Hydrogeology of the Finegayan Basin, Northern Guam Lens Aquifer, Guam

Monday, December 4, 2017: 4:30 p.m.
101 D (Music City Center)
Ida Shalilian , Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam
John Jenson, PhD , Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam
Mark Lander, PhD , Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam
Richard Randall , Marine Laboratory, University of Guam, Mangilao, Guam

The Finegayan Basin of the Northern Guam Lens Aquifer is already well developed, but is expected to undergo additional economic development in the near future, with expansion of US military activities on Guam. The purpose of this study was to better understand the natural plumbing that controls groundwater recharge, transmission, and discharge of the basin, preliminary to exploration for new groundwater production wells. A field survey showed that the single greatest concentration of freshwater discharge from the northwest coast issues from a coastal cave that lies precisely at the end of a major fault. An initial estimate suggested discharge from the cave of up to 5.3 Mgal/day, which would constitute of 32% of basin recharge. Given the 4.8 Mgal/day of withdrawal from the basin, discharge of 5.3 Mgal/day would further constitute 45% of the remaining total basin discharge of 11.7 Mgal/day. The calculated hydraulic gradient along the fault for 5.3 Mgal/day discharge, assuming hydraulic conductivity of 75,000 m/d along the fault, a minimum width of 10 m for the conductive zone, and an average thickness of 30 m for the freshwater lens, is 9 x 10-4. This is consistent with a regional hydraulic gradient of about 3 x 10-4 estimated from previous modeling studies. The estimate of 5.3 Mgal/day for this single discharge point also compares reasonably with a recent finite-element model estimate of 31 Mgal/day discharge from the coastal zone centered around the cave. Additional, more-sophisticated field measurements of discharge are recommended to test the accuracy of the 5.3 Mgal/day estimate for the cave discharge. This presentation offers a hypothesis for the influence of the fault on the drainage of the Finegayan Basin and makes concomitant recommendations for revision of the basin boundaries to reflect the hypothesized influence of the fault, based on the Carbonate Island Karst Model.
Ida Shalilian, Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam
Ida Shalilian is earning an MS in Environmental Science at the University of Guam. She holds a BS in Geology from the University of Houston. Her MS thesis research included mapping and measuring the hydrogeologic properties of one of the most productive and geologically complex basins of the Northern Guam Lens Aquifer.


John Jenson, PhD, Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam
Dr Jenson is Professor of Groundwater Hydrology, and Director of the University of Guam's Water & Environmental Research Institute of the Western Pacific (WERI). His current research interests include development of the Carbonate Island Karst Model and its application to numerical modeling and sustainable management of island karst aquifers.


Mark Lander, PhD, Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam
Dr. Lander is a tropical meteorologist with expertise in tropical weather, tropical climate, typhoon forecasting and prediction, climate variability and climate change, and the ENSO phenomenon. His research efforts include field work on Guam and across the Micronesia region, and operational forecasting of seasonal typhoon risk and ENSO-related drought. Dr. Lander is active in the international community, and has given lectures on tropical climate and typhoon behavior at venues in Thailand, Australia, Fiji, Majuro, Pohnpei, Oman, Hong Kong, Hawaii and China. He also teaches graduate-level courses in the University of Guam’s Environmental Science program.


Richard Randall, Marine Laboratory, University of Guam, Mangilao, Guam
Richard Randall is Professor Emeritus of Marine Biology at the University of Guam Marine Laboratory. He studies the geology of island-arc systems and the systematics of corals, particularly those in Micronesia. He is currently describing several new species of scleractinian corals from the Mariana Islands.


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