RAFTERY, PETER J., RICHARD BROWN, JAMES M. EVENSEN, and TIMOTHY P. GARVEY, Groundwater Technology, Inc., Ventura, CA
Sparging injects pressurized air below the water table. The air moves vertically and laterally through the saturated soil, creating transient air-filled areas. Volatile compounds exposed to this air volatilize and are carried to the vadose zone, where they are removed by vapor extraction.
Several criteria control the effectiveness and applicability of air sparging. The contaminant should have a Henry's constant greater than 10-5. The soil where the air is injected and the overlying soil must be uniformly permeable. Pilot tests must be judiciously performed, and the data carefully applied to system design to assure the system will remove volatiles from groundwater and control volatile vapors.
The subject site is 270 x 180 ft. Leaking underground solvent tanks resulted in groundwater contamination with dissolved perchlorethylene. Soil is medium-grained sand to 35 ft below grade, overlying weathered granodiorite and clayey silt. Groundwater occurs 13 ft below grade.
Groundwater samples contained 40,000 mg/L volatile organics near the former tank location. Concentrations decreased to 1000 mg/L, 100 ft off site. Contaminants had migrated downgradient.
Pilot tests were performed for system design. Data analyses indicated sparging/venting could be successfully applied. The installed system included 15 sparge locations, and eight extraction locations.
Initially, the soil vapor extraction system operated alone. Measurements indicated the vacuum/induced air flow extended off site. Sparging began 1 month after extraction start up. The innermost sparge wells were started first, then periphery wells, once vapor control was confirmed.
An extraction system start up, field readings of the untreated extracted vapor reached 1000 units. Within 1 day, readings dropped to 250 units; this is attributed to rapid evacuation of pore vapors at equilibrium with contaminated soil. After 1 month of extraction, extracted vapor readings had dropped to less than 10 units, and the sparge system was started. Extracted vapor readings rose to 70 units and remained elevated for several months, indicating the removal of dissolved compounds. After 125 days of sparging, volatile compound reduction in groundwater was greater than 98%.
At sites where the criteria for viable air sparging remediation are met, significant, rapid treatment of groundwater will occur. These results indicate that air sparging in conjunction with vapor extraction is a effective, expeditious technology for groundwater remediation.
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Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach,
California, May 5-7, 1993.