Estimating Aquifer Parameters from Analysis of Forced Fluctuations in Well Level - an Example from the Nubian Formation near Aswan, Egypt .3. Diffusivity Estimates for Saturated and Unsaturated Zones

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Journal of Geophysical Research-Solid Earth and Planets
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Jul 10
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Atmospheric pressure variations provide a broadband signal that may force a sympathetic response in well water levels. In this paper, time series analysis techniques are used to estimate the response as a frequency-dependent admittance function, which is then modeled to provide estimates of the fluid transport properties of strata. The data derive from five cased piezometer wells sampling aquifers in the Nubian Formation southwest of Aswan, Egypt. Three shallow wells (100-140 m deep) sample a water table aquifer; a fourth ("W3"; 400 m deep) samples a basal aquifer in the same area that behaves in a confined manner up to a period of several years. The fifth well samples another basal aquifer and shows evidence of partial blockage. Nontidal water level variations in the shallow wells are due almost entirely to barometrically driven flow of air and water. Using a simple model to fit the observed barometric admittance spectra, we obtain estimates of horizontal and vertical permeabilities (for water) in the saturated zone. Local horizontal permeability is constrained by modeling the effects of flow-induced pressure gradients near the screen. For the W3 deep well sampling the basal aquifer, the resulting values (0.15-0.3-mu-m2) are marginally lower than the large-scale (5 km) estimates (0.32-0.43-mu-m2) derived in a previous paper. However, the values for the three wells sampling the water table aquifer, although consistent among themselves (0.2-0.5-mu-m2), are significantly lower than the large-scale estimate (1.0-1.5-mu-m2). This is contrary to what might be expected given that the wells are preferentially screened in clean sandstones. Vertical permeability, estimated by modeling partial confinement effects, is constrained for only one well. A low value was obtained because of the presence of claystone beds in the diffusion path between screen and water table at this well. The effects of air wave diffusion are clearly manifest in the spectra of one well where the water table lay at a depth of about 40 m. The form of the spectra was well fit by ascribing a uniform pneumatic diffusivity of 1.75 x 10(-3) m2/s to the unsaturated zone. However, it was also necessary to include an apparent attenuation of the air wave at the capillary fringe of about 0.5. We propose that the effect is due not to attenuation but that it reflects "compression" of the phreatic surface arising from the presence of trapped air pockets in the underlying saturated zone. A 40-m rise in the water table at the site during the decade prior to the measurements might explain the presence of significant quantities of trapped air. This rise in water table, together with the arid climate, might be taken to suggest that the moisture content of the unsaturated zone is negligible (except within a meter or so of the water table). However, calculation of the intrinsic rock permeability from pneumatic diffusivity assuming zero moisture content yields an estimate which is considered to be too low. The likely explanation is that the assumption of zero moisture content is in error, despite conditions which are as favorable as are ever likely to be realized under field conditions.


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