Northern Guam geospatial information server


This page explains some basic concepts that might help you better understand information provided on this website. It will also help you recognize and interpret natural features and phenomena that can be observed in northern Guam.

What is an aquifer?

An aquifer is an underground body of rocks (or unconsolidated materials such as sand and gravel) from which groundwater can be extracted using water wells. Aquifer rocks (or sediments) must has sufficient permeability and contain enough water to make its extraction economically feasible. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology.

In contrast with the aquifer, which is permeable bedrock capable of containing and transmitting water, there is aquiclude, which is relatively impermeable bedrock that typically represents the boundaries of an aquifer. If the aquifer has an aquiclude on top of it, the water inside the aquifer is said to be confined. This is not the case in Guam. Our aquifer is unconfined and the aquiclude are volcanic basement rocks located beneath the aquifer.

What is limestone?

Limestone is a sedimentary rock consisting mainly of calcium carbonate (CaCO3), which commonly occurs in two mineral forms: calcite and aragonite. It is a soluble rock and often has high porosity. The deposition of limestone occurs in marine and freshwater environments, by accumulation of CaCO3 shells and skeletons of various organisms. When these deposits become exposed to freshwater and atmosphere (as is the case on islands and coasts), or become deeply buried (as in the interiors of continents), they undergo a series of changes and become limestone.

Where does an aquifer's fresh water come from?

When it rains in northern Guam, the water strikes the land surface made of limestone. Because the limestone is highly porous and soluble, water cannot form gullies, streams, and rivers as is typical of the southern half of Guam and most places around the world. Instead, water percolates into the ground and becomes groundwater that replenishes the aquifer. All water in the aquifer originates as rain that percolates downward from the land surface. Once in the aquifer, groundwater flows outward toward the island perimeter and discharges from the aquifer via coastal springs.

How does the water cycle in nature?

Whenever it rains, water percolates into the ground and replenishes the aquifer (or in other areas, trickles over the land surface, runs into streams and rivers, and flows into the ocean). This continues year after year, yet the rain never ceases and water does not run out. This is because the rain is not the original source of water, but merely a step in an endless natural cycle through which water moves. Driven by the heat of the sun, the water in oceans, lakes, rivers and other water bodies warms up and evaporates. It changes from liquid form to gas form called water vapor. As the vapor cools down in the air it condenses to form clouds. When the clouds contain too much condensed water, droplets fall from the sky and create rain. This process is called precipitation. Rain falls back into the ocean from where it rises into the atmosphere again. The rain that falls onto land merely takes different routes: some of it percolates into the ground and flows as groundwater or enters streams and rivers and flows over land surface. Eventually, both surface flow and groundwater flow reach the ocean. Water from the ocean, of course, continuously evaporates and follows the never-ending water cycle.

Diagram above from "Student Atlas of Guam" by Danko Taborosi and David T. Vann, published by BessPress, 2007.

What is karst topography?

Karst is a unique type of topography that develops in limestone and other rocks that are soluble in water. The bedrock is shaped by dissolution and is unable to support surface flow as streams and rivers. Instead, water percolates underground and enlarges its pathways in the process. This creates subterranean networks that conduct most of the drainage in the area and leave the surface without or with few rivers and lakes. Most karst areas around the world display distinctive geological features, with sinkholes and caves being the most common.

Diagram above copyright of McGraw-Hill Companies.