A cone of depression forms in a groundwater table from the pumping of a water well. The pumping well draws groundwater through the aquifer and causes the water table elevation to decrease around the well. This depression in the water table is the cone of depression. Depending on the lithology of a property, the cone of depression may be circular, elliptical or some other shape. Generally, very porous soils such as sand have a wide, shallow cone of depression, while non-porous soils such as clay have narrow, steep cones of depression. A pump test using observation wells is a simple way to measure the cone of depression caused by well pumping.
- Skill level:
Things you need
- Pumping well
- Observation wells (minimum of 3 recommended)
- Water level probe / data logger for each well
- Water level indicator, optional
- Area map
Plan the pump test before you begin so that you obtain usable data. Choose the well that will be the pumping well and the wells that will be the observation wells. Observation wells can be existing residential or monitoring wells. If you do not have access to existing wells, hire a drilling company to install observation wells at several locations. Ideally, the observation wells should be located approximately 300 feet, 700 feet and 1,000 feet from the pumping well and have screens at the same depth.
Install the water level probes and data loggers in the pumping and observation wells and install the pump in the pumping well. Some equipment has a water level meter and data logger in the same apparatus, which makes the installation easier. If the pumping well is too narrow to accommodate both the pump and the data logger, you can take the water level readings in the pumping well manually using a water level indicator. You can rent these instruments, if necessary.
Collect initial water level measurements in all of the wells before you turn on the pump. This data will provide the baseline to measuring the size of the cone of depression.
Turn on the pump at the flow rate that is typical for the well. For example, if your well pump will likely operate at a constant 5 gallons per minute during the lifetime of the well, use that flow rate for your test.
Collect well data at regular intervals during the pumping test. As the test progresses, the depth to groundwater should be greater. Initially, collect data every one to five minutes, then increase the interval over time. Typical pump tests run for 24 to 72 hours.
Evaluate the test data at the end of the test to determine if there was any drawdown in the observation wells and, if so, how much. Plot the locations of the wells on your area map. Calculate the change in the water table elevation by subtracting water level readings at different time intervals from the initial readings prior to the test. Any observation well that does not show a decline in the water table elevation is beyond the cone of influence of the pumping well.
Tips and warnings
- You can use a manual water level to measure the water table elevations rather than an electronic probe; however, to perform the measurements at the proper time, you may need assistants if the wells are far apart.
- Hire a professional geologist to perform the pump test to ensure accurate results.
- A cone of depression will increase in size over time with constant pumping.
- If you are unsure how to interpret the pump test results, consult a geologist at the state environmental office, a local university, or consulting firm that is experienced with groundwater.
- If two pumping wells are located within the same radius of influence, the cones or depression will intersect, affecting the test results.
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