I tried to generate a "story" using Planet imagery: https://www.planet.com/stories/kamtchatka-eco-disaster-4zV9_... The imagery from May 22 seems especially suspicious. Unfortunately there are some large gaps of images available (at least to me!)

edit: apparently I had too many filters on. Here's one with slightly more imagery: https://www.planet.com/stories/more-kamchatka-eco-disater-fr...

A few thoughts: seems to coincide with snowmelt... could be anything in the catchment area. Also, this is a volanic region so there's some possibility that there is some natural deposition of sulfuric acid. The latter also makes for great plausible deniability of course...