EFFECT OF LIQUID DISTRIBUTION ON GAS-WATER PHASE MASS-TRANSFER IN AN UNSATURATED SAND DURING INFILTRATION

Gas-water phase mass transfer was examined in a homogeneous sand with both the gas and water phase mobile: water was infiltrated from the to p of the sand column while benzene-laden air flowed upward from the bo ttom. Mass-transfer limitations for this situation may be important fo r applications of bioventing, where water and nutrients are added at t he ground surface simultaneously with induced air movement to carry ox ygen and volatile organics to microbial populations. Gas- and water-ph ase samples indicate that gas-water phase mass transfer was sufficient ly fast that equilibrium between gas and water phases was achieved at all sampling locations within the porous medium. Lower-bound estimates for the gas-water mass-transfer rate coefficient show that mass trans fer was at least 10-40 times larger than predictions made from an empi rical model developed for gas-water phase mass transfer in an identica l porous medium. A water-phase tracer test demonstrates that water flo w was much more uniform in this study than in those earlier experiment s, which is a likely explanation for the differing rates of gas-water phase mass transfer. It is hypothesized that the liquid distribution i n previous laboratory experiments was less uniform because of preferen tial flow paths due to wetting front instabilities. Gas-water phase ma ss-transfer rate coefficients reported in this investigation are for a n ideal situation of uniform water infiltration: mass-transfer rates i n field soils are expected to be significantly smaller.