Multicomponent Gas Mass Transfer: What Factors Influence Gas Partitioning in Groundwater Systems?

A detailed understanding of the parameters that impact multicomponent mass transfer of gases in groundwater systems is important to improve monitoring strategies and develop accurate numerical models relevant to subsurface gas storage as well as risk assessment and remediation of contaminated sites. A series of one-dimensional experiments were conducted in sand-packed columns to measure the dissolution of trapped gas over time. Experiments were conducted using two trapped gases (i.e., at residual nonwetting saturation), two grain sizes, and three aqueous velocities. Mass transfer characteristics including dissolved gas enrichment, dissolved gas breakthrough times, and changes in gas saturations were assessed by observation and by the ANOVA test to evaluate the overall mass transfer behavior dependence on these three system parameters. Results showed that partitioning of any one gas component is highly dependent on all other gas components and that mass transfer is not consistent with local equilibrium assumptions (LEA), even at aqueous velocities below where LEA was expected. These findings show that the LEA velocity threshold remains uncertain and highlight that multicomponent kinetic mass transfer models that consider the gas-water interfacial area should be used to accurately simulate gas partitioning.