Well, some friends and I had a weekend FTX this last weekend. Where we were going, there was no groundwater to filter, so water had to be carried in.
Since there was an expected 10-15 people (only 8 showed, go figure), and two days' time, the expected water requirement was 20-30 gallons.
We personally have two 35 gallon blue barrels, which originally had soda syrup in them, in addition to several 5 gallon blue "jerry can" style cans. I had set up a bung plug with a standard outdoor type faucet on it, which could simply be screwed into the large bung hole on the barrels to facilitate getting the water from them.
We had filled them and treated them back in late '98 or early '99, and they had been in storage ever since. When we moved in 2000, we took them full, as they had only recently filled them. In the house, one was in a closet under a blanket, and the other was in the dining room, against the wall, next to a window, but not in front of it. Direct sunlight does not ever come in this window.
The plan was to take one of these barrels, along with the faucet adapter. Of course, the "cool factor" was using the water that had been in storage for over 12 yrs.
Since the last thing I wanted was everybody behind trees with the runs from the water, I opened one of the barrels, the one from the dining room, to test this water first. I looked inside with a regular flashlight, and could see all the way to the bottom, seeing the blue of the bottom. I poured some into a glass. It was clear, and after about 5 minutes, some air bubbles were forming on the sides of the glass. It tasted a little "flat", but not bad. It did have a solid hint of Dr Pepper (lol).
To be sure, I got a brighter light (100 lumens), for a better look. Sure enough, some greenish brown algae along about 1/4 of the diameter on the bottom.
Oh, well, no 12yr old water for us.
So, I got the other barrel from the closet. It had been in pretty much total darkness for 12yrs.
It was bad. Lots of strange spiderweb type white milky things floating in it.

Needless to say, both were dumped and the barrels washed and bleached.
However, it was a good test of what happens with very long term storage of water.
Oh, the barrels still smell of Dr Pepper, and the one I took flavored the water of Dr Pepper by about halfway through the weekend.