Distribution of hydrothermal features in Yellowstone as a function of their pH

Distribution of hydrothermal features in Yellowstone as a function of their pH (modified and updated from Nordstrom et al., 2009). There are two distinct groupings: one is more acidic, and the other neutral to basic. Also shown are the pHs of several well-known thermal features in Yellowstone National Park and common fluid substances ranging from acidic (battery acid) to highly alkaline (drain cleaner).