Determination of coefficient of storage by use of gravity measurements.
The purpose of the study was to develop a method to determine the coefficient of storage of a water-table aquifer by correlating change in gravitational field intensity with change in groundwater storage. In theory, this purpose may be accomplished by modifying the Bouguer slab equation to coefficient of storage equals 78.3 times the ratio of change in gravity in milligals to change in water-table elevation in feet. Errors which result from the Bouguer slab assumptions may be corrected through analysis of tilted finite slabs. Field investigations were made to test the theory. The study area is located in the northern Tucson basin, Pima County, Arizona, and lies on unconfined basin-fill deposits and flood-plain alluvium aquifers. The basin-fill aquifer overlies less permeable Rillito beds and is overlain by the flood-plain alluvium. The two upper aquifers are flat-bedded heterogeneous deposits of sand and gravel. The water table through these aquifers slopes westward at a rate of approximately 0.5 degree. Estimates of the coefficient of storage for the basin-fill deposits and the flood-plain alluvium have been previously made by others from laboratory and field tests and by model analyses. The most reliable determinations of the coefficient of storage range from 0.15. to 0.30. The significance of the gravity method lies in determination of the coefficient of storage by measuring the quantities which define it: rise or decline in head and weight of water placed into or removed from storage. Change in gravity was determined by repeated gravity surveys using the same set of field stations through the period, October 1968 to June 1970. Water levels in wells were recorded for the same period. The relationship between change in gravitational field intensity and change in head was determined using a straight line solution method, and the coefficient of storage was computed from the slope of the straight line. At the conclusion of the field investigations, coefficients of storage were computed for 17 field stations. After correction for limited area of water-level decline and for water-table slope, the values of the coefficients ranged from 0.11 to 0.41. An error analysis indicates a maximum probable error in gravity data of ± 26 microgals. This error may be reduced by modifying the survey and reduction procedures and by using a more sensitive gravimeter. Analysis of changes in gravitational field intensity resulting from change of amounts of water in the unsaturated zone indicates that the coefficient of storage computed for field stations near Rillito Creek, the source of the unsaturated-zone water, are too low. Using data from stations least affected by gravity increases after stream recharge, a probable range of 0.25 to 0.29 was determined for the coefficient of storage in the study area. The range for values of the coefficient of storage using the gravity method confirms the larger coefficient of storage estimation made by others for the same area. The study indicates that the gravity method may be used with success over aquifers which have high coefficients of storage and in which the water table rises or declines 20 feet or more. However, large changes in the water content of the unsaturated zone cause gravity data to show large scatter with respect to water-level data. For this reason the gravity method is more suitable for analysis of those portions of a water-table aquifer which are recharged by underflow than for the portions recharged by infiltration from surface sources.
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