Hydrogeochemical Evaluation of Aquifer Storage and Recovery in Edwards Aquifer, New Braunfels, Texas.

This study examines the potential for aquifer storage and recovery (ASR) in the brackish portion of the Edwards aquifer in New Braunfels, Texas. Successful ASR relies on understanding hydraulic properties, aquifer heterogeneity, water geochemistry, and geochemical processes during operations. The research aims to investigate the chemistries of native groundwater and injectant during ASR operation, estimate the hydraulic properties of the aquifer layers, and assess the recovery rate for the recovered groundwater meeting the total dissolved solids (TDS) threshold. The study found that native groundwater is of Na-Cl facies due to halite dissolution and a possible basinal brine migration associated with the zone of greatest fault displacement. High sulfate ions in background native groundwater result from sulfate-bearing minerals' dissolution in the Kainer and Person Formations. The injectant water is of Ca-Mg-HCO facies due to the carbonate-rich composition of the aquifer host matrix and interaction with the Guadalupe River riverbed. During ASR operations, mixing controlled the shift in hydrochemical facies from Na-Cl to Ca-Mg-HCO.The study also suggests a possible connection between Kainer and Person Formations and preferential pathways in the targeted storage zone aquifer. The estimated conductivity values also indicate dominant horizontal flow via possible fracture pathways in both the Person and Kainer Formation storage zones. Recovery of groundwater meeting the TDS of 1000 mg/L requires a recovery rate of 0.03 m/s for 60 days after 40-day storage. This research emphasizes that understanding the hydrogeological conditions and geochemical processes is critical to ASR feasibility in brackish carbonate multi-aquifer fractured systems.