Hj. Cho et al., SIMULATING THE VOLATILIZATION OF SOLVENTS IN UNSATURATED SOILS DURINGLABORATORY AND FIELD INFILTRATION EXPERIMENTS, Water resources research, 29(10), 1993, pp. 3329-3342
This paper describes laboratory and field experiments which were condu
cted to study the dynamics of trichloroethylene (TCE) as it volatilize
d from contaminated groundwater and diffused in the presence of infilt
rating water through the unsaturated soil zone to the land surface. Th
e field experiments were conducted at the Picatinny Arsenal, which is
part of the United States Geological Survey Toxic Substances Hydrology
Program. In both laboratory and field settings the gas and water phas
e concentrations of TCE were not in equilibrium during infiltration. G
as-water mass transfer rate constants were calibrated to the experimen
tal data using a model in which the water phase was treated as two pha
ses: a mobile water phase and an immobile water phase. The mass transf
er limitations of a volatile organic compound between the gas and liqu
id phases were described explicitly in the model. In the laboratory ex
periment the porous medium was nonsorbing, and water infiltration rate
s ranged from 0.076 to 0.28 cm h-1. In the field experiment the water
infiltration rate was 0.34 cm h-1, and sorption onto the soil matrix w
as significant. The laboratory-calibrated gas-water mass transfer rate
constant is 3.3 x 10(-4) h-1 for an infiltration rate of 0.076 cm h-1
and 1.4 x 10(-3) h-1 for an infiltration rate of 0.28 cm h-1. The ove
rall mass transfer rate coefficients, incorporating the contribution o
f mass transfer between mobile and immobile water phases and the varia
tion of interfacial area with moisture content, range from 3 x 10(-4)
h-1 to 1 x 10(-2) h-1. A power law model relates the gas-water mass tr
ansfer rate constant to the infiltration rate and the fraction of the
water phase which is mobile. It was found that the results from the la
boratory experiments could not be extrapolated to the field. In order
to simulate the field experiment the very slow desorption of TCE from
the soil matrix was incorporated into the mathematical model. When des
orption from the soil matrix was added to the model, the calibrated ga
s-water mass transfer rate constant is 2 orders of magnitude lower tha
n that predicted using the power law model developed for the nonsorbin
g laboratory soil.