Taboroši, D.; Jenson, J.W., and Mylroie, J.E., 2013. Field observations of coastal discharge from an uplifted carbonate island aquifer, northern Guam, Mariana Islands: a descriptive geomorphic and hydrogeologic perspective.
Understanding the hydrogeologic properties of carbonate island aquifers requires a conceptual framework within which specific and local properties, especially quantitative phenomena, can be placed and evaluated. In this paper, we present observations and hypotheses for such a qualitative conceptual model based on more than a decade of field studies in Guam. Fieldwork has shown that coastal groundwater discharge features can be usefully classified in terms of their geomorphic context and distinctive, readily observable characteristics: beach seeps and springs, reef seeps and springs, fracture springs, and cave springs. Seeps and springs on beaches and reefs are associated with matrix porosity and diffuse flow or are fed by fissures buried under sediment. Concentrated discharges from fractures and conduits are directly visible on rocky coasts, where springs emerge from dissolution-widened fractures and karst cavities. Although coastal zone conditions make it very difficult to accurately quantify the discharges from each type of feature, the fact that discharge is visibly distributed among these four categories suggests that aquifers of islands such as Guam—composed of uplifted geologically young limestones—can generally be expected to be triple-porosity aquifers, in which matrix, fracture, and conduit porosity each make a substantial contribution to discharge, and presumably to internal flow as well. This is in contrast to noncarbonate aquifers, from which coastal discharge is generally dominated by diffuse flow, and to continental karst aquifers in diagenetically mature limestones, in which flow and discharge are dominated by conduit flow. We propose a conceptual framework that relates these four geomorphic categories of coastal discharge to the evolution and reorganization of matrix, fracture, and conduit porosities in the aquifer, and offer a hypothesis for the general distribution of hydraulic conductivity on uplifted carbonate island aquifers, in particular the juxtaposition of high conductivity in the interior against lower conductivity in the periphery.