HR: 1340h
AN: H33D-1630    [Abstracts]
TI: Mass Transfer from a NAPL Pool in the Presence of an Expanding, Mobile, Discontinuous Gas Phase: Experimental Study
AU: * Mumford, K G
EM: mumforkg@mcmaster.ca
AF: Deparment of Civil Engineering, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4L7, Canada
AU: Smith, J E
EM: smithja@mcmaster.ca
AF: Deparment of Civil Engineering, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4L7, Canada
AU: Smith, J E
EM: smithja@mcmaster.ca
AF: School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4L7, Canada
AU: Dickson, S E
EM: sdickso@mcmaster.ca
AF: Deparment of Civil Engineering, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4L7, Canada
AB: The role of discontinuous, multi-component gas phases in groundwater systems is receiving increased attention. Recent research has demonstrated the significance of these gas phases with respect to non-aqueous phase liquid (NAPL) source zones where they have the potential to change our conceptual model for mass transfer at NAPL-contaminated sites. The spontaneous expansion of a discontinuous gas phase results from the presence of volatile NAPL compounds capable of generating gas-phase partial pressures that, combined with the partial pressures of other dissolved gases, exceeds the sum of hydrostatic and capillary pressures. These expanding gas phases overcome the capillary trapping forces, and are driven vertically over macroscopic length scales by buoyancy forces. This recently reported mechanism has the potential to increase the mass transfer from NAPL pools, and significantly change the dissolved NAPL concentration distribution, as compounds in a mobilized gas phase partition back to the aqueous phase well above the pool surface. Failure to consider this mechanism at sites where it plays an active role could result in the erroneous interpretation of aqueous concentration data and the calculation of inappropriate field-scale parameters based on a two-phase (NAPL-water) assessment alone. This work presents a laboratory investigation of this mechanism using small-scale (1.5 mL) no-flow reactors and an intermediate-scale (60 cm x 70 cm x 1 cm) flow cell. The small-scale reactors provided proof-of-concept information regarding the expected behavior for the expansion of a discontinuous gas phase in the presence of NAPL. The intermediate-scale flow cell was packed with silica sand and contained an emplaced NAPL pool. Multiple sampling ports were used to assess transient changes to the total mass flux and the vertical distribution of dissolved NAPL. In addition, light transmission imaging techniques were used to assess transient changes to the distribution of the discontinuous gas phase, including expansion, mobilization, and re-dissolution.
DE: 1803 Anthropogenic effects (4802, 4902)
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2007 Fall Meeting