I live over an underground "river" called 9 mile run.
Rain barrels are provided free or at a heavy discount to people living over it. They want to keep rain water from running down the street, collecting toxins, and ending up in 9 mile run.
Most people use the collected rain water to water their gardens, and I think this is great. You save water and help the environment.
My concern is the safety of the water itself on my plants though. Is water off my roof going to contain tons of toxins that I am trying to avoid by gardening organically?