Retardation of Dissolved Oxygen Due to a Trapped Gas Phase in Porous Media

Abstract

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., O2 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′(Vg/Vw) where H' is the dimensionless Henry's Law constant for the dissolved gas, and Vg and Vw are the volumes of the trapped gas and water phases, respectively. At 15°C and with Vg/ Vw= 0.05, the predicted retardation factors for He, O2, 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 Vg/Vw increased from 0 to 0.123 and are in general agreement with model predictions except for the larger values of Vg/Vw. 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.

Gas Transport below Artificial Recharge Ponds: Insights from Dissolved Noble Gases and a Dual Gas (SF6 and 3He) Tracer Experiment

“…At 5% air, 88 and 83% of these gases will partition into the air volume. Retardation coefficients calculated using eq 1, the equilibrium model of Fry et al (9), are greater for SF6 than for He and exceed 2 when the amount of trapped air is greater than ∼1% of the pore volume (Table 1).…”

“…Both SF6 and He have high Henry's Law coefficients, respectively, 143 and 95 at 17.5°C (9), and partition strongly into the air. At equilibrium, approximately 60% of the mass of SF6 and 50% of the He will partition into the gas phase when the pore space contains only 1% air (Table 1).…”

“…Therefore, the expected gas transfer and partitioning between infiltrating water and soil air will act to retard (slow) the movement of the gas relative to the water through the unsaturated zone. This retardation has been demonstrated for a number of gases including SF6 and He in laboratory experiments (8)(9)(10).…”

“…Fry et al (9) proposed that the retardation factor, R, for a gas flowing through porous media containing trapped air can be calculated from the dimensionless Henry's Law constant, H, and the ratio between the air, Vair, and water, Vwater, pore volumes In formulating this relationship, Fry et al (9) assumed that gas transfer was sufficiently fast to maintain an equilibrium between the trapped gas and the flowing groundwater. Their equation predicts that the transport of most gases will be retarded in the groundwater systems that contain even small amounts of trapped air.…”

Strategies to Optimize Microbially-Mediated Mitigation of Greenhouse Gas Emissions from Landfill Cover Soils

“…Utilization of methane was modeled with Michaelis-Menten kinetics and methane was considered to be the sole growth limiting substrate. Despite its poor solubility in water (Fry et al, 1995) and low half saturation constant (van Bodegom et al, 2001) comparable to those of methane, oxygen was not considered to be a growth limiting factor in our model due to its abundance in the atmospheric air. With these assumptions, a differential equation was developed describing mass transfer across the biofilm surrounding a spherical packing material.…”