Forced-Gradient Tracer Tests and Inferred Hydraulic Conductivity Distributions at the Mobile Site

ABSTRACT Four single‐well tracer tests and a two‐well tracer test performed in a 21‐m thick confined granular aquifer at a field site near Mobile, Alabama, are described. The data from these tests together with previously published data from a single‐well test and a two‐well test allow one to begin to develop a three‐dimensional picture of the hydraulic properties of the study aquifer. A consistent feature of all the tests is a high hydraulic conductivity zone which appears in the bottom third of the aquifer. This result is in agreement with hydraulic conductivity distributions inferred from previous aquifer thermal energy storage experiments at the same site. In some locations the new tests indicated high hydraulic conductivity zones in the upper third of the aquifer which were not detected in the previous two‐well test and single‐well tests performed at the same site but at different locations. Despite the three‐dimensional spatial variations of hydraulic conductivity indicated by these tests, it was possible to predict the major features of the tracer concentration as a function of time at the withdrawal well in the two‐well test by means of an available numerical model assuming perfect stratification at the test site and using a hydraulic conductivity distribution inferred from a single‐well test. The apparent success of the model prediction presented here, as well as the success of a similar numerical simulation of the previous two‐well test reported earlier, seem to support the conclusion that the study aquifer has an approximately stratified flow field at the test site. These field test results provide further evidence that reliable predictions of solute transport in a real aquifer will depend on a sufficiently detailed knowledged of the major features of the three‐dimensional advection pattern in the aquifer. Based on this study and previous studies, it appears that a healthy mixture of viewpoints and techniques (geologic, hydrologic, geophysical, etc.) will be necessary to characterize the transport properties of natural ground‐water systems.