Vadose zone well (VZW) injection is an effective method utilized in managed aquifer recharge and it plays an important role in semi-arid and arid regions. Accurately prediction of the recharge of VZW injection is still a challenge in the field of hydrology due to the nonlinear nature of the process in the vadose zone. To improve the water budget evaluation and management of VZW injection, three studies are conducted to investigate the influences of time-dependent ground surface flux (GSF) generated by infiltration or evapotranspiration on the VZW injection, the geometric and soil properties control on the recharge of VZW, and the subsurface heterogeneity control on the recharge of VZW. The semi-analytical solutions for the hydraulic head increments and recharge rate are derived for the coupled unsaturated-saturated governing equations by using Laplace-Hankel transforms. The analysis of the solutions indicates that GSF generated by infiltration can improve the recharge of VZW whereas GSF generated by evapotranspiration has the opposite influence. The influences of GSF on recharge of VZW are affected by the properties of the unsaturated zone. A finite-element numerical model based on the Van Genuchten-Mualem soil constitutive model is developed utilizing the COMSOL Multiphysics software to simulate VZW injection in a homogeneous aquifer and several numerical experiments are conducted to estimate the characteristic arrival time and cumulative recharge volume of injected water for different subsurface conditions. The simulation results indicate that coarser soils are beneficial to the recharge and the influences of well geometric properties (screen length and screen depth) on recharge are mainly reflected in affecting the travel distance of the injected water. The injection plan including the injection rate setting and schedule arrangement also has a great influence on recharge when the total volume of injected water is constant. A finite-element numerical model is developed with COMSOL Multiphysics to simulate VZW injection in a heterogeneous aquifer and the subsurface heterogeneity is conceived as the presence of a low permeable layer or lens. The presence of a low permeable layer in the aquifer hinders the infiltration of injected water and reduces recharge rate and cumulative recharge volume. These influences are affected by depth, lateral extension, thickness and hydraulic conductivity of the low permeable layer and the condition of whether the well screen cuts through the low permeable layer or lens. The knowledge of the influences of GSF, soil properties, geometric properties and subsurface heterogeneity on the recharge of VZW provides physically-based guidance for the design and management of VZWs.
ETD Chair
Zhan, Hongbin Holder of Endowed Dudley J. Hughes '51 Chair in Geology and Geophysics
