Northern Guam geospatial information server

NORTHERN GUAM LENS AQUIFER

This page explains the fundamentals of the Northern Guam Lens Aquifer (NGLA), the main source of fresh water on the island.

What is the NGLA?

The Northern Guam Lens Aquifer (NGLA) is the limestone bedrock that underlies the entire northern half of Guam and contains a large and permanent body of fresh groundwater. This body 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.

Where is the NGLA?

The Northern Guam Lens Aquifer encompasses all of northern Guam. It comprises of the limestone bedrock that is located north of the Pago-Adelup fault (geologic fault that separates the volcanic terrain of southern Guam from the limestone plateau in northern Guam). This body of limestone extends to the depth where it meets the volcanic units on top of which it sits. The volcanic units are the base (basement units) of the aquifer and are not a part of the aquifer itself. They are far less porous than the limestone and do not contain significant amounts of groundwater.

Where exactly is the water in the NGLA?

The aquifer is not to be imagined as subterranean lake in a giant underground room. The water is contained within the rocks themselves. Rocks, though solid and dense, are full of holes. There is lots of empty space within them. Rocks that have lots of empty space, like the limestone of northern Guam, can accommodate lots of water. The empty spaces within the northern Guam aquifer include pores and gaps originally made by coral when the limestone was still a live reef, cracks and fractures developed as a result of earthquakes and tectonic movements in general, and karst voids created by water itself as it moves through the rocks and removes some of the limestone bedrock in solution.

What kind of aquifer is the NGLA?

Typical aquifers, such as sandstone, tend to contain water in voids between rock grains, and move it at steady rates and in predictable directions. The NGLA, however, is much more complex. It is a karst aquifer. It contains water within an irregular system of variously sized and variously interconnected pores, fractures, and dissolved-out voids. Water moves through this system at highly variable speeds and in different directions, which may be difficult to predict. For example, in a karst aquifer like the NGLA, water at one point in the rock can be barely moving through tiny pores, while just a few meters away it could be speeding like an underground river through a cave. Therefore, the NGLA is a special type of aquifer in which movement, storage, accessibility, and quality of water are controlled by its karst features.

*******Does water move within the NGLA?

The NGLA is a highly dynamic system and water within it is always on the move. The freshwater lens is replenished by rain seeping downward through the limestone from the land surface. Much of this recharge water flows down via natural shafts that extend downward from sinkholes, many of which have been developed as ponding basins as northern Guam has been subject to urban development. Fresh groundwater naturally discharges from the lens along the coast, which represents the aquifer's perimeter and contains numerous springs that issue fresh groundwater from the NGLA into the surrounding ocean. Shown above is the idealized cross-section of the Northern Guam Lens Aquifer. It depicts overall geology of northern Guam as it influences the distribution and storage of water in the aquifer. Specifically, it shows the basement volcanic bedrock and areas of the freshwater lens that rest directly atop of it (parabasal water), as opposed to areas where fresh water floats atop of the salty groundwater (basal water). Also indicated on the diagram are sites of focused recharge (sinkholes) leading to conduits and fractures that convey meteoric and surface water through the vadose zone to the freshwater lens, and ultimately to the natural discharge points along the coast. ********The situation in northern Guam is very different. Northern Guam is a limestone plateau with virtually no streams or rivers. Rocks in northern Guam are water permeable and soluble, and the rain percolates into the ground instead of flowing on the surface. Driven by gravity, the water moves underground through air-filled fractures, voids, and conduits, and enlarges and connects them by dissolution. It reaches the groundwater level and a fresh groundwater body known as the freshwater lens. (See diagrams on the next page.) This “lens” of fresh water, recharged by rain, floats on top of the underlying seawater, which also easily moves through limestone. This relationship is based on buoyancy: fresh water (1.000g/cm3), is lighter than seawater (1.025g/cm3), and, therefore, floats on it. This freshwater “lens” is thickest in the center of northern Guam, and thins out toward the coastline. The body of rock “holding” the fresh water is known as the Northern Guam Lens Aquifer (NGLA) and is the primary source of drinking water on Guam. NGLA receives water from rain (recharge), and loses it from springs (discharge). The lens is underlain by salty groundwater (basal part of the lens) and volcanic rock (parabasal part of the lens). The aquifer has several hydrologic zones (see diagram below). The unsaturated zone above the groundwater level is called the vadose zone. Rainwater percolates down to the lens through this zone to reach the groundwater table, below which is the saturated, or phreatic zone. It contains the freshwater lens, salty groundwater below it, and the mixing zone between them. Fresh water is extracted from the lens and pumped upwards via production wells to provide island residents with potable drinking water. **** HOW DOES THE WATER MOVE IN NGLA? The Northern Guam Lens Aquifer is recharged by rainwater. This water percolates into the rock and slowly moves downward through holes and cracks in the epikarst (vadose seepage) or flows into sinkholes and caves (vadose flow). Water flowing in caves and other conduits recharges the aquifer quickly; but water moving slowly through the rock can be stored in pores and voids in the vadose zone and can take months to recharge the aquifer. In the phreatic zone, water can also move quickly through karst conduits (conduit flow) and slowly via cracks and pores in rock (diffuse flow). Eventually, groundwater reaches the lens periphery and discharges from coastal springs.

Diagram above from Taborosi, D. (2004) Field Guide to Caves and Karst of Guam. Bess Press, Honolulu.

Why is the NGLA important?

The Northern Guam Lens Aquifer (NGLA) supplies about 80% of Guam’s population with potable water. Protecting this vital resource from contaminants is crucially important both from a public health and sustainable development perspective. Contaminants can come from beneath the aquifer and from the land surface. Salty groundwater that underlies the aquifer is kept in its place by the weight of fresh groundwater sitting atop of it. If too much freshwater is removed by pumping, the amount of fresh water is reduced locally. That causes saltwater to move upward into the aquifer to achieve equilibrium. Contamination by seawater is a major risk in island aquifers. Other sources of contaminants are at the land surface. Domestic wastewater and sewage, industrial spills, and agricultural and stormwater runoff all have the potential of reaching the aquifer and contaminating it. Septic tanks and pit toilets are major sources of contaminants, and several thousands of these are located right above the NGLA. It is estimated that septic tanks alone discharge around 5 million gallons of wastewater into the Guam environment each day. The extent to which contaminants have impacted the chemical and biological integrity of the NGLA is currently unknown, but cannot be ignored, especially considering that the NGLA is a karst aquifer and much water within it moves rapidly through well-connected open spaces without being filtered.