| Oxygen, O(,2), and carbon dioxide, CO(,2), and temperature were measured with depth along a transect of an acid, reclaimed coal-stripmine over a two-year period. Spoil atmosphere O(,2) concentrations decrease with depth but approach zero only in a small portion of the transect. Most of the mine remains well oxygenated (O(,2) > 10% by volume) down to 12 meters depth. CO(,2) concentrations ranged from near atmospheric levels to greater than 15%. At some locations, especially within 2 meters of the surface, variations in O(,2) and CO(,2) are correlated with changes in the spoil temperature. Spoil temperatures in layers below 3 meters remain in a range conducive to iron-oxidizing bacterial activity year around. Flux ratios of CO(,2) and O(,2) and source/sink rates of the two gases indicate that carbonate neutralization of the acid produced by pyrite oxidation is the dominant source of CO(,2).;In a second phase of the study, a numerical model describing the production and removal of acid and acid by-products from reclaimed coal stripmines is presented. Both direct oxygen and bacterially catalyzed pyrite oxidation was considered. The pyrite oxidation rate is assumed to be controlled by first-order, solid-liquid kinetics and simple diffusion of oxidant into reactive, coarse, stone fragments. Oxygen supply into the reclaimed profile is considered to be governed by one-dimensional, gas diffusion. Activity of the iron-oxidizing bacteria is controlled by the ferrous-ferric ratio, from which they obtain their energy, and from a combination of potentially inhibiting factors--the oxygen and hydrogen ion concentrations and the spoil temperature. Under all conditions modeled, excess acid, in the form of free hydrogen, must be removed if bacteria are to play an important role in accelerating pyrite oxidation. Leaching of the spoil by normal precipitation is insufficient in removing the excess hydrogen ion. Neutralization and buffering by the host rock are required for prolonged bacterial activity--the degree of hydrogen ion removal determining the maximum sustainable activity. At optimum conditions, bacteria can greatly increase pyrite oxidation. |
