RETARDATION OF DISSOLVED-OXYGEN DUE TO A TRAPPED GAS-PHASE IN POROUS-MEDIA

Information on the transport of dissolved gases in ground water is needed to design ways to increase dissolved gas concentrations in ground water for use in in situ bioremediation (e.g., O-2 and CH4) and to determine if dissolved gases are conservative tracers of ground-water flow (e.g., He). A theoretical model was developed to describe the effect of small quantities of trapped gas bubbles on the transport of dissolved gases in otherwise saturated porous media. Dissolved gas transport in porous media can be retarded by gas partitioning between the mobile aqueous phase and a stationary trapped gas phase. The model assumes equilibrium partitioning where the retardation factor is defined as R = 1 + H'(V-g/V-w) where H' is the dimensionless Henry's Law constant for the dissolved gas, and V-g and V-w are the volumes of the trapped gas and water phases, respectively. At 15 degrees C and with V-g/V-w = 0.05, the predicted retardation factors for He, O-2, and CH4 are 5.8, 2.4, and 2.3, respectively. The validity of the model was tested for dissolved oxygen in small-scale column experiments over a range of trapped gas volumes. Retardation factors of dissolved oxygen increased from 1 to 6.6 as V-g/V-w increased from 0 to 0.123 and are in general agreement with model predictions except for the larger values of V-g/V-w. The theoretical and experimental results suggest that gas partitioning between the aqueous phase and a trapped gas phase can greatly influence rates of dissolved gas transport in ground water.