Evaluation for air injection remediation technologies for contaminated soil and groundwater: Instrumentation development and field trials

A cylindrical monitoring grid and sample collection system capable of characterizing changes in the subsurface with depth, distance, and orientation from a central injection well were used to investigate the efficacy of an in situ air sparging system (IAS) and an in-well aeration (IWA) system at a gasoline contaminated site in Layton, Utah. Dissolved oxygen (DO) probes manufactured by Technalithics Laboratory, SenSym pressure transducers, and Campbell Scientific, Inc. (CSI) T-type thermocouples were selected for the 27 saturated zone bundles. The bundles also included a stirring blade to mix water near the DO probe. The well points comprising the monitoring grid were installed by direct push technique to minimize problems with disruption of the soil and to ensure intimate contact between the 18-in (46-cm) long screened interval and the soil matrix. A data acquisition system was used to control the stirring blades and record signals from the 70+ in situ sensors. There was not a volume of aquifer that was oxygenated during IAS, but rather, air traveled directly to at least four different monitoring points (MPs) in every trial. The order of air arrival at the MPs was not the same during the first five trials, but once a preferential pathway was established, it did not collapse between trials and was available during successive trials as the shortest distance to the vadose zone. There was no change in the parameters being monitored during two trials (IWA3: 41 days, IWA4: 20 days) of the IWA system. Fluctuations in the depth to groundwater during the trials were similar in all MPs due to seasonal water table changes and rain events. Slug tests across the grid indicated that the K_h is low near the water table (17 cm/day) and decreases with depth to 0.09 cm/day at 20 ft (6.1 m) below grade. The densely spaced, balanced arrangement of monitoring points provided the information necessary to conclude that both IAS and IWA were inappropriate for remediation of the Layton site due to the low horizontal hydraulic conductivity.