The graffiti is on natural stone/rocks, is 100 feet away from the closest dirt road and a quarter mile away from the closest outlet and water source. 

My scope will have to be approved by the State Parks department and the EPA before I can begin the work so chemicals, drainage, etc. will all be a significant factor. 

I was thinking about a wire brush on a high speed grinder or some chemical free use of friction. Any recommendations are greatly appreciated.