Question
i live in the country in stanislaus county, (modesto, ca.). i recently had a tree service grind a black walnut tree stump that was quite close to my cement driveway. the same day the stump was removed, my driveway developed a rust colored type of substance on it and i cant seem to wash it off. can U tell me what this might be and what to use to clean it off?

Answer
The tannic acid or tannin is present in woods like oak, walnut, and mahogany. When the stump was ground the sawdust contained the tannic acid and when it got wet it stained the drive. Here is a web link to information on removing this stain from cement. http://www.ehow.com/how_6225566_remove-cement-caused-black-walnuts.html