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Almagosa River near Agat, Guam




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

Data-Collection Methods

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.

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Photograph of tipping bucket rain gage.

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.

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.

Water Level

Photograph of Odyssey water level transducer logger
Photograph of Odyssey water level transducer logger
Photograph of In-Situ water level transducer logger
Photograph of In-Situ water level transducer logger

Water-level data were recorded in each well using Odyssey transducer loggers and In-Situ Mini-Troll transducer loggers.

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
Photograph of Hydrolab MiniSonde loggers
Photograph of Hydrolab MiniSonde loggers
Photograph of Hydrolab MiniSonde loggers

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
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.

A-2upper-136.77after 6/26/04
B-2lower-139.10after 6/26/04
Dupper middle-117.15after 9/15/04
Clower middle-135.38 
Elower-159.09after 8/4/04
C-22nd lower-142.75after 12/16/04

Specific-Conductance Profiles

Photograph of Ocean Sensors OS200 CTD

Photograph of Ocean Sensors OS200 CTD

Specific-conductance profiles were collected during each site visit using an Ocean Sensors OS200 CTD (conductivity/temperature/pressure sensor).

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).

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