Water pathways in the Northern Guam Lens Aquifer

The NGLA is recharged by rainwater that makes its way down through the rocks to become groundwater. When it rains, most of the water in northern Guam falls directly on the limestone surface. Part of it evaporates back into the atmosphere, and part percolates downward into the ground. This is called autogenic recharge because the surface of the aquifer itself captures recharge water.

Water that moves downward toward the aquifer can slowly percolate through small and poorly connected pores in the rock and take months to years to reach the water table. It can also flow into open fractures and shafts that allow much faster flow and recharge within minutes or hours. In general, such fast flow routes exist beneath sinkholes and other depressions in northern Guam. For that reason, sinkholes are the primary recharge features at the NGLA land surface.

Some rain falls on non-limestone ground. Water that falls on the volcanic terrain of Mt. Santa Rosa, Mataguac Hill, and Palii Hill in northern Guam, and also the volcanic slopes of southern Guam just south of the Pago-Adelup fault, forms small streams that flow downhill across the land surface. That is because the volcanic rocks on Guam, when compared to limestone, are not very permeable. They support surface drainage. Streams can flow over the land surface only as long as the land is volcanic. As soon as they reach the end limestone terrain, the streams sink beneath the ground surface. This is called allogenic recharge because other, non-limestone rocks capture recharge for the aquifer. The sinking streams disappear into sinkholes at the border of volcanic and limestone terrains and continue to move downhill across the volcanic surface beneath the limestone cover. These pathways are called basement conduits and allow for very fast recharge. They can be thought of as streams and rivers that flow on volcanic terrain just like in southern Guam, but merely have a cap of limestone on top of them.