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U.S. GEOLOGICAL SURVEY
Data Series 269

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Ground-Water Temperature Data

Data presented for each well consist of depth-to-water measurements, depth below water level where ground‑water temperature was measured, and ground-water temperature. Distances are reported in inch-pound units (feet) and temperatures in degrees Celsius.

Periodic Ground-Water Temperature Data

Data-Collection Method

Periodic ground-water temperature data were collected with a temperature probe attached to the end of a calibrated electric tape. In general, temperatures in each well were measured annually at depths of 5 and 55 ft below the water surface. If necessitated by well conditions, the depths of ground-water temperature measurements were varied.

Instrumentation

A temperature probe was used to collect periodic ground-water temperature data. The temperature probe protects a thermistor in which electrical resistance varies with temperature. The probe can be used to depths of 1,500 ft below the water surface and has a temperature range of ‑5°C to 50°C (Solinst Canada Ltd., 1995).

The temperature probe was attached to the bottom of a calibrated electric tape. A calibrated electric tape also was also used to measure depth to water prior to temperature data collection. The calibrated electric tape measurements are considered accurate to within 0.1 ft.

Calibration of Temperature Probe

The temperature probe was calibrated annually. The calibration procedure begins by determining the “before” resistance of an electric tape without a temperature probe. The temperature probe is then attached to the electric tape and placed into a water-filled container. Measurements of the resistance of electric tape plus the resistance of the temperature probe thermistor are recorded at various water temperatures ranging from about 0°C to 50°C. During the procedure, water temperature is determined using a National Institute of Standards and Technology (NIST)-calibrated thermometer. After all measurements of the resistance of the electric tape plus the resistance of the thermistor are completed, the “after” resistance of the electric tape without the temperature probe is again measured. The average resistance of the electric tape is the average of the “before” and “after” measurements of electric tape resistance. The average resistance of the electric tape is subtracted from the resistance of the electric tape plus the resistance of the thermistor to determine the resistance of the thermistor at various water temperatures. An equation is used to convert the resistance of the thermistor to a temperature. A regression or best-fit line is fitted to thermistor-derived temperatures versus calibrated thermometer temperatures. The equation of the best-fit line is then used to adjust field-measured temperatures to a calibrated temperature.

Procedure

The general steps used for collecting periodic ground-water temperature data at 5 and 55 ft below the water surface are as follows:

1. Depth to water below measuring point (DTW) for the well is measured using a calibrated electric tape (CET). No correction factor is applied to the measurement. The DTW is then rounded to the nearest foot and recorded as the rounded depth to water below measuring point (DTWR).
2. A “before’’ resistance for a CET is measured before the temperature probe is attached to the CET.
3. The CET is lowered into the well to the depth at which the temperature probe would be 5 feet below the DTWR. The resistance of the CET plus the resistance of the temperature probe thermistor is recorded.
4. The CET is lowered into the well to the depth at which the temperature probe would be 55 ft below the DTWR. The resistance of the CET plus the resistance of the temperature probe thermistor is recorded. The CET is removed from the well.
5. The temperature probe is removed from the CET and an “after’’ resistance for CET is measured.

Periodic Temperature Data

Periodic ground-water temperature data collected in the field were processed using a Microsoft® Excel spreadsheet. The “before” and “after” measured resistances of the electric tape were averaged to determine an average electric tape resistance. The average electric tape resistance was subtracted from the resistance of the electric tape plus the resistance of the temperature probe thermistor to determine the resistance of the thermistor while submerged below water within the well. A raw water temperature is calculated from the resistance of the submerged thermistor. The raw water temperature is corrected for the temperature probe calibration to determine a final calibrated water temperature. Based on data-collection methods, periodic ground-water temperatures are considered accurate to within 1°C.

A table of periodic ground-water temperature measurements collected during calendar years 2000–2006 in 166 wells at and in the vicinity of the NTS is presented in appendix A. The appendix provides information on site location, well construction, hydrogeologic unit, depths-to-water, and temperature data for each of the wells.

