I'm here, now anyways! I was on vacation Friday and I don't have a PC at home.

I have to say, it's really nice to be breathing such clean air, having an entirely new duct system. I was in the crawl Saturday vacuuming work debris from the vapor barrier, then decided to vacuum the whole thing even though I would have to "crawl" as the height gradually diminished. And I noticed something that could shed more light on this whole situation. The crawl takes water. I'd guess about 40% of the earth below the double vapor barrier is wet after a heavy rain we'd had recently (double because I laid a black one when I bought the place 8 years ago and when my HVAC guy put in new heat pump last year he laid a white, thicker mil over the black one). You can easily see it because you can see a shiny translucence of the black plastic when you put weight on the white plastic. I confirmed by pulling it up in a few places and then touching it.

So could it be that all this moisture is what caused the oxidation, and not the MA leaching from either a defective cap or some normal outgassing at all? But then you have to consider, if there's a double-layer of vapor barrier isn't it keeping the moisture from gravitating upwards/through the two layers of plastic? The black mold in the main trunk line could've been because it was old and leaky, and in Alabama we do have a lot of humidity along with the increased humidity from the damp earth below the vapor barriers, plus the pool is only 20 feet away which would serve to potentiate all this humidity assuming it can work its way into the crawl.

So I guess I will never know for sure what caused the incredible oxidation, or if it was a mixture of the two. Since I can't afford $5-10K to have a foundation specialist fix the problem, I'm going to install a dehumidifier in the crawl with a direct line (whose specs are for some 3600 sq. ft. when the crawl is about 1100 sq. ft.) and hope that this will keep everything good and well. It will run a lot, so I probably will have to replace it every couple of years.

Since the new trunk line is commercial grade and is a solid metal unlike the old one which was made of fiberglass which is a perfect environment for mold, and additionally is wrapped, and since all ducts are new and everything is sealed so well along with a UV light, I think I should be fine there--my HVAC guy said nothing will be able to grow in that environ. Upon inspection of the joists I see no rot whatsoever, and the walls are made of concrete block and there is no visible mold on them, so I'm hoping to not have any more nightmares in my lifetime. Statistically, at 55 that's not a crazy amount of time. But you can bet that I will go inspect the crawl on a monthly basis now that I am so aware of the potential for problems.

Even so, my MA and bleach will now be stored in open milk cartons *behind* the pool house, in the open air and in the shade, well away from the upper yard and pool.

Thanks everyone for weighing in!