Field Trip - Northern Guam Lens Aquifer Jun 2014 | Page 20

Stop 2: The Core of the Aquifer –
the Limestone Bedrock
Hydrologic role of the Barrigada Limestone

The Barrigada Limestone is by far the most extensive and important of the three major
limestone units of the aquifer. Even a casual inspection shows that this limestone is, overall,
noticeably porous. A somewhat closer look, however, also reveals that the porosity of this rock
can vary remarkably over the scale of just a few tens of feet. Thus, even though this rock is very
porous at the regional scale, only about one in three or four exploratory wells usually proves
suitable for production. Production from successful wells can be extremely high, however—500
to 750 gallons per minute. Inside the quarry, we will examine the qualities and features of the
rock that determine the success or failure of new wells and the paths by which contaminants may
enter and move through the aquifer.

Barrigada Limestone, DPW Quarry, Dededo

Active quarrying of the limestone here in the “Dededo Coral
Pit” provides some of the island’s most accessible and instructive
exposures of the rock that comprises the core of our aquifer. This
limestone is actually not from coral, but is rather a granular detrital
limestone, deposited in deeper waters mostly from accumulation of
shells and shell fragments left behind over millions of years by tiny
organisms that colonize shallow ocean banks and the water above.
Fresh cuts in the quarry walls provide outstanding examples of the
kinds of porosity that constitute the internal plumbing of the aquifer.

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