An Explanation of _0,3aAnomalous_0,3A Specific Yield in Unconfined Aquifers
Abstract
Interpretations of pumping tests in unconfined aquifers using the theories of Dagan (1967) and Neuman (1974) systematically result in unrealistically low values of specific yield. Addition of Boulton_0,3_0.4s empirical fitting parameter resulted in a partial improvement of specific yield estimates and a better fit of observed data near the water table in few cases (Moench, 1997). However, in was proven that the unsaturated zone does not play an important role in the processes below the water table, and the empirical fitting parameter does not have clear physical meaning. Analysis of numerous sites indicates that data were collected in alluvial aquifers. Such aquifers exhibit a major geological condition that is not represented by neither of above mentioned three existing models- deviation of the aquifer base from aquiclude conditions. This aquifer base can be represented as an aquitard that is capable of vertical water movement inside the aquitard and water release to the upper unit. We present a more realistic model of alluvial aquifer with the aquitard at the base. For typical parameters of the alluvial sand/gravel aquifer and clay aquitard, the piezometer responses differ from the responses predicted by Neuman_0,3_0.4s (1974) model at intermediate times and only at the later times these responses have similar asymptotic behavior. Using this unconfined aquifer/aquitard (AA) model, we investigate the aquitard effect on effective parameter values. Results indicate that (1) presence of the aquitard may strongly affect the aquifer response and data interpretation; (2) use of the contrast in hydraulic conductivity for discriminating aquifer and aquitard units in the steady-state conditions should be replaced with the hydraulic diffusivities, defined as the ratio of the hydraulic conductivity to the specific storage.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.H71A0790Z
- Keywords:
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- 1829 Groundwater hydrology;
- 1875 Unsaturated zone