The influence of water-rock interaction on the chemistry of thermal springs in western Canada

A comparison of new data with historical records indicates that the chemist ry of thermal springs from the Canadian Cordillera is constant through time , suggesting that water compositions develop equilibrium with the host rock . A thermodynamic model is used to evaluate the influence of water-rock int eraction on the chemistry of thermal spring waters. An isotope mass-balance approach Is used to evaluate biological controls on the S and C cycles in the springs. A comparison of mineral stability with water compositions suggests that the activities of major cations are controlled by equilibrium reactions with c ommon rock forming minerals and alteration products. Sulfur has a complex r edox history in thermal springs. Sulfate derived from dissolution of evapor ite minerals is reduced by bacteria, causing the production of HS-. The los s of HS- from the system appears to be minor, instead it is reoxidized to S O4 as the spring water ascends to surface, Calculations indicate that the a mount of SO4 that is reduced and reoxidized varies from 0 to 53%. There is an inverse relationship between the proportion of biological cycling of SO4 and the concentration of SO4, indicating that SO4 is not a limiting nutrie nt in hydrothermal systems. In low alkalinity thermal springs, HCO3 is deri ved from either dissolution of carbonate minerals or oxidized organic matte r. However, for high alkalinity springs (>100 mg/l) HCO3 is dominantly deri ved from carbonate dissolution. (C) 2000 Elsevier Science Ltd. All rights r eserved.