A multi-tracer push-pull diagnostic test for in situ air sparging systems

This document describes the development and initial application of a multi-tracer push-pull test designed to provide near real-time point-specific measures of contaminant volatilization and aerobic biodegradation rates during in situ air sparging (IAS) operation. Measured biodegradation and volatilization rates are specific to the tracers used, so the results provide relative measures useful for identifying spatial differences in treatment performance and changes in performance with changes in system operation and design. The diagnostic test involves injecting a solution containing multiple tracer compounds into the target treatment zone through a monitoring well, piezometer, or drive point. The injected solution is initially deoxygenated and can contain: (a) a nondegradable, non-volatile conservative tracer, (b) one or more nondegradable, volatile chemicals, (c) an aerobically biodegradable, nonvolatile compound, and (d) a visible dye. After some predetermined hold time, an excess quantity of groundwater is extracted from the same injection point and the change in the concentrations of the tracer compounds is measured. Volatilization and oxygen utilization rates are then estimated from mass balances on the tracer components. The development of this diagnostic tool was conducted in a controlled physical model study and then initial field tests were conducted at the U.S. Navy Hydrocarbon National Test Site (HNTS) in Port Hueneme, California. Spatial variations in oxygenation and volatilization rates were observed, with oxygenation rates varying from 0 to 51 mg-O ₂/L-water/d, and tracer volatilization rates varying from 0 to 47%/d. Acetate and sulfur hexafluoride (SF ₆) were used as tracers in the initial testing, and it was discovered that these are not ideal choices due to the potential for anaerobic acetate biodegradation and SF ₆ partitioning into trapped gas in the aquifer.