Not sure to be honest,it was from when i used to clean fascia and gutters,i`ve washed my car with it in diluted form for years,just that this driveway is quite new so the stains are niticeable,they are fadeing but won`t jet wash out completely,plus once the grounds wet i cannot see the stains at all,bit of a pain when you can`t see what your cleaning,ha ha,maybe a few frosts will get it out,thanks anyway,dan