Field monitoring and performance evaluation of a field-scale in-well aeration system at a gasoline-contaminated site

Several in-well aeration (IWA) technologies have been used since the early 1990s, but few field studies have been performed to evaluate the extent of water circulation around IWA systems. In this study, 27 discrete monitoring points (MPs) were installed at a gasoline-contaminated site to assess the efficacy of IWA. Pressure transducers and dissolved oxygen (DO) probes were sealed into the MPs, allowing them to be used to characterize subsurface changes in total head and DO with depth, distance and orientation from a central injection well. No change in DO or in hydrocarbon total mass or distribution occurred across the site during two trials (41 and 20 days) of the system. Water level fluctuations during the trials were similar in all MPs, and were due to seasonal water table changes and rainfall events. No circulation cell was established around the IWA well after 41 days of operation, and the impact of the well extended less than 90 cm from it. Groundwater only circulated through the sand pack around the well. Little, if any, recharge occurred through the lower screen. Silt accumulated in the well, limiting its operation time, even with a fabric filter sock over the lower screen, Obviously, IWA was ineffective at this site, probably because the horizontal hydraulic conductivity (Kh) of the soil opposite the lower screen was low (0.09 cm per day) and because the distance between the two screens was short relative to the borehole radius. Long remediation times would likely make IWA unattractive at this or other sites where the Kh of the soil is so low that the air injection rate would have to be low to prevent blowing the well dry. (C) 2001 Elsevier Science B.V. All rights reserved.