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Drywell infiltration and hydraulic properties in heterogeneous soil profiles
Citation:
Sasidharan, S., S. Bradford, J. Simunek, AND S. Kraemer. Drywell infiltration and hydraulic properties in heterogeneous soil profiles. JOURNAL OF HYDROLOGY. Elsevier Science Ltd, New York, NY, 570:598-611, (2019). https://doi.org/10.1016/j.jhydrol.2018.12.073
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 of groundwater storage in the subsurface aquifer. This investigation of hypothetical scenarios informed by site specific data associated with the drywell at the Army Fort Irwin, California, explores the impact of unknown heterogeneity in the physical properties of the unsaturated soils on water movement. A physics-based computer model was used to simulate water flow through a large number of realizations of possible distributions of variably permeable soil horizons. The research demonstrated that a constant head hydraulic test, rather than a falling head test, will be most effective at representing an effective hydraulic conductivity of the soil profile integrating the layered heterogeneity that exists. The effective hydraulic conductivity, might be effective for asses
Description:
Drywells are increasingly used to capture stormwater runoff for surface infiltration and aquifer recharge, but little research has examined the role of ubiquitous subsurface heterogeneity in hydraulic properties on drywell performance. Numerical experiments were therefore 29 conducted using the HYDRUS (2D/3D) software to systematically study the influence of subsurface heterogeneity on drywell infiltration. Subsurface heterogeneity was described deterministically by defining soil layers or lenses, or by generating stochastic realizations of soil hydraulic properties with selected variance (σ) and horizontal (X) and vertical (Z) correlation lengths. The infiltration rate increased when a high permeability layer/lens was located at the bottom of the drywell, and had larger vertical and especially horizontal dimensions. Furthermore, the average cumulative infiltration (I) for 100 stochastic realizations of a given subsurface heterogeneity increased with σ and X, but decreased with Z. This indicates that the presence of many highly permeable, laterally extending lenses provides a larger surface area for enhanced infiltration than the presence of isolated, highly permeable pockets. The ability to inversely determine soil hydraulic properties from numerical drywell infiltration results was also investigated. The hydraulic properties and the lateral extension of a highly permeable lens could be accurately determined for certain idealized situations (e.g., simple layered profiles) using constant head tests. However, variability in soil hydraulic properties could not be accurately determined for systems that exhibited more realistic stochastic heterogeneity. In this case, the heterogeneous profile could be replaced with an equivalent homogeneous profile and values of an effective isotropic saturated conductivity (K_s) and the shape parameter in the soil water retention function (/alpha) could be inversely determined. The average value of K_s for 100 stochastic realizations showed a similar dependency to I on σ, X, and Z. Whereas, the average value of /alpha had large confidence interval for soil heterogeneity parameters and played a secondary role in drywell infiltration. This research provides valuable insight on the selection of site, design, installation, and long-term performance of a drywell.
