The short answer is no problems. There has been controversy over treated pine in the past because it has used arsenic and copper to preserve it. If burned, the compounds of these elements are released into the atmosphere. If buried in refuse tips, the concern was them leaching into ground water. As a result, different, less toxic preservatives are being used by many manufacturers now. Irrespective, the old treated pine was used in a lot of kids’ outdoor playgrounds and to the best of my knowledge, no problems have been substantiated.
You can always ask what they use in the wood you intend to buy. If not happy with at, then you can use the traditional soaking layer of creosote, with a bit of kero to help it penetrate, for that section of pine going below ground.
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