Production of sulfuric acid in vapor-dominated hydrothermal systems is primarily a bacterial process. The rate of production of sulfuric acid was measured in springs in several acid-altered areas in Yellowstone National Park. Most of these springs lack surface water flow, but water enters and leaves these springs at approximately constant rates via underground seepage. The rate of water exchange in these steady-state systems was measured by enriching the springs with sodium chloride and measuring the rate at which the chloride ion was diluted. In all cases, the added chloride was diluted at an exponential rate, and half-times for dilution were calculated. The rate at which sulfuric acid was being produced was calculated from a knowledge of dilution rate and volume of the springs and from measurement of the sulfuric-acid concentrations of the waters. In several small springs, flow rate was measured more directly by draining the springs and measuring the rate at which water returned.

These studies showed that water in acid springs enters as cold acid ground water, which is steam heated within the source pool. It was possible to estimate how much of the sulfuric acid in a given spring could have been produced in situ, and how much entered by underground seepage. In springs with pool volumes of 2,000 1 or less, most of the sulfuric acid was produced outside the spring, probably by bacteria present in the nearby acid-altered soil. In springs with pool volumes around 106 1, most of the sulfuric acid was produced in situ by resident bacterial populations. The techniques used may have wider utility in biogeochemical investigations.

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