Tg. Poulsen et al., Predicting soil-water and soil-air transport properties and their effects on soil-vapor extraction efficiency, GR WATER M, 19(3), 1999, pp. 61-70
Accurate prediction Of water and air transport parameters in variably satur
ated soil is necessary for modeling of soil-vapor extraction (SVE) at soil
sites contaminated with volatile organic chemicals (VOCs). An expression fo
r predicting saturated water permeability (k(l,s)) in undisturbed soils fro
m the soil total porosity and;the field capacity soil-water content was dev
eloped by fitting a tortuous-tube fluid flow model to measured water permea
bility and gas diffusivity data: The new k(l,s) expression gave accurate-pr
edictions when tested against independent k(l,s) data. The k(l,s) expressio
n was implemented in the Campbell relative Water permeability model to yiel
d a predictive model for water permeability in variably saturated, undistur
bed soil. The water permeability model, together with recently developed pr
edictive equations for gas permeability and gas diffusivity, was-used in a
two-dimensional numerical SVE model that also included non-equilibrium mass
transfer of VOC from a separate phase (nonaqueous phase liquid [NAPL]) to
the air phase. SVE calculations showed:that gas,permeability is likely the
most important factor controlling VOC migration and vapor extraction effici
ency. Water:permeability and gas diffusivity effects became significant at
water contents near and above field capacity. The NAPL-air mass transfer co
efficient also had large:impacts On simulated vapor extraction efficiency.
The calculations suggest that realistic VE models need to include predictiv
e expressions:for both convective, diffusive, and phase-partitioning proces
ses in natural, undisturbed soils.