MODELING CO2 DEGASSING AND PH IN A STREAM-AQUIFER SYSTEM

Final Creek, Arizona receives an inflow of ground water with high diss olved inorganic carbon (57-75 mg/l) and low pH (5.8-6.3). There is an observed increase of in-stream pH from approximately 6.0-7.8 over the 3 km downstream of the point of groundwater inflow. We hypothesized th at CO2 gas-exchange was the most important factor causing the pH incre ase in this stream-aquifer system. An existing transport model, for co upled ground water-surface water systems (OTIS), was modified to inclu de carbonate equilibria and CO2 degassing, used to simulate alkalinity , total dissolved inorganic carbon (C-T), and pH in Pinal Creek. Becau se of the non-linear relation between pH and Cn the modified transport model used the numerical iteration method to solve the non-linearity. The transport model parameters were determined by the injection of tw o tracers, bromide and propane. The resulting simulations of alkalinit y, C-T and pH reproduced, without fitting, the overall trends in downs tream concentrations. A multi-parametric sensitivity analysis (MPSA) w as used to identify the relative sensitivities of the predictions to s ix of the physical and chemical parameters used in the transport model . MPSA results implied that C-T and pH in stream water were controlled by the mixing of ground water with stream water and CO2 degassing. Th e relative importance of these two processes varied spatially dependin g on the hydrologic conditions, such as stream flow velocity and wheth er a reach gained or lost stream water caused by the interaction with the ground water. The coupled transport model with CO2 degassing and g eneralized sensitivity analysis presented in this study can be applied to evaluate carbon transport and pH in other coupled stream-ground wa ter systems. (C) 1998 Elsevier Science B.V. All rights reserved.