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

Manual measurements in nonflowing wells 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

Manual measurements in nonflowing wells 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

Manual measurements in nonflowing wells 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

Manual measurements in nonflowing wells Float method –A float is attached to the end of a steel tape –Read the depth off of the steel tape

Manual measurements in nonflowing wells 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)

Manual measurements in nonflowing wells 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

Manual measurements in nonflowing wells