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Water Level MonitoringMethod in the Madness
Borehole Users ConferenceLoughborough, 5 Oct 2017
Peter Dumble
2 hours
5 Weeks
Groundwater Monitoring Cycle
Network design and data collection
Data management, review and action
Adapted from Fig 2.1 UNESCO / IGRAC 2006, Guideline on Groundwater monitoring for general reference purposes
Groundwater Level Measurement
Measuring Devices– Water Level Dip Tapes, Loggers– How they work / practical issues
• Accuracy / Precision– Can you trust the manufacturer?
• Validation of measurements– Reducing uncertainty / increasing
confidence
Electric Water Level Meter
Tape on a reel withweighted probe
• Manual method• Tape contains electric cable• Electrical circuit completed when probe reaches
water• Measures depth to water level• Often used to measure depth to the bottom of
the borehole• Probes need to be pressure sealed to do this
(otherwise water gets into the electrics)• Probe is usually not heavy enough to use as a
plumb line for depths >30m?
Accuracy decreases with depth.
Measurements with a “Dip Tape”Depth to water level
• Measure from a fixed datumposition (e.g. top of headworks)
• Describe / photograph / mark the datum position
Adjusting measurements toDepth below Ground Level
• Measure the “stick-up” heightHeight above Ordnance Datum (or elevation relative to any other known datum)
• Survey the level of the fixed datum positionCan also measure
• Depth to base of well or borehole• Diameter of boreole
“Data Loggers” (Pressure Sensors)
Electronic device /programmable / variants
• Installed below water level within borehole
• Measures the pressure of water above the sensor• May need to compensate for
atmospheric pressure (depends on type of logger)
Instrument accuracy is expressed as a % of the pressure range
Sensor
Barometric sensor inclogger & battery
Logger includingmemory & battery
Non-Vented(Total Pressure) Systems
Suspension orDirect Read Cable
Types of Loggers and Configurations
Vented Systems
Vented cable
Sensor
Logger includingmemory & battery
Air Vent
Logger includingmemory & battery
As Demonstrated
Different Datums – Different Measurements
Head of Water
Logger Measurement
Dip Tape Measurement
Datum point is at the sensor
Datum point is at the top of the well
A manual water level measurement taken at the same time as a logger reading measures the difference in height between the sensor and dip tape datums. This dimension can then be used to convert all subsequent logger “head” readings to “depth to water”.
Depth to Water
USE OF DATA LOGGERS4.6.1 It is essential that careful attention be paid to the resolution and accuracy of the equipment … and calibration by manual dipping will still be necessary
Validation / Verification?
BSI ISO 14686: 2003Pumping Tests for water wells - guidelines
Use of Data Loggers for PumpingTests
Validating Logger Records
What the logger recorded
Base of Well (at 9.95m)
Manual Dip Measurements
Adjusted (validated) record
Validating logger records is time-consuming. This cannot be done confidently without good quality manual dip measurements.
Highest Expected Water Level
Water Level at time of installation
Lowest Expected Water Level
Logger suspension depth needs to be below the lowest expected water level
Dept
h of
Logg
er in
Wel
l
Dept
h of
Wel
l
Logger rangeneeds to be sufficient to accommodate the highest expected water level
Min
imum
Logg
er ra
nge
Barometric pressure range
(+ barometric pressure change if using non-vented loggers)
Logger Positioning in a Well and Instrument Range
Sometimes you get it wrong….
Borehole is being pumped for water supply daily – water level falls below logger during pumping / logger needs to be deeper.
Depth of logger in borehole
Well is 30m deepPump is at 25m
Corrected!
Data on previous slide
Access tubes
BSI ISO 14686: 2003Pumping Tests for water wells – guidelines. Section 4.5.4: Any instruments used in the test well during the test should be lowered through a specially inserted tube which extends at least 2 m below the pump intake level. For water-level measurement, this tube need only be three times the diameter of the probe or the dipper.
Pump
Access tube(to 2m below pump intake)
Access Tube ID Probe OD1 inch / 25 mm 8.3 mm
¾ inch / 37.5 mm 12.5 mm2 inch / 50 mm 16.7 mm
Access / Diameter• Is there sufficient access?• What other equipment is present?
What could possibly go wrong?
Thanks to Prof Richard Carter for images
Thanks to Jeff Davies of BGS for image
Caption competition“I know I said we needed good QC on this one, but are three dip tapes really necessary?”
Water Level MeasurementWhat Accuracy is required?
