MASS TRANSPORT BY GAS BUBBLE FLOW DURING SUBSURFACE THERMAL REMEDIATION

Gas bubble formation and movement in subsurface saturated media during heating (e.g., during thermal groundwater remediation) can significantly increase mass transfer at temperatures well below the boiling point of water. Originally these bubbles were believed to be immobile, however recent studies have shown that trapped bubbles are not necessarily static; they can grow, fragment, and mobilize. Under heated conditions, subsurface bubbles can move upwards into cooler regions where they can collapse and re-dissolve in the aqueous phase. This cooling effect can lead to an accumulation of mass and result in the contamination of previously clean groundwater zones.

To simulate discrete bubble behavior during thermal remediation, a 2D finite-difference mass transport model, which incorporates a macroscopic invasion percolation (MIP) model to simulate gas movement, was developed. The model was used to study the effect of varying permeabilities and entry pressures on the mass removal from an aqueous-phase source at different remediation temperatures. Simulations showed that discrete bubble mobilization occurred in all soil types investigated (fine to medium sands), with mobility being limited by lower temperatures and higher entry pressures. The effect of clay lenses on bubble movement and formation was examined, focusing on the effect of lower permeable zones on bubble trapping.

In addition to providing insight into mass removal from source zones consisting of only aqueous-phase mass, the model also showed that thermal remediation of non-aqueous phase liquids (NAPLs) may result in increased bubble formation and mobilization. To further investigate this, 1D and 2D heated experiments were performed using carbon tetrachloride (CT) as the NAPL. The data indicated that the rate of bubble formation was greatest when the contact area between the NAPL and water was high. With sustained bubble formation, continuous preferential vertical pathways were established that allowed significant mass transport to the water table.