Experimental observations of aquifer storage and recovery in brackish aquifers using multiple partially penetrating wellsExperimentelle Untersuchungen zur Speicherung von Süßwasser in brackischen Grundwasserleitern mittels Multilevel-BrunnenObservations expérimentales du stockage et de la récupération en aquifère dans des aquifères saumâtres à l’aide de puits multiples à pénétration partielleObservaciones experimentales sobre el almacenamiento y la recuperación de acuíferos de aguas salobres mediante pozos múltiples parcialmente penetrantes咸水含水层中基于多个非完整井的含水层储水和恢复的实验观测Observações experimentais de armazenamento e recuperação de aquíferos em aquíferos salobras usando vários poços parcialmente penetrantes

Aquifer storage and recovery systems using multiple partially penetrating wells (MPPW-ASR) can form a viable solution to the problem of freshwater buoyancy when using brackish aquifers for freshwater storage. This study presents the result of a series of laboratory experiments that aimed at visualizing the shape of freshwater bodies injected into a brackish aquifer and determining the effect on the recovery efficiency (RE) of several MPPW-ASR operational variables. A model aquifer was built in a Plexiglas tank using glass beads and water was injected and abstracted through point and vertical wells, which were operated in various combinations. Numerical models were used to support the interpretation of the time-lapse photographs, and showed that three-dimensional flow effects had to be considered for a correct interpretation of the visible dye patterns. Upward migration of both fresh (during injection) and brackish water (during recovery) along the vertical wells was observed, indicating that the role of well infrastructure as conduits is a critical design criterion for real-world systems. Gravitational instabilities formed when freshwater did not extend all the way to the top of the aquifer, and this negatively impacted the RE by causing greater mixing. The positive freshwater buoyancy led to freshwater bodies that became narrower with depth, and the formation of thin, elongated buffer zones along the aquifer top in multicycle experiments. Up-coning below abstraction wells resulted in lower RE values, reinforcing the potential of scavenger wells to enhance MPPW-ASR system performance.