freeway, or maybe from exhaust? Bottom line is it's HOT and I want to jump in this thing! I'd rather not get sick from the water though. Thoughts?