Field Trip - Northern Guam Lens Aquifer Jun 2014 | Page 18

Stop 1: The Floor of the Aquifer
– The Volcanic Basement Rock
Hydrologic role of basement rock

Named the Alutom Formation, this hydrologically “tight” volcanic rock unit forms

subterranean hills, ridges, and valleys beneath the overlying limestone bedrock. Accurate
knowledge of the basement topography is the first prerequisite to successful groundwater
exploration on northern Guam. Beneath about one-fifth of the plateau surface the basement
partitions the aquifer into six semi-contiguous groundwater basins. Descending groundwater
concentrates along the axes of valleys above sea level and may even be impounded in some small
subterranean reservoirs. Although very difficult to locate, such streams and patches of suprabasal water are the freshest water in the aquifer, and are invulnerable to saltwater contamination.
The water descending down the hills and valleys to sea level is concentrated into a rim of parabasal freshwater that is underlain by volcanic basement rock rather than saltwater. Because it
is thus very fresh and much less vulnerable to saltwater contamination than the surrounding
basal water of the freshwater lens, the para-basal zone has long been the zone of choice for
development and production of groundwater.

Alutom Formation, Mt. Alutom

Here, around the crest of Mount Alutom, are the island’s
best outcrops of t H