Groundwater recharge from drywells under constant head conditions

Drywells are widely used as managed aquifer recharge devices to capture stormwater runoff and recharge groundwater, but little research has examined the role of subsurface heterogeneity in hydraulic properties on drywell recharge efficiency. Numerical experiments were therefore conducted on a 2D-axisymmetric domain using the HYDRUS (2D/3D) software to systematically study the influence of various homogenous soil types and subsurface heterogeneity on recharge from drywells under constant head conditions. The mean cumulative infiltration (mu I) and recharge (mu R) volumes increased with an increase in the saturated hydraulic conductivity (K-s) for various homogeneous soils. Subsurface heterogeneity was described by generating ten stochastic realizations of soil hydraulic properties with selected standard deviation (sigma), and horizontal (X) and vertical (Z) correlation lengths. After 365 days, values of mu I, mu R, and the radius of the recharge area increased with s and X but decreased with Z. The value of mu R was always smaller for a homogeneous than a heterogeneous domain. This indicates that recharge for a heterogeneous profile cannot be estimated with an equivalent homogeneous profile. The value of mu R was always smaller than mu I and correlations were highly non-linear due to vadose zone storage. Knowledge of only infiltration volume can, therefore, lead to misinterpretation of recharge efficiency, especially at earlier times. The arrival time of the wetting front at the bottom boundary (60 m) ranged from 21 to 317 days, with earlier times occurring for increasing s and Z. The corresponding first arrival location can be 0.1-44 m away from the bottom releasing point of a drywell in the horizontal direction, with greater distances occurring for increasing s and X. This knowledge is important to accurately assess drywell recharged performance, water quantity, and water quality.