Soil Property and Subsurface Heterogeneity Control on Groundwater Recharge of Vadose Zone Injection Wells

Vadose zone well (VZW) injection is an effective method of managed aquifer recharge (MAR), and it plays an important role in semiarid and arid regions. Accurate estimation of the recharge of VZW injection remains a challenge in the field of hydrology due to the nonlinear nature of the process in the vadose zone. To improve knowledge of VZW injection, a finite-element numerical model was developed by COMSOL Multiphysics to evaluate the soil property and subsurface heterogeneity control on the recharge of VZWs. Several numerical experiments were conducted to estimate the characteristic arrival time and cumulative recharge volume of injected water for different subsurface conditions. Simulation results indicate that soil properties have a profound influence on the recharge of VZWs and saturated hydraulic conductivity plays a more important role than the Van Genuchten –Mualem soil constitutive model parameters $\alpha$ and $n$ in the influences on the recharge. Subsurface heterogeneity has large impacts on the recharge of VZWs as well. The presence of a low permeable layer in the aquifer hinders the infiltration of injected water and reduces recharge rate $Q_{wt}$ and cumulative recharge volume $V_{wt}$. These influences are affected by depth $D_c$, lateral extension $R_c$, thickness $B_c$, and hydraulic conductivity $K_s$ of the low permeable layer. Whether the well screen cuts through the low permeable layer is a factor that influences the effects of these parameters in two forms. When the well screen cuts through the low permeable layer, $Q_{wt}$ and $V_{wt}$ decrease substantially as $D_c$ increases, and the change of $B_c$ has a greater impact on recharge than $K_s$; when the low permeable layer is below the well screen, the change of $D_c$ only has slight influence on recharge, and the cases with the same hydraulic conductance value of $K_s/B_c$ have similar $Q_{wt}$ and $V_{wt}$. For the low permeable lens, when the well screen cuts through it, the increase of $R_c$ leads to slight decrease of $Q_{wt}$ and $V_{wt}$, which are more obvious in the later stage of VZW injection. When the low permeable lens is below the well screen, the increase of $R_c$ makes $Q_{wt}$ decrease in the early stage of VZW injection, and then $Q_{wt}$ for different values of $R_c$ reach similar asymptotic values in the later stage. Understanding the influences of soil properties and subsurface heterogeneity on the recharge of VZWs provides physically based guidance for the design and management of VZWs.