THERMODYNAMIC INTERPRETATION OF WATER CHEMISTRY FROM SPRINGS IN THE BIG CHICO CREEK ECOLOGICAL RESERVE

Multidisciplinary research at the Big Chico Creek Ecological Reserve (BCCER) in Butte County, California, includes collection of field chemical data and major element analyses for water from diverse low discharge springs emerging from the Chico Fm (Cretaceous near shore and deltaic sedimentary rocks) and the Tuscan Fm (Pliocene lahar and volcanogenic sedimentary rocks). Thermodynamic interpretation of spring water chemistry provides an understanding of equilibrium aqueous speciation, controls on major element concentrations, saturation states with respect to minerals and gases, stability of pH and aqueous carbonate speciation in samples, oxidation potential disequilibrium, and other hydrochemical characteristics. BCCER spring waters are dilute (ionic strength = 10^-2 to 10^-3) with near neutral pH. Temperatures vary seasonally from 11 to 20°C. Ionic contents of Chico Fm springs are dominated by sodium bicarbonate and Tuscan Fm springs by calcium bicarbonate. Tuscan Fm springs have higher silica contents near equilibrium with amorphous silica. Chico Fm springs have higher ionic concentrations and are undersaturated with respect to amorphous silica. Tuscan Fm springs mainly have pH < 7 and are undersaturated in calcite; Chico Fm springs mainly have pH > 7 and are saturated in calcite. Lower pH corresponds to relatively high calculated dissolved CO₂ contents, exceeding atmospheric pressures, which occurs particularly during seasonally wet conditions. Oxygen fugacities are 10^-44 to 10^-28 bar based on electrode measurements and are generally lower in Chico Fm springs. However, measured dissolved oxygen contents approach atmospheric O₂ pressure. These chemical data and thermodynamic interpretations contribute to understanding gas-water-rock interactions such as chemical weathering, groundwater recharge and flow, controls on surface water and groundwater quality (e.g., mixing), spring environment ecology, and other critical zone processes.