Volcanic CO2 vents produce shallow - water gradients of pH across tens of meters, reaching values as low as 6.6 — 6.8 in the Mediterranean, for example around the most active vents in Ischia and Vulcano Islands, Italy (Hall - Spencer et al. 2008; Johnson et al. 2013) and pH values < 7.7 (as low as 7.21) in volcanic seeps in Papua New Guinea (Fabricius et al. 2011). (link.springer.com)
Those atop active vents can bubble and overflow during an eruption, spilling boiling water down the volcano's slopes. (scientificamerican.com)
Using the signals, the researchers were able to confirm the location of active vents on Veniaminof and Pavlof. (sciencedaily.com)