Abstract
This paper presents the results of an investigation of the flow of air into and out of the unsaturated zone at Picatinny Arsenal, New Jersey, in response to changing atmospheric pressure. An extensive site investigation occurring over 4 years was conducted that included measuring subsurface air pressure, moisture content, water-table elevation, field-scale air permeability, and the collection of soil cores for laboratory air-permeability experiments. This information is used in developing a one-dimensional airflow model driven by atmospheric pressure changes. Barometric pressure was obtained from Newark International Airport where it was measured hourly. Subsurface air velocities simulated by the airflow model are used to track hypothetical markers placed at varying depths in the unsaturated zone. This code allows for visualization of air movement into and out of the vadose zone at the site and allows calculation of the depth of soil gas evacuation and surface air penetration during atmospheric pressure cycles. A large atmospheric pressure drop measured in December 2001 is used to illustrate the effect of different model parameters on airflow into and out of the vadose zone at the site. Results indicate that air-permeability and moisture-content values have little effect on airflow in a single-layer, shallow unsaturated zone system, producing a maximum depth evacuated of 12.5 cm for the parameter values simulated. Single-layer airflow is increased in systems having deeper unsaturated zones. Simulations involving a low-permeability, high moisture-content layer overlaying a more permeable, low moisture-content layer show increased subsurface airflow, with a maximum depth of 65.0 cm evacuated for the 3-m unsaturated zone.
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