Water table response to barometric pressure changes : a laboratory investigation.

Abstract

The water level in water-table wells and in evapotranspirometers has been observed to fluctuate in response to barometric pressure changes. It has been suggested that air bubbles entrapped in the porous media cause the water level changes. It was to test that hypothesis that this study was undertaken. Previous studies of barometric effects are reviewed. Possible sources and consequences of gas in aquifers are discussed. Experiments using gamma-attenuation equipment are described which allow measurement of the amount of air contained in a sample of water-saturated sand at different pressure levels. The theory and practice of gamma-attenuation measurements and the applicability of this method to hydrologic studies are explained. The experimental results demonstrate that air bubbles in the water-saturated sand do change in size in response to pressure changes and that this size change accounts in large part for water level changes. However, an exact correlation between air content and water level was not found, due in part to the difficulty in measuring the small changes which occur. Additional mechanisms which may cause water-level fluctuations are suggested.