Natural discharge from the Northern Guam Lens Aquifer

The Northern Guam Lens Aquifer (NGLA) naturally discharges freshwater along its perimeter, which coincides with the coastline of northern Guam. Groundwater discharge features are readily observable in many places along the coast. They are particularly visible during low tides, when freshwater can be seen emerging from beach sand and shallow reefs, as well as fractures and openings in coastal bedrock. The fact that coastal discharge can be diffuse (from wide regions of beach or reef), or emerges concentrated from fractures and cave-like openings of conduits suggests that the NGLA is a triple-porosity aquifer. In it, matrix, fracture, and conduit porosity each make a substantial contribution to discharge (and presumably to internal flow as well). The matrix-fracture-conduit model is fully interactive, with fractures and caves that discharge water along the lower-permeability peripheral parts of the island are themselves fed by matrix flow arriving from the higher-permeability island interior.

Variable-permeability island aquifers like the NGLA are fundamentally different from non-carbonate coastal aquifers, from which discharge is generally dominated by diffuse flow, and from continental karst aquifers in diagenetically mature limestones, in which flow and discharge are dominated by conduit flow.