Effects of Unsaturated Zone on Aquifer Test Analysis in a Shallow‐Aquifer System

Abstract

AbstractA comparison between two hypothetical flow models of an unconfined aquifer, one saturated and the other variably saturated, indicates that the variably saturated model which explicitly models drainage from the unsaturated zone provides a better conceptual framework for analyzing unconfined aquifer test data and better estimates of the lateral and vertical hydraulic conductivity in fine‐grained sands. Explicitly accounting for multiple aquifers, well‐bore storage, and the effects of delayed drainage from the unsaturated zone increases confidence in aquifer property estimates by removing some assumptions and allowing for the inclusion of early time data and water‐table observations in an aquifer test analysis. The inclusion of the unsaturated zone expands the number of parameters to be estimated, but reasonable estimates of lateral and vertical hydraulic conductivity and specific storage of the unconfined aquifer can be obtained. For the cases examined, only the van Genuchten parameter a needed to be determined by the test, because the parameters n and 9r had a minimal effect on the estimates of hydraulic conductivities, and literature values could be used for these parameters. Estimates of lateral and vertical hydraulic conductivity using MODFLOW were not as good as the VS2DT based estimates and differed from the known values by as much as 30 percent.The hydraulic properties of a surficial aquifer system were estimated through a series of aquifer tests conducted at Cecil Field Naval Air Station in Jacksonville, Florida. Aquifer test results were analyzed by calibrating a variably saturated, radial flow model to the measured drawdowns. Parameter estimation was performed by minimizing the difference between simulated and measured drawdowns with an optimization routine coupled to VS2DT and was constrained by assuming that the hydraulic properties of each aquifer or confining unit were homogeneous. Given the hydrogeologic conditions at the field site, estimating the hydraulic properties of the aquifers and confining units with analytically derived type curves would have been inappropriate. Estimates of the lateral hydraulic conductivity from the VS2DT solution were more consistent with the observed geology than estimates from Theis analyses, which ranged from 20 to 80 percent more than the final estimates. The unsaturated zone affected an aquifer test conducted in a leaky aquifer about 100 feet below land surface more than the other two aquifer tests because about half of the pumped water came from the overlying, unconfined aquifer.