Yeah, look up how they weather sealed utility poles in the 80s and earlier. Probably creosote. Pretty nasty stuff. Sure does work, though!
Back in my construction management days, one of our clients was the owner of a high rise downtown. Their lead engineer was an older guy who'd been there for decades. He liked to tell me about how he spent the beginning of his career sourcing asbestos fireproofing for the steel as the building was being erected and now he's spending the end of his career taking it all back out.
Same thing happened to my dad. Spent his early career building a nuclear plant and the end of it taking that same plant apart.
Wouldn't want you to suffer the same fate with your bog.
Yeesh! You're so right!
The reason I thought about using that pole to begin with is that I noticed how well the wood has held up the past 20 years (or more, since it was already out of service when we bought the place). I don't think it's creosote; I would have recognized the smell. But
something has certainly preserved it, and whatever it is could well leach into the pond, even if it's on the outside of the bog. The bog will be a good three meters away from the pond, but it's on a higher level. Now it's possible that after all these years, whatever could leach out of the wood has already done so, but why take a chance?
So probably I'll just go with cinderblocks for the inside construction and rock for the facade, and the pole will live to light another day (or not, since there's no current going to it). In any case, this decision will make His Honor the Chef happy, as he tends to get nervous whenever I pick up a chainsaw. Hides in his office mostly, but that's because there's no room under the bed.