Mechanisms Controlling Air Stratification Within a Large Diameter Borehole and Atmospheric Exchange

Abstract

Large boreholes dot the landscape across much of the arid low latitudes. Here we explore air dynamics within these features to understand their significance to gas transport at the Earth-atmosphere interface. We instrumented a large-diameter (3.4 m) borehole and the borehole-atmosphere interface to explore under natural conditions the role of atmospheric variability on gas transport down to the water table at a depth of 59 m. Two independent tracers that naturally existed inside the borehole, water vapor and CO 2 , were used to map diffusive and advective transport regions and rates. We hypothesize that temporal variations in atmospheric and borehole air temperature determine the transport regime; thus, we conducted two separate 1-month observations, during summer and winter. Of several potential air transport mechanisms known to act within cavities, thermal-induced convection (TIC) was found to be the dominating advective mechanism inside the borehole. During winter, TIC circulated atmospheric air throughout the entire borehole down to the water table. During summer, however, atmospheric air reached only down to the middle of the borehole, indicating stable stratification below that depth; TIC was limited to the upper mixed layer overlying a diffusive transport layer. During times when TIC was suppressed, the stratified borehole air developed relatively low temperatures, and accumulated elevated CO 2 concentrations and high vapor content, which were then vented to the atmosphere during convective active periods.