A theoretical analysis of mixed convection in aquifer storage and recovery: How important are density effects?

Aquifer storage and recovery (ASR) involves the injection of freshwater into a confined aquifer and represents an alternative to surface water storage. This paper investigates the hydrogeologic parameters governing density effects in a mixed convection system during the injection, storage and recovery of a freshwater bubble in a saline (isotropic and homogeneous) aquifer. The vertical interface between injected freshwater and ambient saltwater is inherently unstable due to the density difference between the two fluids. This instability can potentially lead to macroscopic rotation of the interface, leading to premature breakthrough of salt at the bottom of the well and hence reduce the volume of water that is recoverable at an acceptable quality. Significance of density effects during injection was found to depend on the mixed convection ratio (a ratio of the characteristic velocities from forced convection due to pumping, to free convection due to concentration differences), with the greatest interface tilt occurring with high density contrasts plus slow injection rates and high hydraulic conductivities. During storage, the extent of the density effect depends on the storage duration and hydraulic conductivity. During recovery, the breakthrough curve shape is profoundly different in density-dominated cases compared with advection and dispersion-dominated cases. Dispersive mixing was found to attenuate density effects in all three phases of ASR. This study demonstrates that the density contrast alone is insufficient as a decision parameter when choosing whether density effects are negligible. Even when modelling ASR scenarios where the density contrast is low and would traditionally be considered negligible, the decision to include or neglect fluid density in the model should be based on a full mixed convection analysis, as considered in this study. Density effects in ASR are shown to depend on the relative influences of the density difference, hydraulic conductivity, pumping rates, injected radius, storage duration, and dispersivity. (C) 2007 Elsevier B.V. All rights reserved.