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 chemistry 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 interaction 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 common rock forming minerals and alteration products. Sulfur has a complex redox history in thermal springs. Sulfate derived from dissolution of evaporite minerals is reduced by bacteria, causing the production of HS−. The loss of HS− from the system appears to be minor, instead it is reoxidized to SO4 as the spring water ascends to surface. Calculations indicate that the amount 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 nutrient in hydrothermal systems. In low alkalinity thermal springs, HCO3 is derived from either dissolution of carbonate minerals or oxidized organic matter. However, for high alkalinity springs (>100 mg/l) HCO3 is dominantly derived from carbonate dissolution.