Rate of Sulfuric Acid Formation in Yellowstone National Park

Abstract

Sulfuric acid forms near sulfurous hot springs as the result of oxidation of hydrogen sulfide exhalations by atmospheric oxygen. This strong acid rapidly alters the surrounding rocks and can destroy man-made structures and contaminate streams. Four tracts of acid-altered ground in Yellowstone National Park were studied in order to determine the rate at which sulfuric acid is forming. Although the size of the hot-spring areas varied by as much as a factor of 19, acid production was nearly uniform at about 10 grams per square meter of area per day. The near constancy of acid production per unit area implies that the area of land surface is a major control of the oxidation reaction of sulfide to sulfate. This is consistent with a biological origin for the acid by aerobic sulfur-oxidizing bacteria living close to the land surface. Laboratory rates of acid production for sulfur-oxidizing bacteria are as much as 200 times greater than the rates measured in Yellowstone National Park. A strictly biological origin for the acid is, therefore, quantitatively feasible. The data gathered in this study, however, do not rule out the possibility of the formation of natural sulfuric acid in hot springs by inorganic processes.