Field Trip - Northern Guam Lens Aquifer Jun 2014 | Page 14

Cross-section of the NGLA
along the Yigo Trough
This cross section depicts the actual proportions of the freshwater
lens and geologic units along the axis of the basement valley that
heads between Mount Santa Rosa and Mataguac Hill and extends
southwest beneath Dededo to Tumon. Called the Yigo Trough by
local hydrologists, it collects and carries fresh water, much like
a surface stream valley, beginning in the area around Yigo, and
discharges it into the ocean in Tumon Bay. It should be noted
that beneath the first few tens of feet beneath the water table, the
freshwater lens, as depicted by the blue layer in this diagram,
generally becomes increasingly brackish downward.

BASAL WATER

PARA-BASAL WATER

SUPRA-BASAL WATER

~75%

<5%

~20%

SALTWATER

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of northern Guam

of northern Guam
Mt. Santa Rosa

Tumon Bay

Mean sea level

of northern Guam

Water table
FRESHWATER LENS

NOTE: Vertical scale on this diagram is realistic. The following values are used:
horizontal and vertical scale: 1 inch = 3168 ft (1 km); cross-section width: 7.5 miles;
elevations: Mt. Santa Rosa (MSR) +830 ft, central plateau surface +400 ft, volcanic contact at
the coast -800 m, volcanic contact offshore -1500 ft, water table +4 ft, 50% isochlor -200 ft.

Saltwater toe

LIMESTONE
VOLCANIC ROCK
FRESH WATER
SALTWATER

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