Effect of trapped gas on dissolved oxygen transport - Implications for in situ bioremediation

In situ bioremediation of contaminated aquifers is often limited by the concentration of dissolved oxygen in the ground water. Various methods have been used to increase dissolved oxygen concentrations in ground water, but the effect of a trapped gas phase on the distribution and transport of dissolved oxygen needs to be understood. The two-dimensional transport of dissolved oxygen is investigated in experiments conducted in a large-scale physical aquifer model (2 m X 4 m X 0.2 m) where a gas phase is trapped in the pore spaces of an otherwise-saturated porous medium. The transport of dissolved oxygen is shown to be retarded up to 11.2 times the transport of the bulk water due to the mass transfer of oxygen between the aqueous phase and the trapped gas phase. The theoretical model for dissolved gas transport in the presence of a trapped gas phase is evaluated in a two-dimensional ground-water flow field using the U.S.G.S. numerical model MOC. The results show that dissolved oxygen transport can be modeled with the advection-dispersion equation with linear equilibrium mass transfer but only when the longitudinal dispersion is increased compared to the value determined using a bromide tracer of the water flow. Increased longitudinal dispersion of the dissolved oxygen plume may be due to a temporally or spatially varying retardation factor or rate-limited mass transfer. The presence of even a small amount of a trapped gas phase in an aquifer will significantly affect the distribution and transport of dissolved oxygen (trapped gas filling only 5% of the pore space will cause a retardation factor for oxygen of 2.6 at T = 15 degrees C) and thus should be considered when designing ways to increase the dissolved oxygen concentration in ground water for in situ bioremediation.