Modeling CO2 degassing and pH in a stream–aquifer system

Abstract

Pinal Creek, Arizona receives an inflow of ground water with high dissolved 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 that CO 2 gas-exchange was the most important factor causing the pH increase in this stream-aquifer system. An existing transport model, for coupled ground water-surface water systems (OTIS), was modified to include carbonate equilibria and CO 2 degassing, used to simulate alkalinity, total dissolved inorganic carbon (C T), and pH in Pinal Creek. Because of the non-linear relation between pH and C T , the modified transport model used the numerical iteration method to solve the non-linearity. The transport model parameters were determined by the injection of two tracers, bromide and propane. The resulting simulations of alkalinity, C T and pH reproduced, without fitting, the overall trends in downstream concentrations. A multi-parametric sensitivity analysis (MPSA) was used to identify the relative sensitivities of the predictions to six 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 CO 2 degassing. The relative importance of these two processes varied spatially depending on the hydrologic conditions, such as stream flow velocity and whether a reach gained or lost stream water caused by the interaction with the ground water. The coupled transport model with CO 2 degassing and generalized sensitivity analysis presented in this study can be applied to evaluate carbon transport and pH in other coupled stream-ground water systems.

Structural characterization of terrestrial microbial Mn oxides from Pinal Creek, AZ

“…Stream-water chemistry is characterized by high dissolved solids (e.g., 11-14 mM SO 4 À2 and 8-12 mM Ca 2+ ) and elevated dissolved Mn, Co, Ni, Cu, and Zn. Stream water pH increases from about 6 near the head of perennial flow to about 7.8 at Inspiration Dam due to CO 2 outgassing (Choi et al, 1998). Dissolved Mn concentrations in stream and shallow ground water increased from background levels in 1984 to levels approaching 1.3 mM by 1990 with little subsequent change through 1999 .…”

Effects of groundwater flux on open‐channel estimates of stream metabolism

“…Flux of groundwater also must be considered in streams where the effect of groundwater on oxygen mass balance is large relative to rates of biological processes and reaeration, or metabolism will be estimated with error. Odum (1956) considered the potential effects of groundwater flux on the mass balance of oxygen in streams, and more recent work (Choi et al 1998;Jones and Mulholland 1998) has demonstrated empirically that flux of groundwater can greatly affect gas concentrations in streams, but the effects of flux on estimates of stream metabolism have not been examined explicitly. Although practitioners of the open-channel method (e.g., Marzolf et al 1994;Uehlinger and Naegeli 1998;Fellows et al 2001) often have attempted to confine their work to reaches where rates of groundwater flux are low, the potential for error in open-channel estimates of metabolism has not been studied.…”

Variability of dissolved CO<sub>2</sub> in the Pang and Lambourn Chalk rivers

“…While physically-based models for simulation of conservative natural tracers may be used as a starting point in the context of CO 2 (e.g. Choi et al, 1998), parameterisation of such models is difficult (Genereux and Variability of dissolved CO 2 in the Pang and Lambourn Chalk river s Hemond, 1992) and extraneous processes of CO 2 production, CO 2 degassing and carbonate precipitation are still not addressed. Indeed, Finlay (2003), suggests that even the presence of canopy cover above streams can affect the temporal dynamics of dissolved CO 2 significantly.…”