Stratigraphic and Sedimentologic Considerations in Vapor Extraction System Design

Abstract

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 20C. 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. Laboratory data concerning organic compound concentrations are obtained through analyses of sediment and vapor samples collected during remedial investigation. These results are extrapolated to map the distribution of organic compounds within subsurface lithosomes. volumes of free, dissolved, adsorbed, and vapor phases or organic ages 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. Anmore » 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.« less