The removal of volatile organic compounds from groundwater by air sparging
(AS) is well-established, although reliable methods for predicting the time
required to reach site closure have not been established. To develop an im
proved understanding of mass transfer processes that limit AS performance,
two extended controlled shutdowns of an operating AS system were performed.
Monitoring of TCE concentrations in source zone groundwater indicated "tai
ling" and "rebound" behavior similar to that observed for pump-and-treat sy
stems. A simple two-compartment model provided a reasonable description of
the 3-year AS history, using parameters calibrated from data collected duri
ng the first shutdown period. Comparison of the calibrated rate constants w
ith parameters estimated from laboratory soil columns suggests that, for th
e study site, aqueous diffusion to discrete air channels has a stronger inf
luence on system performance than rate-limited desorption. Predictions base
d on the calibrated model indicated that restoration of the source zone to
drinking water standards would require approximately 1 decade for the curre
nt AS system.