Use of an SF6-Based Diagnostic Tool for Assessing Air Distributions and Oxygen Transfer Rates during IAS Operation

Abstract

A diagnostic test designed to assess air distribution and oxygen delivery rate to the aquifer during in situ air sparging (IAS) is described. The conservative tracer gas, sulfur hexafluoride (SF6), is added upstream of the air injection manifold during steady IAS operation and groundwater samples are collected from the target treatment zone after some time period (usually 4 to 24 h). The appearance of SF6 in groundwater is used to characterize the air distribution in the target treatment zone, while the SF6 concentration increase with time is used to assess oxygen transfer rates to the target treatment zone. Conversion from SF6 concentration to oxygen mass transfer rate involves correcting the SF6 concentration increase over time for differences in the relevant chemical properties and injection air concentration. Data presented from a field demonstration site illustrate the utility of this test for identifying air distribution details not readily identified by deep vadose zone helium and groundwater pressure transducer response tests. Oxygen transfer rates at this site ranged from 0 to 20 mg-O2/L-H2O/d. Finally, a comparison of short-term SF6 test data with longer-term dissolved oxygen data illustrated this test's utility for anticipating long-term dissolved oxygen distributions.