Vadose Hydrology at Jinapsan Cave, Northern Guam

Monday, December 4, 2017: 4:50 p.m.
102 B (Music City Center)
Kaylyn Bautista , Water and Environmental Research Institute, 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 and Environmental Research Institute, University of Guam, Mangilao, Guam
Timothy Righetti, PhD , College of Natural and Applied Sciences, University of Guam, Mangilao, Guam

Six years of monthly data were analyzed from an active tropical limestone cave in Guam, the southernmost Mariana Island in the western Pacific Ocean. The purpose of this study was to characterize rates and variability of vadose percolation in the Plio-Pleistocene Mariana Limestone, which occupies about 75% of the surface of the Northern Guam Lens Aquifer (NGLA). A ground survey grid was established on the surface above the cave, a vegetated talus slope beneath the >150-m-high cliff behind the cave. Cave and vadose zone 3-D models were constructed from the surface survey and a cave interior survey. Cross sections display talus slope features, inferred epikarst and vadose layer dimensions, cave floor slope, and structural and geomorphic features of the cave, including a brackish water-table pool at the cave bottom. A plan-view map displays significant boulder talus and limestone-forest trees, cave entrance location, and the underlying cave boundary and fractures mapped on the cave ceiling. Thicknesses of the talus and vadose bedrock sections range from 1.3 to 17.0 meters and 1.7 to 46.4 meters, respectively. Drip rate and discharge rate data from 7 cave stations are presented in graphs showing varying responses between percolation and changes in rainfall during wet (Jun-Nov) and dry (Dec-May) seasons. Six stations exhibited seasonal drip responses to wet-dry rainfall. One (the slowest) displayed mostly perennial dripping, with several overflow occurrences. Average drip rate, plotted on a log scale, divided stations based on order-of-magnitude into inferred hydrologic preferential pathway categories: fracture flow (fast; 103-104 drips/hr); fracture-fissure (fast; 102-103 drips/hr); small fissure flow (medium; 101-102 drips/hr); and matrix flow (slow; <101 drips/hr).
Kaylyn Bautista, Water and Environmental Research Institute, University of Guam, Mangilao, Guam
Kaylyn K. Bautista is a recent graduate from the Water and Environmental Research Institute at the University of Guam. Although her graduate studies emphasized karst hydrology, she had the fortunate opportunity to collaborate on projects in Guam involving field work on well water chemistry and instrumentation, coastal discharge site documentation - springs, fractures, and caves, contact spring instrumentation, and cave surveying and mapping. She resides in Guam with her husband, John, and three daughters.


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 and Environmental Research Institute, University of Guam, Mangilao, Guam
Dr. Mark Lander is a professor of meteorology at the Western and Environmental Research Institute at the University of Guam.


Timothy Righetti, PhD, College of Natural and Applied Sciences, University of Guam, Mangilao, Guam
Dr. Timothy Righetti is an Associate Professor of Biology at the College of Natural and Applied Sciences at the University of Guam.


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