The turquoise lake in the crater of the Kawah Ijen volcano looks serene and inviting. It also happens to be the world's largest acidic lake. The water in the crater lake has a pH less than 0.3 on a scale of 0 to 14 (7 is neutral). For comparison, lemon juice has a pH of 2; battery acid has a pH of 1. That acidity affects the chemistry of nearby river ecosystems, including the river Banyupahit.
Originating at the lake, the Banyupahit delivers acidic water to populated areas downstream. According to a 2005 research paper, the river water that local farmers use to irrigate crops has a pH between 2.5 and 3.5. The normal range for stream water, according to the U.S. Geological Survey, is between about 6 and 8.
The plume drifting west from the crater likely comes from fumaroles, which release hot gases from underground magma. The plume could also come from hot springs and mud pots, according to Erik Klemetti, a volcanologist at Denison University. The plume is white, so it is likely mostly steam with some other volcanic gases like carbon dioxide and sulfur dioxide mixed in.
Image courtesy of NASA.