acquisition and interpretation of water-level data travis von dessonneck

Acquisition and Interpretation of Water-Level Data

Travis von Dessonneck

Importance of Water-Level Data

• The acquisition and interpretation of ground-water data are essential for environmental site assesment

• Can be used to determine hydraulic head in formations

• Used to make 3D flow patterns

Water level and Hydraulic-head relationships

• Hydraulic head varies spatially and temporally

• Piezometer– Monitoring device for measuring water levels– Hollow vertical pipe with a screen

• Elevation head– The elevation of the bottom of the

well/piezometer

Water level and Hydraulic-head relationships

• Pressure head– The height of the water above the bottom of

the well

• Total hydraulic head– Elevation head + Pressure head

Hydraulic Media and aquifer systems

• Aquifer is not “a water-bearinglayer of geologic material, which will yield water in a usable quantity to a well or spring” in this instance

• Aquifer is where water lies with respect to the top of a geologic unit

Design features for water-level monitoring systems

• Takes into account water-level monitoring and sampling

• 2 phases– Site data collection– Monitoring for changes and proper placement of wells

• Can also be used to determine if monitoring system is not set up correctly

• Site geology must be known– Heterogeneous sites require more monitoring than

homogeneous sites

Piezometers or wells

• Piezometers are generally not used to gather water samples– Small diameter pipe– Can accommodate pressure transducers

• Wells are designed for sampling– Larger diameter

Approach to system designs

• What to consider– Boring and well logs– Surficial geology– Topographic maps– Drainage features– Cultural features (well fields, irrigation, pipes)– Rainfall– Recharge

Approach to system designs

• Review the data to get– Depth and characteristics of high and low K

areas– Depth to water, intermittent or perched zones– Flow direction– Vertical hydraulic gradients– Possible causes and frequency of fluctuation– Existing wells that may be incorporated

Number and placement of wells

• Dependant on size and complexity of site

• Minimum to establish direction and rate of flow

• Larger sites usually have a grid of six to nine wells to get direction

• Take into account screen depth and length

Water-level measurement precision and intervals

• Need to accurately located wells vertically and horizontally– Survey/GPS– Accuracy to 0.1 and 0.01 ft

• Need to know what you are looking for– Seasonal changes– Diurnal changes

Reporting of data

• Monitoring installations– Geologic sequence– Well construction features– Depth and elevation of well casing

• Water-level data– Date and time of measurement– Method used– Other conditions that might affect the well

level

Manual measurements in nonflowing wells

• Wetted chalked tape method– Weight attached to bottom of tape– Coat bottom 2-3ft of tape with carpenter’s

chalk– Accurate to 0.01ft (USGS 1980)– Disadvantages

• Stretching of the tape• Need to know approximate depth to water


• Air-line submergence method– Insert a small diameter tube below the water

surface– Pump the water out the bottom by hand or

electric pump– Ending psi * 2.31 gives feet

• Subtract the calculated distance from length of tube


• Electrical methods– Whistler

• Open circuit is completed when it comes in contact with the water and beeps at you

• Wires are at the end of a measuring tape• Read the tape to determine depth


• Pressure transducer methods– Measures the pressure in the well at the

sensor– Open to the atmosphere by a small capillary

tube– Usually have a sealed data logger– Sensor is lowered a known distance into the

water when installed


• Float method– A float is attached to the end of a steel tape– Read the depth off of the steel tape


• Sonic or audible methods– The classic “drop the pebble in the well

approach” only with a tape attached to the pebble

– Drop a battery powered probe down the beeps when it is in the water (whistler)


• Ultrasonic/radar/laser methods– A sonar type device– Calculates the reflection time– Can get depth to water and total depth of the

well

Manual measurements in flowing wells

• Manometers and pressure gauges– Well is sealed and a pressure gauge is

installed in the top– Mercury can be accurate to 0.005ft– Pressure gauges can be accurate to 0.2 ft

Methods of Continuous measurement

• Mechanical: float recorder systems– A float attached to a seismometer type drum

• Electromechanical: Iterative Conductance Probes (dippers)– Probe is lowered to the water surface by a stepping

motor– Sensor like on a whistler tells the motor to stop– Motor reverses and repeats at set intervals

• Data loggers

Analysis, Interpretation, and Presentation of Water-level data

• Water-level can be effected by recharge and discharge conditions– Water flows down during recharge and up

during discharge

Approach to Interpreting Water-level data

• Conduct a thorough site analysis

• Review monitoring wells features

• Establish groundwater flow direction and magnitude– Monitor for several days to see long term

fluctuations

Transient Effects

• Water level can change due to many things– Seasonal precipitation– Irrigation– Well pumping– River stage– Tidal fluctuations

• These can reverse flow direction

Contouring water level elevation data

• Made like a topo map, only of the water table and not the surface elevation

• May require cross sections in areas with high vertical flow

acquisition and interpretation of water-level data travis von dessonneck

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