THE EFFECTS OF ENTRAPPED GAS BUBBLES ON PHYSICAL FLOW AND DISSOLVED GAS TRANSPORT - A SAND TANK EXPERIMENT

Groundwater tables undergo natural fluctuations due to a variety of processes like snow melt, rain infiltration, aquifer recharge/discharge and river stage fluctuations. Water table fluctuations result in the entrapment of discontinuous air bubbles below the water table that will affect the physical properties of the soil and potentially the groundwater geochemistry. For example, oxygen in the air bubbles can dissolve into the groundwater and be a source of dissolved oxygen.

This research describes a series of lab-scale sand tank experiments. Initial experiments involved measuring the change in water content and hydraulic conductivity of the sand under saturated and five water table fluctuation scenarios. As the water table fluctuation level increased, the amount of entrapped air increased, resulting in a decrease in water content and hydraulic conductivity of the entrapped air zones. Bromide tracer tests under fully saturated and 29 cm and 45 cm water table fluctuations identified stratified velocity and dispersivity profiles across the sand tank.

Subsequent experiments quantified dissolved argon and oxygen concentrations in the sand tank under saturated, 29 cm and 45 cm water table fluctuation scenarios. Experimental results were simulated with a reactive transport model (MIN3P). Experimental and simulation results show that the quantity of entrapped bubbles, equilibration of gas between the aqueous and gas phases, and the velocity profiles are the primary controls on the depletion of dissolved argon and oxygen concentrations, and the transport of these gases through the sand tank.