The surface of northern Guam is composed of diagenetically immature, highly porous limestone. Because such rock cannot support surface water flow, northern Guam lacks streams, rivers, ponds, and other surface water bodies. (The only exceptions are volcanic inliers of Mt. Santa Rosa and Mataguac Hill, and clay-rich limestone in the southern part of the plateau, where a few streams do occur.)

Drainage in northern Guam is subterranean, via pores in the rock, fractures, and karst conduits. The limestone bedrock of the northern Guam plateau contains the Northern Guam Lens Aquifer (NGLA). The aquifer underlies the entire northern half of Guam and contains a large and permanent body of fresh groundwater.

The body of groundwater within the Northern Guam Lens Aquifer is approximately lens-shaped, being the thickest in the island's interior and thinnest along the island's perimeter. Because fresh water is less dense than seawater, this fresh groundwater lens floats upon saltwater that permeates the bedrock beneath it. Freshwater in the aquifer is replenished by rain seeping/flowing downward through the limestone from the land surface. Fresh water naturally discharges from the aquifer along its perimeter via coastal springs and is also extracted by people via production wells where water is pumped upward to the land surface and then distributed to provide island residents with potable drinking water.

Factsheet:

Click below to open full-sized version.

Maps:

Use the menu to the right to view relevant maps.