Groundwater Cleanup by In-Situ Sparging. VI. A Solution/Distributed Diffusion Model for Nonaqueous Phase Liquid Removal

Abstract

A microcomputer model for the sparging of aquifers contaminated with volatile organic compounds (VOCs) is presented which includes the kinetics of solution of nonaqueous phase liquid (NAPL) droplets and of diffusion from low-permeability porous layers. The well configuration modeled is a horizontal slotted pipe. Modeling results lead to the following conclusions. 1) The presence of low-permeability porous lenses of clay, till, silt, etc. results in marked increases in cleanup times. The extent of the increases depends strongly on the thickness of the structures. NAPL droplets of large size also result in marked increases in cleanup time. 2) Increases in air flow rate do not yield corresponding decreases in cleanup time if the system is limited by solution/diffusion kinetics. 3) The rate of induced water circulation plays a minor role in determining cleanup time. 4) Wells should be sufficiently deep and operated at an air flow rate such that air is delivered to the entire zone of contamination. 5) The spatial distribution of the VOC has little effect on the cleanup time as long as air is delivered to the entire contaminated zone. 6) Cleanup times increase roughly proportionally to increasing initial VOC concentration. 7) The terminal phase of cleanup typically shows substantial tailing as water containing VOC must circulate into the zone of aeration for the VOC to be stripped. 8) VOCs having Henry's constants of 0.05 or larger can readily be removed by sparging.