New Observations of Gas‐Phase Expansion above a Dense Nonaqueous Phase Liquid Pool

The partitioning of volatile dense nonaqueous phase liquid (DNAPL) compounds to a discontinuous gas phase results in the repeated expansion, fragmentation, and vertical mobilization of gas clusters. This process has the potential to significantly affect the dissolution of DNAPL source zones and the characterization of DNAPL‐contaminated sites, but has not been included in common conceptual models. This study presents new observations of discontinuous gas‐phase growth above a 1,1,1‐trichloroethane pool in a two‐dimensional flow cell packed with 1.1‐mm diameter sand. In contrast to the behavior observed in coarse glass beads, these visualization results show that the gas phase evolves as a collection of macroscopic fingers, composed of multiple trapped and disconnected gas clusters, and that the growth rate of these fingers is faster at the leading edge of the DNAPL pool due to the stripping of dissolved gases. These results provide valuable information for the incorporation of discontinuous gas phases in our evolving conceptual models of DNAPL source zones.