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AN INVESTIGATION OF SITE CHARACTERISTICS CONTROLLING AIRFLOW INTO AND OUT OF THE SHALLOW UNSATURATED ZONE IN RESPONSE TO ATMOSPHERIC PRESSURE CHANGES
Citation:
Tillman Jr., F. D. AND J W. Weaver. AN INVESTIGATION OF SITE CHARACTERISTICS CONTROLLING AIRFLOW INTO AND OUT OF THE SHALLOW UNSATURATED ZONE IN RESPONSE TO ATMOSPHERIC PRESSURE CHANGES. Presented at 16th National Underground Storage Tank Conference, New Orleans, LA, March 1-3, 2004.
Impact/Purpose:
To assess quantitative and qualitative methodologies evaluation of vapor intrusion, including quantification of uncertainties, to be used in the decision making process.
Description:
Atmospheric pressure near the land surface is constantly changing, due both to short-term diurnal temperature fluctuations as well as longer-term cycles due to the passage of high-and-low-pressure weather systems. Depending upon soil properties, such as air-filled porosity and air-permeability and site characteristics such as depth to the water table, barometric pressure changes at land surface may be transmitted deep into the unsaturated zone. As subsurface gas responds to changing surface pressure, gas is "breathed" in and out of the unsaturated zone - a process known as "barometric pumping". Periods of falling atmospheric pressure above a leaking underground storage tank site can withdraw unsaturated zone soil gas, carrying with it potentially harmful organic vapors. By this process contaminants will escape from the shallow unsaturated zone faster than they would by pure diffusion alone. Periods of increasing atmospheric pressure increase the rate of contaminant diffusion by lowering the surface where the contaminant concentration is zero below the true ground level (i.e. reducing the diffusion length). Additionally, oxygen from land surface may penetrate the subsurface during rising atmospheric pressure, providing increased supply of electron acceptors to bacteria degrading organic compounds. This study addressed the need for information about how site characteristics affect soil-gas flow in response to natural changes in atmospheric pressure by incorporating time-series surface-pressure data with subsurface porosity, moisture content and air permeability data into a one-dimensional air-flow/particle tracking model. Results indicate that air-permeability and moisture-content values have little effect on airflow in a single-layer, 3.0-m unsaturated zone system, producing a maximum evacuated depth 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.