Analysis and Interpretation of Single-Well Tracer Tests in Stratified Aquifers

This study deals with the definition and measurement of the dispersive properties of a stratified aquifer based on the single-well tracer test. Knowledge of such dispersive properties are of fundamental importance to the evaluation, analysis, and simulation of contaminant migration in groundwater, a subject of great interest in recent years. In the single-well test, tracer is pumped into the formation for a period of time and then pumped out. Concentration data are obtained from the injection-withdrawal well and from one or more sampling-observation wells which may be multilevel. Our analysis of such a test is based on a Lagrangian-Eulerian numerical model which considers the depth-dependent advection in the radial direction and local hydrodynamic dispersion in the vertical and radial directions. Results show that the movement of an injected tracer in a stratified aquifer may be accurately simulated without resorting to the use of a scale-dependent, full aquifer dispersivity if the flow field is known in sufficient detail. When the advection process is simulated accurately, the values of local dispersivity will be small, constant, and on the order of those measured in the field or laboratory at individual levels in the aquifer. The full-aquifer breakthrough curves measured in observation wells in a single-well test in a stratified aquifer are determined by the hydraulic conductivity profile in the region between the injection-withdrawal well and the observation well if the travel distance between these wells is typical of most test geometries. However, the relative concentration versus time data recorded at the injection-withdrawal well during the withdrawal phase is primarily a measure of mixing in the aquifer due to local dispersion which has taken place during the experiment. The amount of mixing will depend on both the hydraulic conductivity distribution in the aquifer and the size of the experiment. As the experiment scale increases, the effects of local vertical dispersion will become larger compared to the effects of local radial dispersion. Local vertical dispersion will cause a solute traveling in a high-conductivity layer in an aquifer to migrate into adjacent low-conductivity layers where its movement will be relatively slow in comparison. In the initial design of a tracer test it is important to have some idea of the type of nonhomogeneity with which one is dealing. More information of a broad nature concerning the types or classifications of nonhomogeneities that exist in natural aquifers would be very useful. Future research in this area is needed.