That is a very interesting question. if we have any experts of toxicity of Arsenic in the membership please way in on this discussion.'
I would first plant a buffer hedge along the fence and perhaps have the soil tested to see if there is Arsenic in the soil.
Just as a safe estimate after you plant the buffer hedge I would say give it at least 10 to 15 feet. But, definately get the soil tested, Arsenic poisoning is quite nasty.