It is U.S. Forest Service wilderness and has never been logged or burned yet. 150-200 foot Ponderosa Pines, often only 10-20 feet from eachother, smaller trees, oaks and thorny bushes make up the rest with huge granite boulders and debris from mudslides and rain. Of course, there are old fallen dead trees everywhere, some with trunks 4 feet across. Arroyos (dry creek beds) criss-cross the area It is also at around a 20% grade. No vehicle could ever make it up there, unless a bulldozer preceded it. Walking is a challenge. There is a remnant of a road which was cut in to install the well, but again, fallen trees, live new trees, arroyos, and debris make the road (what is left of it) impassible.
I do like the idea of ground contact pressure treated lumber. My yard has it, but only KDBT (Kiln Dried Before Treatment) and won't cut it, because it has a high moisture contact (they have SawStop saws), so it will shrink like crazy, which is OK for posts, but probably not for a structure. I've heard of KDAT (Kiln Dried After Treatment), but none of the yards have it here. Regular pressure treated is kiln dried at my yard. So I think we are going with that.
The well is forty feet horizontally so any leeching or evaporating sealants will not go near the aquifer.
Given the advice so far, I am back with ordinary pressure treated lumber and sealants, be it paint, black jack, or flex seal. I can wrap the back of the structure where it makes direct contact with the mountain in EPDM rolled roofing membrane. Putting a membrane on the bottom 2x6 seems like it would be futile.
Bear in mind that what was there from 1980 is still there, albeit pretty rotted.
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