Periodic ground-water temperatures measured in wells at and in the vicinity of the NTS ranged from about 10.7°C to 50.4°C. The minimum temperature was measured in well TTR Antelope Mine 1 at 11 ft below land surface and the maximum in well ER ‑3 ‑1 ‑2 (shallow) at 55 ft below land surface. An example of the results from periodic ground-water temperature collection is shown in figure 2.

Ground-Water Temperature Profile Data

Data-Collection Method

Ground-water temperature profile data were collected with temperature loggers attached to an electric tape. Temperatures were collected at multiple depths throughout the water column.

Instrumentation

Sealed underwater temperature loggers were used to collect ground-water temperature profile data. The loggers were depth rated to 1,000 ft, but have been known to work at pressures equivalent to 10,000 ft of water (Onset Computer Corporation, 1996). The temperature range of the loggers is -20°C to 50°C, but they will record temperatures as much as 70°C.

The sealed temperature loggers were attached to an uncalibrated electric tape at 20-foot intervals. The tape was believed to be accurate to at least the nearest foot based on its previous use as a calibrated electric tape. This accuracy was sufficient for the ground-water temperature profiles.

Calibrated electric tapes were used to measure depth to water prior to temperature-data collection. The calibrated electric tape measurements are recorded as being accurate to within 0.1 ft.

Calibration of Temperature Loggers

Prior to field use, temperature loggers were checked for accuracy, precision, resolution, and the time needed to equilibrate to surrounding fluid temperature. To evaluate accuracy, precision, and resolution, multiple temperature loggers were activated and submerged into a container of water. The water was stabilized at various temperatures within the range of temperatures expected to be encountered in the field. Temperature loggers were allowed to equilibrate at these stable temperatures for at least 10 minutes. The accuracy of an individual logger was determined by comparing its recorded temperatures with temperatures from an NIST-calibrated thermometer. The precision of an individual logger was determined by comparing its recorded temperatures with the recorded temperatures from other loggers. The resolution of an individual logger was determined by noting the minimum difference between temperature readings that each logger could record. Each logger has 255 different temperature steps in its measurement range that can be recorded (Onset Computer Corporation, 1996).

The accuracy, precision, and resolution of the loggers are temperature dependent. The measured accuracies of the temperature loggers, which are the differences between logger temperature readings and measurements from an NIST-calibrated thermometer, range from ± 0.2°C to ± 0.7°C. The measured accuracies are within ± 0.3°C of the manufacturer’s specifications of ± 0.4°C from 0°C to about 30°C and ± 0.5°C from about 30°C to 50°C.

The temperature logger precisions, which are the maximum differences between temperatures measured simultaneously by multiple loggers, are between 0.3°C and 0.6°C. Measured resolutions are within 0.1°C of manufacturer’s resolution specifications of ± 0.3°C from about 0°C to 25°C and ± 0.4°C from 25°Cto 50°C.

Based on data-collection methods and the accuracy, precision, and resolution testing, ground-water temperature profile measurements between 0.5°C and 50°C are considered accurate to within 1°C. Temperatures measured greater than 50°C may be less accurate.

The equilibration time needed for the loggers to record a consistent temperature was measured. Temperature loggers were quickly moved from a container of relatively cold, warm, or hot water and placed into a container with water of different temperature. The amount of time the logger needed to equilibrate after these changes was recorded. Equilibration times ranged from 7 to 16 minutes. Equilibration times when temperature changes were less abrupt were as short as 2 minutes.

Procedure

The general steps used for collecting ground-water temperature profile data are as follows (fig. 3):

