Field Trip - Northern Guam Lens Aquifer Jun 2014

Idealized profile of a Carbonate Island Karst Aquifer Vertical scale is greatly exaggerated. Please see next page for a diagram that is drawn to scale. T H R E E G R O U N D W A T E R BASAL WATER • • • • underlain by saltwater vulnerable to saltwater contamination variable quality water easy to find (underlies most of northern Guam) Z O N E S PARA-BASAL WATER • underlain by basement rock • resistant to saltwater contamination • ”upstream” from surface threats • more affected by wet-dry cycles than basal water • hard to find SUPRA-BASAL WATER • • • • • underlain by basement rock and standing above sea level invulnerable to saltwater contamination very high quality water — catchment “headwaters” more responsive to wet-dry cycles than para-basal water very hard to find (luck is important, even with map...) Interior highlands Sinkholes Coastal lowlands Reef LIMESTONE Water table Mean sea level OCEAN Mixin SALTWATER g zone 1:40 FRESHWATER LENS Saltwater toe VOLCANIC BASEMENT Over geologic time, porosity of periphery decreases and discharge concentrates into fissures feeding coastal springs. 12 12 13 13

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