TEST WELL4.6.1 The resolution of measurement in the test well should be 10 mm or better …..
OBSERVATION WELL4.6.1 The resolution of measurement in the observation well should be 5 mm or better …..
The measurement datum should be clearly marked on each well.
BSI ISO 14686: 2003Pumping Tests for water wells - guidelines
How accurate are water level measuring devices?
Accuracy is often expressed as a– % of a measuring tape’s length or– % of the instrument range.
Aspirational accuracy objectives may not always beachievable. Be wary of logger manufacturer’s claims – fieldconditions are always more challenging than the lab.
0.10% of 10 m = 1 cm100 m = 10 cm
0.05% of 10 m = 0.5 cm100 m = 5 cm
0.02% of 10 m = 0.2 cm100 m = 2 cm
Loggers – but there is no universal testing standard
High Quality Dip Tapes – e.g. EU Class II manufacturing standard
Checking Dip Tape Accuracy
8 cm at 30m 16 cm at 60m 27 cm at 100m
Poor Quality Dip Tape Better (0.02%)
Best (<0.02% - Encapsulated steel tapes)
6.000
Error: ± 6 mm in 30m
1.6 cm difference (the dip tape has shrunk – probably
during manufacture)
5.984
Error: -1.6 cm / 6 m = -0.27%
Density - Saline BoreholesAccuracy will depend on:• Where the logger is positioned in the
water column• Variation over time in the water
density above the logger• The density value programmed into
the logger
From: Post & von Asmuth, 2013, Hydrogeology Journal 21: 737-750
The density of most natural
waters varies over time and by depth.
Example: If the logger is programmed with a density value of 1000 kg/m³ and then submerged into seawater (density 1024 kg/m³), the denser (heavier) seawater would lead to an error in water level measurement by 2.4 cm for every 1 m change in water level
Seawater Density approx. 1.024 kg/m³
Pure water density approx. 1000 kg/m³
Pumping TestLogging at 1 minute Intervals
Group 3 - Pumping Well - Conductivity
12.0
12.5
13.0
13.5
14.0
14.5
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0Time (mins)
Dept
h to
Wat
er (m
)
700
740
780
820
860
900
Cond
uctiv
ity (u
S/cm
)
G3A LevelG3A ECs
DRAWDOWN RECOVERY
Electrical Conductivity (uS/cm)
Pumping Test Record Logarithmic Programme Schedule
22Data from In-Situ Level TROLL 700 vented Logger
DRAWDOWN
Logging at 1 sec intervals or less
RECOVERY
Logging reducing to 1 minute intervals
Temperature
What to look for in a good logger record?
What to look for in a good hydrogeologist?
“it should become standardpractice in hydrogeology toprovide quantitative estimatesof the measurement error.”
Post V.E.A. & von Asmuth J.R. 2013. Review: Hydraulic head measurements – new technologies, classic pitfalls. Hydrogeology Journal 21, 737-750
Accuracy & Precision• Accuracy is how close a measured value is to the actual (true) value.
• Precision is how close the measured values are to each other.
https://www.mathsisfun.com/accuracy-precision.html
High AccuracyLow Precision
Low AccuracyHigh Precision
High AccuracyHigh Precision
So, if you are playing football and you always hit the left goal post instead of scoring, then you are not accurate, but you are precise
ResolutionResolution is the smallest distinguishable measurement that can be recorded with an instrument.
The specified resolution has no relation to the accuracy of the instrument
Example: A 20m range logging device records a water level to 4 decimal places as 2.3584 metres, this is the instrument resolution. It does not mean the device can measure to an accuracy of 0.1 mm!It may have an instrument accuracy given as 0.05% of its range (i.e. 1 cm or 0.01 m) so the reading should be stated as 2.36 ± 0.01 m and the reading therefore lies somewhere between 2.35 and 2.37 metres.
Measurement ErrorsMeasurement Error (also called Observational Error) is the difference between a measured quantity and its true value. It includes systematic error (caused by a mis-calibrated instrument that affects all measurements) and random error (naturally occurring errors that are to be expected with any experiment) – including human error
Examples of Random Errors when using pressure loggers and dip tapes
• Fluctuations in water density• Physical movement in the position of the logger (or barologger,)• Mis-timing of manual water level measurements for applying datum
adjustment for validation of records• Using an inaccurate or damaged dip tape for validation• Mis-reading of dip tapes• Typographical errors in field notes ….. AND LOTS MORE!
