Hydrogeologic Map of the Northern Guam Lens Aquifer

Jenson, John

Hydrogeologic Map of the Northern Guam Lens Aquifer

Thursday, December 6, 2018: 9:40 a.m.

Exhibit Hall- C4 & C5 (Las Vegas Convention Center)

Nathan Habana, PhD , 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

David Vann , Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam

Vivianna Bendixson , Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam

Douglas Roff , AECOM, San Diego, CA

Christine Simard , Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam

Rob Schumann , AECOM, San Diego, CA

Stephen Gingerich, Ph.D. , Oregon Water Science Center, U.S. Geological Survey, Portland, OR

The Northern Guam Lens Aquifer (NGLA) produces 90% (42/45 MGD) of Guam’s potable water for its 160,000 civilian and military residents and more than 1.3 million annual visitors. Plans are underway to increase the military presence, with concomitant increases in economic activity and demand for water. The NGLA is an uplifted Carbonate Island Karst aquifer, in which the porous, water-bearing limestone bedrock is partitioned into six groundwater basins by the much lower-permeability volcaniclastic basement rock that forms subterranean rises, ridges, and valleys above sea level beneath the limestone. Within each basin, freshwater is found in three groundwater zones: 1) the basal zone, in which the buoyantly-supported freshwater lens flows from the basement partitions in the interior toward the coast atop underlying saltwater; 2) the para-basal zone, at the head of the lens, where freshwater is underlain by the basement aquiclude and whence it enters the basal lens; and 3) the supra-basal zone, in which perched water flows down the aquifer-aquiclude contact into the para-basal zone ringing the flank of the basement where it rises above sea level. Field and modeling studies indicate that the porous matrix of the young (Miocene to Pleistocene) limestone supports storage of 0.20 and higher. Flow paths can be strongly controlled by regional-scale fractures and conduits in this triple-porosity aquifer. Successful modeling, exploration, well design, production management, and aquifer protection strategies depend on having a reliable hydrogeologic map of the aquifer, showing the zones within the basins and the expected flow paths. This presentation shows a start-of-the art map with the modeled water table and inferred boundaries of the basins and zones based on basement topography derived from borehole and geophysical data. The NGLA map has become an essential tool for local water developers and managers, federal and local regulators and planners, and water resources educators.

Nathan Habana, PhD, Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam

Dr Habana is Assistant Professor of Groundwater Hydrology at the University of Guam's Water & Environmental Research Institute of the Western Pacific (WERI). His duties at WERI include serving as Coordinator for the Guam Hydrologic Survey Program, which compiles and analyzes basic data on Guam’s freshwater resources. His current research interests include modeling of vadose and phreatic transport, and the management of salinity and nitrate contamination in island aquifers.

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.

David Vann, Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam

Graduate of University of Guam Environmental Science Program. Lead author of WERI Technical Report 142.

Vivianna Bendixson, Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam

Graduate of University of Guam Environmental Science program. Lead author of WERI Technical Report 141.

Douglas Roff, AECOM, San Diego, CA

Senior Hydrogeologist, AECOM.

Christine Simard, Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam

Graduate of University of Guam's Environmental Science program. Lead author of WERI Technical Report 143.

Rob Schumann, AECOM, San Diego, CA

Senior Hydrogeologist, AECOM.

Stephen Gingerich, Ph.D., Oregon Water Science Center, U.S. Geological Survey, Portland, OR

Dr. Stephen Gingerich has been a research hydrologist for the U.S. Geological Survey since 1995, and is now located in Portland, Oregon. Dr. Gingerich has extensive experience in the hydrology and water resources of oceanic islands - he has worked on groundwater resources issues on islands all over the world, with a focus on Hawaii. He performed field studies to characterize island coastal aquifers, recharge, and hydrogeology, and he is an expert on numerical modeling of seawater intrusion in coastal aquifers.