Along one side of my vegie patch and seperated by a gravel path is a retaining wall of treated timber sleepers. It was not my choice of material and I kinda worry about run off getting in to the beds close to it. I have been thinking about having it redone in stone.
But if I am interpreting your post correctly if there is enough Potassium, sodium, calcium etc. available the plants will not take up the arsenic even if it is there in excessive levels.