CP-2005-01
Validity of the generalized Richards equation for the analysis of pumping test data for a coarse-material aquifer
El-Kadi, Aly I.
This paper presents an examination of the validity of the generalized Richards equation (GRE), which includes unsaturation and compressibility effects, in the analysis of a well-documented, three dimensional aquifer test. The potential effects of wellbore storage and monitoring-well delayed response were included in the analysis. The uniqueness of the solution was also examined by testing the potential success of fully saturated models in simulating the drawdown measurements. The solution of the GRE closely matched the field measured drawdowns with some parameters that were close to their independently measured values. The aquifer-test analysis can thus provide accurate estimates for some average aquifer parameters, namely, horizontal and vertical hydraulic conductivities and specific storage. However, the model is not fully validated due to the need for calibrated soil hydraulic parameters. In general, it is possible to account for early time discrepancies by using an inflated fitting value for the specific storage. However, good accuracy was obtained using a physically based value for such a parameter when wellbore storage is considered. Sensitivity of results to values of saturated conductivity again confirmed the great importance of obtaining accurate estimates of such values. Finally, the study showed that saturated flow models did not provide results as accurate as those provided by the GRE model. Classifying the aquifer material as coarse can be misleading, considering that its effective soil properties is that of a finer texture. As such, unsaturated flow effects should not be overlooked.