Northern Guam geospatial information server

"Plumbing" of the NGLA

Recharge involves fresh water making its way from the land surface (where it arrives as rain) down through the unsaturated bedrock all the way to the water table. Some of this water percolates through small and poorly connected pores in the rock. Water subject to this vadose percolation can takes months to years to reach the water table. However, a lot of water flows down via rapid pathways that extend from solution sinkholes at the land surface and continue via widened fractures and natural shafts downward to the water table. Water involved in vadose fast flow reaches the water table in minutes to hours. Many solution sinkholes that lead to preferential pathways for water in the subsurface have been developed as ponding basins as northern Guam has been subject to extensive urban development.

Storage and flow of water within the saturated zone below the water table can also be slow or rapid. It can be diffuse (slow) via pores in the rock or poorly or somewhat better interconnected series of voids, but also turbulent (rapid) through widened fractures and possibly networks of cave-like conduits.

Discharge from the aquifer occurs naturally along the coast. The coast represents the outer perimeter, margin of the aquifer where water leaves the aquifer and enters the ocean. This freshwater flow can be observed in numerous seepage zones and springs along beaches and rocky shores in northern Guam.

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