Drywell infiltration and hydraulic properties in heterogeneous soil profiles

Authors: SASIDHARAN, SALINI - University Of California - Cooperative Extension Service; Bradford, Scott; SIMUNEK, JIRI - University Of California; KRAEMER, STEPHEN - Environmental Protection Agency (EPA)

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/26/2018
Publication Date: 1/11/2019
Citation: Sasidharan, S., Bradford, S.A., Simunek, J., Kraemer, S.R. 2019. Drywell infiltration and hydraulic properties in heterogeneous soil profiles. Journal of Hydrology. 570:598-611. https://doi.org/10.1016/j.jhydrol.2018.12.073.
DOI: https://doi.org/10.1016/j.jhydrol.2018.12.073

Interpretive Summary: Spatial variations in soil properties are expected to influence the ability of drywells to infiltrate captured storm water and recharge groundwater, but little research has previously addressed this issue. Numerical experiments were therefore conducted to investigate the influence of soil heterogeneity on drywell performance. Results demonstrate that drywell infiltration is generally enhanced by soil heterogeneity, especially when conductive lenses with large lateral extensions occur near the bottom of the drywell. Observations of drywell infiltration behavior can also be used to estimate soil properties that can be used to predict drywell behavior. These simulation results will be of interest to scientists and engineers concerned with managing drywells for runoff water capture and aquifer storage.

Technical Abstract: 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 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 (s) 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 s 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 lenses. 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 () and the shape parameter in the soil water retention function () could be inversely determined. The average value of for 100 stochastic realizations showed a similar dependency to I on s, X, and Z. Whereas, the average value of 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.