Science Inventory

Groundwater Recharge from Drywells Under Constant Head Conditions

Citation:

Sasidharan, S., S. Bradford, J. Simunek, AND S. Kraemer. Groundwater Recharge from Drywells Under Constant Head Conditions. JOURNAL OF HYDROLOGY. Elsevier Science Ltd, New York, NY, 583:124569, (2020). https://doi.org/10.1016/j.jhydrol.2020.124569

Impact/Purpose:

The construction of drywells, or gravity-fed wells located in the unsaturated soils above the water table, is occuring in the arid and semi-arid USA to infiltrate captured stormwater and provide recharge into groundwater storage in the subsurface aquifer. In Los Angeles, for example, groundwater is a significant source of local drinking water supply. This investigation of hypothetical scenarios informed by site specific data associated with the drywell at the Army National Training Center at Fort Irwin, California, explores the impact of unknown heterogeneity in the physical properties of the unsaturated soils on water movement. A previous article described the use of a physics-based computer model, HYDRUS 2D/3D, to simulate water flow through realizations of possible distributions of variably permeable soil horizons. The previous research demonstrated that a constant head hydraulic test, rather than a falling head test, will be effective at evaluating the system performance. In this paper, the efficiency of the drywell system for translating infiltration into recharge is investigated, and the importance role that vertical and horizontal heterogeneities in soil properties in lateral spread of infiltrating water and the time for the infiltrating water to reach the water table is analyzed. The implications for site characterization and design and monitoring of drywells is discussed.

Description:

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 (μI) and recharge (μR) 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 (σ), and horizontal (X) and vertical (Z) correlation lengths. After 365 days, values of μI, μR, and the radius of the recharge area increased with σ and X but decreased with Z. The value of μR 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 μR was always smaller than μI 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 21-317 days, with earlier times occurring for increasing σ 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 σ and X. This knowledge is important to accurately assess drywell recharged performance, water quantity, and water quality.