Rainfall,
water-level, and specific-conductance data were collected from
four locations in the Yigo-Tumon sub-basin of the Northern
Guam Lens. Data were collected from May 17, 2004, to February 23,
2005. Periodic field visits were made at roughly six-week intervals
to service and maintain data loggers and collect a continuous profile
of specific conductance with depth for each well, and make manual
measurements for quality assurance / quality control purposes.
During each visit, data from the rainfall, specific-conductance
and water-level loggers were downloaded, and the loggers
were calibrated if necessary, reset, and re-installed.
The maintenance of each logger resulted in up to 24 hours of missing
record. In addition, some of the loggers either failed resulting
in loss of all data for the segment between visits, or ran out
of battery voltage resulting in a loss of record during the latter
part of the segment.
The rainfall,
water-level, and specific-conductance data were loaded into the
USGS National Water Information System. Guidelines and
USGS standard procedures, much of which is described in two USGS
Water-Resources
Investigations Reports (Wagner and others, 2000; Sauer, 2001) were
followed to properly document, load into the database, calibrate,
adjust and rate the data. Specific-conductance profile data were
documented and archived on computers at the USGS Pacific Islands
Water Science Center in Honolulu, Hawaii.
Any use of trade, product, or firm names is for descriptive purposes
only and does not imply endorsement by the U.S. Geologic Survey
or the U.S. Government.
Rainfall Rainfall was recorded using tipping bucket rain gages. At EX-7
and EX-10, the rain gage was only a few feet from the well. At EX-6,
the rain gage was placed about 100 feet away from the well, and
at GHURA-Dededo, the tipping bucket rain gage was placed next to
the existing GHURA-Dededo rain gage, which is about 200 yards from
the well. The existing GHURA-Dededo rain gage is an accumulating-can
style rain gage.
![Photograph of tipping bucket raingage. The black plastic part in the middle rocks back and forth, and is calibrated to tip at 0.01 inches of rain collected.](images/raingage2.gif)
Photograph
of tipping bucket rain gage. The black plastic part in the middle
rocks back and forth, and is calibrated to tip at 0.01 inches
of rain collected.
Water Level
Water-level data were recorded in each well using Odyssey transducer
loggers and In-Situ Mini-Troll transducer loggers.
![Photograph of Odyssey water level transducer logger](images/odyssey.gif)
Photograph
of Odyssey water level transducer logger
![Photograph of In-Situ water level transducer logger](images/minitroll.gif)
Photograph
of In-Situ water level transducer logger
Water-level
data from the loggers were adjusted to manual measurements taken
during each visit. Manual measurements were made when the
logger was installed or redeployed, and when the logger was removed
for downloading. At the beginning of the data collection period,
it is likely that the cables that suspended the water-level data
loggers stretched slightly, thus the water-level record at the
end of the segment of data (between visits) is probably more accurate
than at the beginning of the segment.
Specific-Conductance
Hydrographs
Specific-conductance
hydrographs were recorded at selected depths within each well.
Eleven Hydrolab MiniSonde loggers were used in
the four wells, with two loggers in EX-6, and three loggers each
in EX-7, EX-10 and GHURA-Dededo. An additional two Solinst Levelogger
LTC loggers were used in EX-7, for a total of five specific-conductance
loggers in that well.
![Photograph of Hydrolab MiniSonde loggers](images/hydrolab.gif)
Photograph of Hydrolab MiniSonde loggers
![Photograph of Solinst LTC levelogger](images/solinst.gif)
Photograph of Solinst LTC levelogger
The diagram
below shows how the instrumentation was installed in the wells.
The loggers were hung using stainless-steel fishing
wire and stainless-steel carabiners. It is assumed that the stainless-steel
wire stretched during the first part of the record at the beginning
of the project. The wire also may have stretched or relaxed
during the beginning of each subsequent deployment. The wires supporting
the specific-conductance loggers were re-measured at the end of
the project, and these lengths from the end of the study were used
to determine the altitude of the conductivity sensors.
![Diagram showing how instrumentation was installed in the wells](images/fig.x.wellschematic.gif)
Diagram showing how instrumentation was installed in the wells
The table below
shows the altitude of the loggers in the wells. In well EX-6,
the loggers were set too deep initially, such that both loggers
read high specific conductance values. In addition, the conductivity
profile in this well shows a very sharp transition zone. In an
attempt to have more intermediate values of specific conductance,
the suspension wire for these loggers was shortened by 1 foot.
At the GHURA-Dededo well, a shortened line was mistakenly used
during the last segment of data.
Altitudes of specific-conductance loggers |
|
Well |
Logger |
Relative Location |
Altitude, in feet |
Notes |
|
EX-6 |
A-1 |
upper |
-137.77 |
|
|
B-1 |
lower |
-140.10 |
|
|
|
|
|
|
|
A-2 |
upper |
-136.77 |
after 6/26/04 |
|
B-2 |
lower |
-139.10 |
after 6/26/04 |
|
|
|
|
|
|
EX-7 |
A |
upper |
-114.19 |
|
|
D |
upper middle |
-117.15 |
after 9/15/04 |
|
B |
middle |
-120.37 |
|
|
C |
lower middle |
-135.38 |
|
|
E |
lower |
-159.09 |
after 8/4/04 |
|
|
|
|
|
|
EX-10 |
A |
upper |
-105.86 |
|
|
B |
middle |
-116.59 |
|
|
C |
lower |
-124.52 |
|
|
|
|
|
|
|
GHURA-Dededo |
A |
upper |
-131.69 |
|
|
B |
middle |
-137.11 |
|
|
C-1 |
lower |
-143.07 |
|
|
C-2 |
2nd lower |
-142.75 |
after 12/16/04 |
|
Altitudes based on measured cable length at the
end of the project, and revised measuring-point altitudes. |
Start date was on or about May 17, 2004, and
end date was on or about February 24, 2005. |
Specific-Conductance
Profiles
Specific-conductance
profiles were collected during each site visit using an Ocean
Sensors OS200 CTD (conductivity/temperature/pressure
sensor).
![Photograph of Ocean Sensors OS200 CTD](images/ctd.gif)
Photograph of Ocean Sensors OS200 CTD
The CTD was lowered through the water column in each well using
a stainless-steel line and an electric deep-sea fishing winch.
The same wire was used for all of the profiles. The wire was measured
against a calibrated tape near the end of the study. Marks used
for measuring were also used as depth-calibration points during
the profiling. The specific conductance-depth logs are adjusted
to these calibration points, and the depths determined from the
pressure sensor on the CTD are used to determine intermediate depths
between the calibration points.
Specific conductance in the wells ranged from about 0.450 mS/cm
at the top of the water column, to over 51.0 mS/cm at the bottom
of
some
of the wells. These values correspond to freshwater and saltwater
in the wells. Chloride concentrations, from previous sampling in
1992, suggests that the freshwater was about 40 mg/L and salt water
was about 18,000 to 19,000 mg/L chloride (from USGS well files
in Honolulu).
|