If a person were really interesting in collecting rainwater and had a few extra dollars, the simplest thing to do would be to collect some clean (after the first hour of rain, say) water and take it in to some lab that specializes in rainwater. First, ask them how to collect it properly (they may want it in glass, which is quite stable). Then tell them your worries and ask them to test it for the things your roof/area is likely to be contaminating the water.
You could call your local Cooperative Extension Service office (see
http://www.csrees.usda.gov/Extension/) or look in your phone book under Water Analysis & Testing.
All the guessing and worrying in the world isn't going to give you the answers you want.
Also keep in mind that rainwater is twenty times more pure than the purest groundwater.
Sue