Just my opinion, but I would say you have nothing to worry about. As Thomas said, with the age of the wood, any toxins would most likely be gone by now. I read a study years ago about test being done on treated wood and any leaching was limited to the immediate contact point with the ground and the plant uptake of the toxins was extremely minuscule. I believe you take in more harmful chemicals with every breath you take than you would ever get from treated wood.
