BAROMETRIC AND EARTH TIDE INDUCED WATER-LEVEL CHANGES IN A RIGID SANDSTONE AQUIFER, SOUTHWESTERN INDIANA
the sun and moon pass over a point on the Earth, their gravitational
attraction generates a dilational force on the bedrock, increasing pore
space, and decreasing the potential of the water in the aquifer (a).
After the sun and moon pass, the gravitational force decreases the
aquifer (pore space) contracts, thus increasing the pore water potential
BY: Dr. Paul Doss, Paul Inkenbrandt, and Robert Brown
Water levels from a deep-shallow piezometer nest in the Inglefield sandstone depict a dynamic ground-water system. Water levels at both the 60 foot and 110 foot depths fluctuate up to 0.5 feet in the matter of hours. Most of this fluctuation is driven by responses to atmospheric pressure change. A remarkable inverse correlation exists between ground-water levels and barometric pressure. Calculated barometric efficiency for this aquifer ranges from 0.85 to 0.94, indicating a rigid aquifer skeleton. Following successful quantification and removal of the barometric effects on water-level data, the residual hydrographs suggested an additional, smaller amplitude periodicity was still present in the water-level records. These fluctuations were hypothesized to result from Earth-tide induced crustal deformation stresses. Evaluation of barometric-corrected head data by a Fast Fourier Transform method identified periodicities of water-level changes at 12.01 and 12.4 hours. These periodicities correlate well with solar and lunar tide stressors, respectively. Other periods that were indicated by Fourier analysis (54 day and 2.5 hour) have not yet been interpreted and may be artifacts of the evaluated period of record. Whereas barometric fluctuations of water levels are driven through the well-water column and do not result from potential changes within the aquifer, Earth-tide induced fluctuations are the result of changes in aquifer potential. Further, these stress induced changes are suggestive of a confined system, yet simple stratigraphy suggests the aquifer is unconfined. Lithologic variability within the sandstone, specifically a finer-grained and mica-rich shallow zone, likely generates confined behavior. It is hoped that future work can identify the source of the other observed periodicities and whether this aquifer responds to seismic stresses associated with the Wabash Valley Fault System.
We appreciate the help of Tom Pickett for teaching us about the physics involved and performing the Fast Fourier Transforms. We are grateful for help from Todd Rasmussen, for explaining the barometric influence removal process. We thank the USI RISC program for research and travel support to Inkenbrandt and Brown. Thanks to William Wilding for help with statistics. We would also like to thank the weather for working with us and being dynamic. The sun and moon deserve gratitude for their great mass and proximity.
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