Water that rains in northern Guam and does not evaporate moves underground to recharge the aquifer. Much of it infiltrates directly into the ground, or flows short distances overland into the nearest surface depression. Normal overland flow occurs only on top of volcanic terrain and also leads to surface depressions in the surrounding limestone. Many depressions are karst sinkholes that lead to high permeability pathways in the subsurface and can accommodate large amounts of water. Due to blockage of pathways by debris and/or accumulation of insoluble materials (clay, etc.) at the bottom of the depression, a sinkhole may not be able to convey underground all water that it takes in. That results in temporary ponding of water. Such water is called perched water.

Water in sinkholes generally moves as fast flow downward through vadose shafts, basement conduits at the boundary of volcanic rocks and overlying limestone, and other open spaces in the bedrock. Such flow recharges the aquifer quickly and without filtration. Water elsewhere descends through slow percolation. It moves via small pores and relatively unconnected open spaces in the bedrock and recharges the aquifer slowly and after filtration.

Factsheet:

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Once in the saturated zone, water diffuses or flows within the aquifer and discharges along the coast. There exist definite preferential pathways that conduct water horizontally from inland areas toward the discharge margin of the lens. Studies by dye tracing mark water at recharge points inland and detect its discharge at specific points along the coast and after specific time periods. This type of research gives information about the layout and efficiency of water pathways within the aquifer. For example, a dye trace study of the prominent sinkhole in Harmon has revealed subsurface connections to major springs in Tumon and Hagåtña bays.

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