Gravitational and Dispersive Mixing in Aquifers

Analytical techniques are developed to describe the mixing between two fluids of different density in a confined aquifer, in which one fluid is introduced to the aquifer by well recharge. The immiscible displacement process in radial flows is analyzed and the effects of longitudinal and lateral dispersion are included using a boundary layer approximation. The theoretical results demonstrate the effect of hydrodynamic dispersion in retarding gravity segregation due to density differences. The theoretical results are compared with observations of aquifer mixing in a radial flow laboratory model. During recharge, excellent agreement between the theoretical predictions and experimental results is found. Theoretical predictions of recovery efficiency during a recharge-storage-withdrawal cycle show trends similar to those observed, but are somewhat lower than those observed. Direct theoretical predictions of recovery efficiency are developed for an immiscible system.