Air sparging in a sandy aquifer (Florence, Oregon, U.S.A.): Actual and apparent radius of influence

Injection of air into the saturated zone of a contaminated aquifer, air sparging, is a currently popular remediation alternative to the conventional approach of groundwater extraction followed by surface treatment. The air flow behavior around a single air sparging well was investigated under natural field conditions in a relatively homogeneous aquifer consisting of well-sorted dune sand. The response of conventional monitoring data (including water table mounding, soil gas pressure, soil gas composition and tracer gas response) was compared with the results of cross-borehole electrical resistance tomography (ERT), which provides a sensitive indication of the pattern of air flow in the saturated zone. Compared with the region of air flow imaged by ERT, conventional monitoring data provided an ambiguous indication of the region of air flow in the saturated zone, possibly leading to an overestimation of the radius of influence by a factor of two to eight. Critical evaluation of the conventional monitoring data indicates that the shortcomings are a function of various factors which include the transient aspects of pressure and water table changes, and differences in the controls on air flow within the saturated and unsaturated zones.