13 THE DEVELOPMENT OF A RAPID LABORATORY
SCREENING PROCEDURE FOR SOIL AIR-SPARGING
WITH AN AUTOMATED PURGE AND TRAP SYSTEM
George E. Hoag, Associate Professor of Civil Engineering and Director of the
Environmental Research Institute
Farhad Nadim, Research Assistant
Department of Civil and Environmental Engineering
Shili Liu, Laboratory Manager
Department of Civil and Environmental Engineering
Amine M. Dahmani, Laboratory Manager
Department of Civil and Environmental Engineering
University of Connecticut
Starrs, Connecticut, 06269-3210
INTRODUCTION
Air sparging is a method of site remediation that introduces air (or other gases) into the saturated zone contaminated with volatile organic compounds (VOCs). In addition to volatilization
of VOCs, air sparging promotes the growth of aerobic bacteria in saturated zones and may oxidize reduced chemical species. In this study biological degradation of the volatile compounds
was minimized utilizing helium as the sparging gas.
Hinchee, 1994, categorizes air sparging as two distinct technologies, in-well aeration and the
injection of air into the saturated zone. In-well aeration is the injection of air below the water
table in a well, producing a physical change in the ground water system known as the "airlift
pump effect." As the air bubbles move upward in the well and reach the water surface, they create a relative vacuum behind in their flow paths. This pressure difference causes the groundwater
to move into the well from the bottom screens and return to the aquifer from the shallower portion of the screens.' This movement of groundwater into the bottom of the well results in VOCs
entering the well as they are stripped from solution with the sparging gas. A major drawback in
this system is the gradual buildup of bacteria and inorganic precipitation in the vicinity of the
well screens that can significantly reduce the circulation of water and the sparging efficiency.
The objective of the second air sparging technique is to inject pressurized air into the aquifer material through the well screens or diffusers. It has been reported that the air injected into the
aquifer migrates in stable channels and only the contaminated soil within these channels is exposed to the injected air.2 If stable air channels form in the aquifer by air injection, the only removal process of the contaminants not in direct contact with the air flow paths would be liquid
phase diffusion.
Equilibrium partitioning of the contaminants at the gas/water interface can be written as:
C = C H, where C is the gaseous concentration in terms of mg/L, C is the aqueous concentration in mg/L, ana H is the dimensionless Henry's law constant. This relation can be used to
describe the partitioning of gas and water in the sparging system.' The efficiency with which air
sparging transfers oxygen to water is measured by determining the rate at which the dissolved
oxygen concentration of groundwater increases. Increase in oxygen concentration depends on the
partial pressure of oxygen in the sparging gas, which in part depends on the gauge pressure of
the air pump. In a study conducted by Hoag. 1994. a gauge pressure of 0.96 atm. was required to
induce air sparging 8 meters below the water table in an aquifer formation of silt and sand.3
51st Purdue Industrial Waste Conference Proceedings, 1996. Ann Arbor Press. Inc., Chelsea. Michigan 48118.
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