1. Prior to data collection, the temperature loggers are attached to an uncalibrated electric tape (UET). The temperature loggers are spaced at 20-foot intervals with the initial logger to enter the well located 5 ft above the bottom of the UET and the final logger to enter the well located 185 ft above the bottom of the UET.
2. DTW for the well is measured using a calibrated electric tape. No correction factor is applied to the measurement. The DTW value is then rounded to the nearest foot and recorded as the DTWR.
3. The length of water column is estimated by subtracting the DTWR from the most recent measured accessible well depth. The length of the water column is needed to estimate the length of UET that will be lowered into the well or borehole.
4. The UET is lowered into the well. Each temperature logger is activated and time synchronized before being lowered into the well.
   • If the estimated water column length is less than 200 ft, the UET is lowered to the maximum accessible depth of the well. The UET is then raised so that the nearest whole number foot marking on the UET above that accessible depth lines up with the measuring point (MP). The whole number foot marking is recorded as the hang point (HP).
   • If the estimated water column length is equal to or greater than 200 feet, the UET is lowered to a HP such that the temperature logger located 185 feet above the bottom of the UET will be 5 feet below DTWR and the temperature logger located 5 feet above the bottom of UET will be 185 feet below DTWR (fig. 3). If the maximum accessible depth is reached before the temperature loggers are located at these depths, then the procedures used for an estimated column length of less than 200 feet should be used.
5. The UET is secured at the HP and a start time recorded. The UET is not moved from the HP for at least 10 minutes to allow the temperature loggers to equilibrate to the surrounding water temperature. Before the UET is moved, an end time is recorded.
6. The UET is moved to a different HP.
   • If the water column length was determined in step 4 to be less than 200 ft, the UET is moved so that one temperature logger measures ground-water temperature about 5 ft below the water surface. Other temperatures will be measured in 20-foot intervals below the logger at 5 ft below the water surface.
   • If the water column length was determined in step 4 to be equal to or greater than 200 feet, the UET is lowered so that the HP is 200 feet greater than in step 4 (fig. 3). If a maximum accessible depth is reached before the HP is lowered the additional 200 feet, then the UET is raised so that the nearest whole number foot marking on the UET above that maximum accessible depth lines up with the MP.
7. The UET is secured at the HP and a start time recorded. The UET is not moved from the HP for at least 8 minutes to allow the temperature loggers to equilibrate to the surrounding water temperature. Before the UET is moved, an end time is recorded.
8. In wells that have not reached maximum accessible depth by the end of step 7, steps 6 and 7 are repeated until maximum accessible depth is reached in the well, or the UET has been extended to its maximum length.
9. The UET is raised out of the well. Data is collected from each temperature logger. Each temperature logger is then deactivated.

Temperature Profile Data

Temperature profile data collected in the field were processed using a Microsoft® Excel spreadsheet. The spreadsheet used the HP, data collection times at each HP, and the position of the logger on the UET to determine the depth of each logger below the water surface and the temperature at that depth (K.J. Halford, U.S. Geological Survey, written commun., 2004). Temperature correction factors only slightly affected temperature logger readings and were not applied to these values.

A table of the ground-water temperature profiles collected in 73 wells at and in the vicinity of the NTS is presented in appendix B. The appendix provides information on site location, well construction, hydrogeologic unit, depths-to-water, and temperature data. Two graphs of temperature versus depth below land surface for each well with temperature profiles greater than 50 ft in length also are presented in appendix B. The scale chosen for one of these graphs is customized for each well to illustrate detailed changes in temperature with depth. The scale chosen for the other graph is standardized for all wells to illustrate the relative temperature profile length and change in temperature at each well when compared to all collected temperature profiles. An example of the ground-water temperature profile graphs is shown in figure 4.

Temperatures measured while collecting ground-water temperature profiles in wells at and in the vicinity of the NTS during the period of record ranged from about 18.8°C to 59.0°C. The minimum temperature was measured in wells ER- 2-1 piezometer (deep) and UE- 4t 1 (1906–2010 ft) at 5 feet below land surface and the maximum in well PM-1 at 3,565 feet below land surface. The deepest temperature measurement was at 3,982 feet below land surface and the longest ground-water temperature profile was 3,977 ft.

When available, ground-water temperature profile measurements in a well were compared to periodic temperature measurements closest in depth and measurement date in the same well. The ground-water temperature profile measurements were collected at or within 10 feet of the periodic measurements and, except in three cases, within 400 days. The maximum difference between temperature profile and periodic temperature measurements at similar depths was 1.1°C, but was less than 0.5°C in about 90 percent of the compared measurements.

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