Impact of Atmospheric Pressure Fluctuations on Vadose-zone Contaminant Plumes
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Impact of Atmospheric Pressure Fluctuations on Vadose-zone Contaminant Plumes Wayne C. Downs Department of Civil Engineering, Brigham Young University Provo, UT Chang H. Oh, Todd Housley, and Jeff Sondup Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID ABSTRACT Field and laboratory studies have been underway at the INEEL and BYU to investigate the effect of atmospheric pressure fluctuations on the migration of contaminant plumes. In the field, two vadose-zone piezometer nests were instrumented to measure soilpressures and carbon tetrachloride concentrations at depths of roughly 78-ft, 112-ft., and 150-ft., below ground surface in wells 25-ft. apart. At land surface, a manifold was constructed to systematically rotate from port to port at 15-minute increments of time. Contaminant concentrations were measured at each rotation, and soil pressures were measured at all ports at 15-minute increments. Atmospheric pressures were also recorded. Results showed that atmospheric pressure changes propagated downward lagged in time and dampened in amplitude. Carbon tetrachloride concentrations were observed to decrease with pressure increases at each port, suggesting that pressure increases pushed the plume downward. Increases in concentration were observed with decreases in pressure. Concentrations were observed to change at a port by as much as an order of magnitude. Modeling is underway to assist in isolating the transport mechanisms involved and assist in quantifying the effect of varied pneumatic permeability on pressure propagation. Laboratory work is underway at BYU to more carefully isolate the mechanisms of diffusive transport and pressure fluctuation in unsaturated conditions. A sealed soil column has been packed with sand and instrumented with a contaminant reservoir at one end and concentration sensor at the other. A mechanical syringe is used to vary the pressure within the tube, and pressure sensors have been emplaced. An affiliation has been established with researchers at the DOE Sandia laboratory to utilize a novel non-invasive vapor detector in our sealed-column laboratory work. INTRODUCTION Barometric pressure changes and uneven distribution of atmospheric pressures are known to induce movement of parcels of air and winds in the upper atmosphere and at land surface. It is not unreasonable to assume that these atmospheric pressure changes may be propagated in the void spaces beneath land surface and induce movement in soil air as well. We will show that atmospheric pressure fluctuations are propagated vertically in the vadose zone and are correlated with changes in the location or shape of vadose-zone contaminant plumes. That vadose-zone plumes may be dynamic in nature has strong implications on the use of monitoring data for management strategy and remediation decisions. BACKGROUND Buckingham1 proposed that gaseous diffusion, a process that he studied in the laboratory, would be the dominant transport process at depth in the vadose zone. Several other investigators
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