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-6 |
133034144500871 |
309.41 |
309.06 |
445 |
3.88 |
EX-7 |
133119144491771 |
283.31 |
282.93 |
499 |
3.68 |
EX-10 |
133224144495271 |
348.54 |
348.24 |
488 |
3.28 |
GHURA-Dededo |
133120144505471 |
393.90 |
395.87 |
777 |
3.83 |
|
Map showing
location of the four monitor wells and the Yigo-Tumon sub-basin
in Northern Guam.
|