PNNL

Abstract

A side effect of in situ groundwater remediation techniques that operate by establishing reducing conditions within an aquifer is that anoxic water exits these zones, posing a potential risk to aquatic organisms inhabiting areas of groundwater discharge downgradient from the site. A number of processes have been identified that can attenuate an anoxic plume in an unconfined aquifer with a fluctuating water table. The hypothesis that water table fluctuations increase oxygen transfer from air to water, through enhanced exchange from entrapped air, is tested in an intermediate-scale, fluctuating water table experiment. A dual-energy gamma radiation system was used to measure water saturations while dissolved oxygen (DO) concentrations were measured with flow-through oxygen microelectrodes. A hysteretic multifluid simulator was used to test whether the experimentally obtained water and entrapped air saturations, as well as DO concentrations, could be predicted using the assumptions of two-phase flow and equilibrium partitioning between the gas and the aqueous phases. The experimental results show that zones with entrapped air, formed during the imbibition portions of the experiment, were instrumental in re-oxygenation of the water in the zone of fluctuation. The fluctuating water table system also caused significant amounts of dissolved oxygen to be transported deeper into the flow cell. The simulator was able to predict water saturations, entrapped air saturations, and dissolved oxygen concentrations reasonably well.