9.1: It's All Underground
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Well Yield is the amount of water a certain well can produce over a specific period of time. Typically well yield is expressed as gallons per minute (gpm). During the drilling of a well, pump tests are performed to determine if the underlying aquifer has the ability to supply enough water. A well yield test involves a comparison of the maximum amount of water that can be pumped and the amount of water that recharges back into the well from the surrounding aquifer. Continuous pumping for an extended period of time is usually performed and the yield is calculated based on the amount of water extracted. Well yields are typically measured in the field with a flow meter. Water levels in the well are then measured to determine the specific capacity and drawdown of the well.
Specific Capacity is helpful in assessing the overall performance of a well and the transmissivity (horizontal flow ability) of the aquifer. The specific capacity is used in determining the pump design in order to get the maximum yield from a well. It is also helpful in identifying problems with a well, pump, or aquifer. The specific capacity is defined as the well yield divided by the drawdown, expressed as gallons per minute per foot of drawdown.
- Specific Capacity = gpm/ft
In order to understand the term drawdown, you must also understand static water level and pumping water level as these measurements provide valuable information regarding the well and underlying aquifer. The static water level is defined as the distance between the ground surface and the water level when the well is not operating. The pumping level is defined as the distance between the ground surface and the water level when a well is pumping. Therefore, the pumping water level is always deeper than the static water level. The difference between these two levels is the drawdown. Depending on the aquifer, static water levels can be 20 feet below ground surface (bgs) or several hundred feet bgs.
The diagram above shows a well casing penetrating into the ground, the relationship between static and pumping water levels, and the drawdown.
- Pumping Water Level – Static Water Level = Drawdown
- 50 ft – 20 ft = 30 ft
- Drawdown + Static Water Level = Pumping Water Level
- 30 ft + 20 ft = 50 ft
- Pumping Water Level – Drawdown = Static Water Level
- 50 ft – 30 ft = 20 ft
Since static and pumping water levels are field measurements, drawdown is typically the calculated value.
Calculating Specific Capacity
Once you have the drawdown, the specific capacity of the well can be calculated, as long as you know the well yield.
Flow Rate = 1,000 gpm
Drawdown = 30 ft
Specific Capacity = 1,000 gpm/30 ft = 33.3 gpm/ft
Solve the following problems.
A well has a static water level of 23 ft bgs and a pumping level of 58 ft bgs. What is the drawdown?
A groundwater well has a base elevation of 1,125 ft above sea level. If the drawdown on this well is 44 ft and the pumping level is 80 ft bgs, what is the static water elevation above sea level?
A deep well has a static water level of 122 ft bgs. A drawdown has been calculated out to be 65 ft. What is the pumping level of the well?
A well has an hour meter attached to a water meter totalizer. After 3 hours of operation, the well produced 279,000 gallons. Water is the well yield in gpm?
When a well was first constructed it was pumping 1,750 gpm. The efficiency of the well has dropped 35%. In addition, the drawdown has decreased by 15%. If the original drawdown was 42 ft what is the current specific capacity?
A well pumped 538 AF over a one year period averaging 10 hours of operation per day. For half the year the static water level was 25 ft bgs and half the year 42 ft bgs. The pumping level averaged 55 ft bgs for half the year and 68 ft bgs the other half. What was the average specific capacity for the year?
A well has a specific capacity of 42 gpm per foot. The well operates at a constant 1,500 gpm. What is the drawdown?
A well has a calculated specific capacity of 30 gpm per foot and operates at a flow rate of 1.08 MGD. If the static water level is 28 ft bgs, what is the pumping level?