ESTIMATING THE STORAGE PROPERTIES OF AN AQUIFER FROM BAROMETRIC AND EARTH TIDE INDUCED WATER LEVEL CHANGES

The Inglefield Sandstone, a locally important domestic aquifer in southwestern Indiana, displays a high barometric efficiency (0.95). Despite the aquifer's pronounced response to barometric pressure changes, which suggests that this is a rigid sandstone aquifer, the Inglefield displays a measurable Earth tide response. Following the removal of the barometric influence on ground-water levels, a Fast-Fourier Transform analysis verified the Earth-tide response. Four primary (most influential) gravitational frequencies (O₁, K₁, M₂, S₂) were clearly defined by the transform. These frequencies represent the periodicities of tidal stresses applied to the aquifer skeleton. The responses of water levels to the barometric stress and to the periodic gravitational stresses are essentially passive aquifer tests, and can be used to estimate the storage characteristics of the observed aquifer.

Porosity of the measured aquifer horizon was determined gravimetrically and by thin-section point count as 0.24. Compressibility of the aquifer was calculated from head-change and pressure-change relations as 4.55 X 10^-10 Pa^-1. Specific storage for the Inglefield sandstone was estimated from tidal response by applying the method of Bredehoeft (1967). The value for specific storage estimated using tidal fluctuations (3.36 X 10^-6 m^-1) corresponds well with the value derived from Jacob's relationship (5.53 X 10^-6 m^-1) and the value derived using barometric efficiency (1.16 X 10^-6 m^-1). Single wells that display measurable Earth tide responses in water levels can be effectively used to estimate the local storage properties of an aquifer.