Virus transport from drywells under constant head conditions: a modeling study

Authors: SASIDHARAN, SALINI - University Of California; Bradford, Scott; SIMUNEK, JIRI - University Of California; GERBA, CHARLES - University Of Arizona; KRAEMER, STEPHEN - Environmental Protection Agency (EPA)

Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2021
Publication Date: 3/12/2021
Publication URL: https://handle.nal.usda.gov/10113/7313432
Citation: Sasidharan, S., Bradford, S.A., Simunek, J., Gerba, C.P., Kraemer, S.R. 2021. Virus transport from drywells under constant head conditions: a modeling study. Water Research. 197. Article 117040. https://doi.org/10.1016/j.watres.2021.117040.
DOI: https://doi.org/10.1016/j.watres.2021.117040

Interpretive Summary: Drywells are increasingly used for stormwater capture and aquifer recharge, but there is a potential risk of microbial contamination. Numerical experiments were therefore conducted to study virus transport from a drywell under different removal and soil layering scenarios. The setback distance between the bottom of the drywell and the groundwater table to protect groundwater quality was found to be much larger than the currently recommended guideline under many instances. Results show conditions that influence the risk of groundwater contamination by the virus. The risk of virus groundwater contamination can be mitigated by intermittent use of the drywell and in-situ soil treatment with iron compounds. These results will be of interest to scientists, engineers, water managers, government regulators, and public health officials concerned with microbial risks to groundwater during recharge operations.

Technical Abstract: Many arid and semi-arid regions of the world face challenges in maintaining water quantity and quality needs of growing populations. A drywell is an engineered vadose zone infiltration device that is widely used for stormwater capture and aquifer recharge. To our knowledge, no prior studies have quantitatively examined virus transport from a drywell, especially in the presence of subsurface heterogeneity. Axi-symmetric numerical experiments were conducted to systematically study virus fate from a drywell for various virus removal and subsurface heterogeneity scenarios under steady-state water flow conditions. Subsurface domains were homogeneous or had stochastic heterogeneity with selected variance (sigma) and horizontal (X) and vertical (Z) correlation lengths. Low levels of virus concentration tailing can occur even at a setback distance of 22 m from the bottom of the drywell, and 6-log10 virus removal was not achieved when a small detachment rate (kdet=1×10'5 min'¹) is present in a homogeneous domain. Improved virus removal was achieved at a depth of 22 m in the presence of horizontal layering (e.g., X=10, Z =0.1, sigma=1) that enhanced the lateral movement and distribution of virus. In contrast, faster downward movement of the virus with an early arrival time at a depth of 22 m occurred when considering a vertical correlation in permeability (X= 1 m, Z= 2 m, s= 1). Therefore, the general assumption of a 1.5–12 m separation distance to protect water quality may not be adequate in some in- stances, and site-specific microbial risk assessment is essential to minimize risk. Microbial water quality can potentially be improved by using an in situ soil treatment with iron oxides to increase irreversible attachment and solid-phase inactivation.