Hydrogeologic Map of the Northern Guam Lens Aquifer
Thursday, December 6, 2018: 9:40 a.m.
Exhibit Hall- C4 & C5 (Las Vegas Convention Center)
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.