Field Trip - Northern Guam Lens Aquifer Jun 2014 | Page 24

Stop 4: The Roof of the Aquifer –
the Water Catchment System

Hydrologic role of the aquifer surface

All of the water that recharges the freshwater lens begins as rainwater that falls on the
surface of the plateau. Some 70 percent of Guam’s rainfall arrives during its wet season, from
July through December. Ongoing studies of cave dripwaters suggest that hardly any of the other
30 percent that falls during the dry season contributes to recharge of the lens. Consistent with
these findings, another recent study of recharge suggests that about 50 percent of total annual
rainfall evaporates or is taken up and transpired by vegetation growing on the plateau, while
the other 50 percent descends through the limestone—either directly to the water table or onto
the flanks of the basement hills and valleys that occupy the 20 percent of the aquifer within the
supra-basal zone. Past studies suggest that over the long term up to about 30 percent of recharge
may descend via fast flow routes, such as observed at the previous stop. Quantifying aquifer
recharge remains one of the outstanding challenges of hydrological research on Guam.

Vista of entire aquifer, Summit of Mt. Santa Rosa

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This final stop provides a spectacular view of the entire
aquifer surface, including each of its six basins. The Hagatña
Basin can be seen at the extreme southwest. The Finegayan Basin
occupies the far side of Mataguac Hill, to the west. The Agafa
Gumas Basin occupies the northern-most portion of the plateau,
seen to the northwest from here. The Andersen Basin, named for
the Air Force base, is readily seen just to the north. The Mangilao
Basin starts on the southern flank of Mount Santa Rosa and runs
southwest along the Pacific coastline to the southeastern flank of
Barrigada Hill. This view provides a particularly good perspective
of the Yigo-Tumon Basin, which heads between Mount Santa Rosa
and Mataguac Hill and run ́ͽ