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Abstract
The Northern Guam Lens Aquifer (NGLA) is Guam’s primary freshwater source, with a lens that responds dynamically to recharge, drought, and tidal influences. Building on the 2019 pilot study by Dougher et al., which focused on the Tomhom Basin, this study extends the analysis to the Machanao, Hagåtña, and Pågu Aquifer Basins. Using data from nine Deep Observation Wells (DOWs), this study examined the aquifer’s historic response to drought and storm-related recharge. Conductivity, temperature, and depth (CTD), rainfall, tide, and climate data from 2000 to 2024 were compiled to develop phreatic hydrographs for each DOW and evaluate freshwater lens response to recharge and drought. Results showed that lens thickness varied among DOWs, reflecting differences in recharge and drought response that may be influenced by local geology and DOW location. DOWs in the Barrigada Limestone, including those in the Tomhom and Machanao Basins, generally exhibited thicker and more stable freshwater lenses, whereas DOWs in the argillaceous Mariana Limestone of the Hagåtña and Pågu Basins showed thinner, more complex, and more erratic fresh to brackish lenses. Understanding these dynamic factors may contribute to improved groundwater model development for evaluating Guam’s water supply under different climatic and hydrogeologic conditions. The findings provide insight into aquifer behavior and may support future groundwater management strategies for Guam’s water security.
Key Words
Northern Guam Lens Aquifer, freshwater lens, Deep Observation Wells, recharge, drought, water security
