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

Information on the transport of dissolved gases in ground water is nee ded 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 det ermine if dissolved gases are conservative tracers of ground-water flo w (e.g., He). A theoretical model was developed to describe the effect of small quantities of trapped gas bubbles on the transport of dissol ved gases in otherwise saturated porous media. Dissolved gas transport in porous media can be retarded by gas partitioning between the mobil e 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 f or the dissolved gas, and V-g and V-w are the volumes of the trapped g as 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 d issolved oxygen in small-scale column experiments over a range of trap ped gas volumes. Retardation factors of dissolved oxygen increased fro m 1 to 6.6 as V-g/V-w increased from 0 to 0.123 and are in general agr eement with model predictions except for the larger values of V-g/V-w. The theoretical and experimental results suggest that gas partitionin g between the aqueous phase and a trapped gas phase can greatly influe nce rates of dissolved gas transport in ground water.