Aquifer Recharge Using a Vadose Zone Infiltration Well

Vadose zone infiltration well (VZW) is an important method to implement managed aquifer recharge (MAR) when groundwater table is relatively deep or sufficiently permeable soils at ground and/or sizable land areas for surface infiltration systems are not available. However, the theoretical model for this kind of artificial recharge has rarely been developed. A mathematical model for the coupled unsaturated-saturated flow process induced by a VZW is presented in this study with the purpose of understanding the overall hydrodynamics of unsaturated flow and recharge efficiency. Semianalytical solutions for hydraulic heads and recharge rate are derived using Laplace-Hankel transforms. The solutions are tested using a finite-element numerical solution built with COMSOL. The performance and capacity of VZW are assessed for different unsaturated zone parameters and well structures. For the case of an unsaturated zone with a small storage capacity, a greater proportion of injected water will be stored in the saturated zone, thus is a favored situation for the implement of MAR using VZWs. Both anisotropy and infiltration well structures have nonnegligible effects on the recharge rate of the saturated zone at early times but have negligible influence at later times. The proposed semianalytical solutions can be used to estimate unsaturated parameters and can also be used to assess the effectiveness of VZWs for the purpose of MAR.