Introduction
Rainfall,
water-level, and specific-conductance data were
collected from four locations in the Yigo-Tumon sub-basin of
the Northern Guam Lens (NGL). The data was collected to investigate
how the lens thickness and the specific-conductance distribution
with depth responds to recharge events.
The Northern Guam Lens provides nearly 40 million gallons per
day (about 80 percent of the total) of drinking water to the island
of Guam (Jocson and others, 2002). Within the NGL, the Yigo-Tumon
sub-basin is the principal water-producing sub-basin, accounting
for about 40 percent of the total production and probably as much
of the remaining freshwater reserve. It is also the most heavily
developed area, containing over half of the island's population,
and nearly all of its civilian commercial and industrial activities.
Saltwater intrusion has affected several wells in the Yigo-Tumon
sub-basin owing to high pumping rates and well completion depths
that are too deep relative to the freshwater-lens thickness, aquifer
properties, and recharge. A better understanding of the response
of the freshwater lens to pumping, as well as to short-term variations
in recharge, is needed to provide a basis for appropriate well
design and proper production management.
Rainfall, water-level,
and specific-conductance data were collected in cooperation with
the Guam Environmental Protection Agency (GEPA) from May
17, 2004 to February 24, 2005. Personnel from the Water and Environmental
Research Institute (WERI) of the University of Guam and the GEPA
assisted with the collection and review of the data.
A description
of general hydrologic principles, the island’s ground-water
systems, as well as Guam’s rainfall, surface water, and
geology, is provided in USGS
Water-Resources Investigation Report 03-4126 (Gingerich 2003).
Map showing location of the four monitor wells and the Yigo-Tumon
sub-basin in Northern Guam.
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