Stratigraphic and Sedimentologic Considerations in Vapor Extraction System Design
GARVEY, TIMOTHY P., JAMES M. EVENSEN, JR., and ROBERT J. MENZIE, JR., Groundwater Technology, Inc., Ventura, CA
Vapor extraction (VE) is a commonly used remediation technique for vadose zone sediments that contain volatile and semivolatile organic compounds with vapor pressures greater than 0.1 mm Hg at 20 degrees C. Proper design of an effective VE system requires complete site-specific evaluation of (1) stratigraphy; (2) distribution, volume, and phase of organic compounds; and (3) the subsurface airflow dynamics when vacuum is applied to a VE well.
Most vadose zone sediments were deposited in alluvial fan, fluvial (e.g., braided river, delta plain, etc.), or shoreface environments. These depositional environments produce predominantly laterally and vertically heterogeneous stratigraphic sequences with characteristically irregular lithosome geometries.
Laboratory data concerning organic compound concentrations are obtained through analyses of sediment and vapor samples collected during remedial investigations. These results are extrapolated to map the distribution of organic compounds within subsurface lithosomes. Volumes of free, dissolved, adsorbed, and vapor phases or organic compounds in the subsurface can be calculated using weighted averages of analytical results.
During VE from subsurface sediments, preferential airflow occurs through permeable lithosomes. Stratigraphy and air conductivity of respective lithosomes are characterized from results of (1) physical analyses of sediment samples, and (2) vapor extraction tests that measure induced vacuum pressure drawdown from extraction and observation wells with vapor monitor probes isolated within select stratigraphic intervals.
Results of stratigraphic, chemical, and pressure drawdown data analyses are used in conjunction with modified Thiem and Theis equations to model the three-dimensional airflow influence of specific applied vacuums in subsurface sediments. An accurate three dimensional depiction of airflow areas of influence yields design information needed for well construction and spacing which, in turn, are required for effective remediation of all vadose zone stratigraphic intervals.
AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)