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Almagosa River near Agat, Guam

GUAM LENS

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Groundwater monitoring, northern Guam Lens, May 2004-February 2005

Data-Collection Sites

Four monitoring wells that penetrate the transition zone between freshwater and saltwater in the Yigo-Tumon subbasin of the Northern Guam Lens were selected for monitoring. The well locations are shown on the map below.

These wells, as well as four other deep monitor wells, were drilled in the late 1970’s and early 1980’s as part of a program to monitor and explore freshwater lens systems on Guam (Ayers and Clayshulte, 1984). The wells were refurbished in 1999 by removing obstructions and installing new 5- or 6-inch PVC casings to the bottom of the well.

Well measuring-point altitudes were revised by the USGS using vertical level surveys from existing National Geodetic Survey (NGS) benchmarks in November 2004. The altitudes of the benchmarks were updated by the NGS in May 2005 from leveling data collected in June 2004. The table below summarizes the well depths, previous and revised well measuring-point altitudes, and water levels measured in May 2004.

The previous measuring points on the wells were the tops of the steel casings. The differences between the tops of the steel casings and the new measuring points on the PVC casings for each well are also shown below in the table.

Well Name USGS Well Number Previous measuring point altitude, in feet Revised measuring point altitude, in feet Total depth of well, in feet Water level above mean sea level, in feet May, 2004
EX-6133034144500871309.41309.064453.88
EX-7133119144491771283.31282.934993.68
EX-10133224144495271348.54348.244883.28
GHURA-Dededo133120144505471393.90395.877773.83

Map showing location of the four monitor wells and the Yigo-Tumon sub-basin in Northern Guam.
Map showing location of the four monitor wells and the Yigo-Tumon sub-basin in Northern Guam.

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