U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

A Multispecies Transport Model to Simulate Vapor Extraction and Bioventing Remediation of Unsaturated Zones Contaminated with Gasoline

Abstract

Vapor extraction is an established method for removing spilled gasoline from the unsaturated zone. The technology involves the establishment of an unsaturated zone air-flow field by withdrawing and injecting air through dry well sand trenches. The success of a vapor-extraction installation depends fundamentally on the establishment of an air-flow field that intersects the contaminant and on the partitioning of the contaminant into the gaseous phase. Bioventing is a term applied to simultaneous venting and enhanced biodegradation of a contaminant. Aerobic biodegradation is stimulated by oxygen delivered to the unsaturated zone by air injection and withdrawal. A model to predict the transport and reaction of multiple chemical species in the gaseous phase under conditions of induced air flow has been developed to simulate the performance of vapor-extraction-based remediation. The model consists of a flow module and a reactive transport module. The flow module, named AIR3D, based on an adaptation of a ground-water flow simulator, defines the three-dimensional air-velocity field within which the chemicals are transported and react. The transport module solves a coupled system of conservation-of-mass equations, one for each chemical. Advection-domination transport is assumed, which allows the conservation-of-mass equations to be defined along streamlines. In this paper, model applications and assumptions are summarized.