Over the years I had the occasion to ask several caretakers at various location about how they treated their water. Off the top of my head, Roaring Brook BSP - No treat, source is pipe upstream off of Roaring brook; Chimney Pond - No treat, pipe up to spring along Cathedral Trail, RMC - No treat - spring between Gray Knob and Crag. Liberty Spring - No treat - spring. Guyot - No treat - Spring. One major exception was the RMC Log Cabin - there was a specific warning posted to treat the water as its in the water shed of at least the Gray Knob outhouse and possibly Crag camp. Despite these devices being solar powered composters, they like many composters have issues with excess liquids and on occasion they have to drain it some. They do bleach the liquid but ultimately its disposed of in the watershed. Unlike Guyot the other locations have little human use while the Guyot water source is surface water draining though a fairly small area with potential for human waste upstream.
Almost all had the disclaimer that they recommend treatment
The background for backcountry treatment is a federal law from quite a few years ago that mandated treatment of surface waters for public water sources. The definition of pubic water sources was quite broad. Many community water systems were abandoned rather than take on the liability for water quality. Frequent testing is required and if its not done the party in control gets sued. AMC elected to drill ground water wells at all the huts rather than deal with the regulations. This was an expensive proposition as they had to fly in a well drilling rig in pieces to each hut. Even with so called ground water wells, I have tasted some sort of strong chemical treatment at LOC and Zealand. Unfortunately folks with sensitivity to water contaminants may be exposed to something in the water so the easiest approach is to officially recommend treatment to the public and then allow the employees to ignore at their peril.
I know folks who have "drank water all their lives out of streams" with no issues and gotten Giardia. There is a yearly debate on whiteblaze on thru hiker water treatments, plenty of folks proudly announce they didn't treat but frequently others indicate that they got giardia and lost a few days off the trail because of it. There is also the rather icky argument to some that the primary source of Giardia is poor sanitation and the fecal oral route is the typical vector. It may be a vector but it means its in the general population for someone is bringing it into the woods. Luckily backcountry waterborne virus's are not an issue in the US but I have talked to some international hikers who are advised to filter and then chemically treat due to the potential for some really nasty viruses endemic in the water supply. Sadly Norovirus outbreaks are now a yearly occurrence on the AT but this is usually a surface contact issue.
