Does Enhanced Mixing Improve Groundwater Quality? Evaluating the Impact of Injection-Extraction Engineering on Redox Conditions, Contaminants of Emerging Concern, and Antibiotic Resistance Genes

Redox (reduction-oxidation) reactions play a crucial role in determining groundwater quality, as they strongly influence the behavior of Contaminants of Emerging Concern (CECs). The extent of mixing between electron donors and acceptors largely governs the occurrence of these reactions. Additionally, increased spatial and temporal variability in redox potential (Eh) has been shown to enhance the likelihood of CEC removal. Numerical modeling studies suggest that injection and extraction engineering can improve mixing, leading to greater Eh variability and, in turn, a higher potential for CEC removal. However, these studies often neglect the critical role of biofilm, as most subsurface redox reactions occur primarily within the relatively immobile biofilm where microbial activity dominates.

In this study, we explored the effects of enhanced mixing in the subsurface, achieved through the use of extraction and injection dipoles in a pilot-scale Managed Aquifer Recharge system, on redox potential distribution and the behavior of CECs and ARGs. Redox potential was continuously monitored using a network of over 30 probes, while CECs and ARGs removal and their associated toxicity (evaluated with zebrafish embryos) were assessed before and after the implementation of EIE. The application of three dipoles in a system with stratified geochemistry caused significant alterations in Eh, initiating new geochemical processes such as iron precipitation. Surprisingly, concentrations of both CECs and ARGs, along with associated toxicity, increased following EIE. This rise was attributed to biofilm detachment, which likely released sorbed CECs and ARGs into the aqueous phase, thereby amplifying their ecotoxicological effects.