Impact of Density-Dependent Flow and Aquifer Heterogeneity on Virus Transport and Removal During Aquifer Storage and Recovery

Aquifer storage and recovery (ASR) is increasingly used worldwide to maintain, enhance and secure freshwater availability. However, its implementation presents challenges due to the potential risk of virus contamination from injected water sources such as stormwater runoff and treated wastewater, as well as premature breakthrough of native groundwater caused by density-dependent flow. This study incorporates the virus transport and removal processes into a 2D-axisymmetric numerical model of coupled density-dependent groundwater flow and salt transport, accounting for physical heterogeneity with varying connectivity features, correlation lengths and layer structures. Geochemical heterogeneity is modeled using Colloid Filtration Theory (CFT), linking attachment rate coefficients to permeability distribution. The results reveal that density-dependent flow enhances virus removal, particularly during the storage phase, by distorting virus plume and increasing virus attachment. Neglecting density effects leads to an underestimation of virus removal, which in turn overestimates the required post-treatment intensity, especially under stricter potable standards. Aquifer heterogeneity exerts a coupled and dual control on density-driven virus removal, enhancing it through high-permeability connectivity during storage but reducing it through preferential flow and limited attachment during recovery. This study underscores the potential of native brackish-to-saline groundwater conditions to enhance virus attenuation in ASR systems. The findings contribute to existing guidelines for site selection and ASR system design, along with considerations for pre-/post-treatment and/or desalination facilities, by emphasizing the importance of density-dependent flow, aquifer heterogeneity, and project-specific objectives of ASR.