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Is My Water Well Going Dry?

Posted Oct. 1, 1998

We have been through a period of extreme drought, watching ponds and streams deteriorate to the point of going dry in some places. Those that are fortunate enough to have wells as water supplies are certainly concerned about the well maintaining yield necessary for an adequate supply. To discontinue using the well to save the water is not the correct approach. You need to know its actual characteristics.

Certainly some aquifers do have limited water and now is the time to determine those characteristics. Running a simple pump test will provide facts about the well for future planning. These long periods of usage with little to no rainfall for recharge provide the opportunity to collect necessary information during extreme conditions. The test should build confidence in future decisions rather than feeling around in the dark.

There is sophisticated equipment out there to facilitate an accurate pump test with significant expense. However, there are ways to conduct these tests with equipment you have on your farm. It requires a heavy string or light rope of adequate length to get to the water table, a small weight (a nut makes a good weight), and a five gallon bucket or a barrel depending on the volume of the well. Use a string that retains water so that when the weight goes into the water, the exact top of the water level on the rope can be determined.

To begin, disconnect the well from the distribution system. After moving the well seal to the side of the well casing, check the water level with the string and weight. Let the weighted string down inside the well casing to the water table and measure the depth to the water. Record the depth.

Then turn on the pump to begin production. Measure the yield of the pump by collecting the output in the known volume container. Record the beginning time of when the output was diverted into the container and then record the time it is full. The yield is the volume collected in that unit of time. For accuracy, the process should be repeated until you are confident of the numbers.

Divide the unit of time it took to fill the container into 60 (seconds per minute), then multiply that number by the capacity of the container. For example, if it took 10 seconds to fill a five gallon bucket with water, 10 divided into 60 gives a multiplier of six. Multiply five gallons times six, to determine the pump is producing 30 gallons per minute.

Before all the calculations, check the pumping level with the weighted string while the pump is on. To verify the numbers, it is wise to repeat the process. During wet periods, it is important to pump the well for days to obtain a stable reading. Under drought conditions, there should be almost immediate indication of the stability of the water table. Where the well has been intensively used, the results of the test should be immediate.

Further, by dividing the gallons per minute yield by the number of feet difference between when the pump was on and off (draw down), it provides well yield in gallons per minute per foot of draw down. If the total depth of the well is determined, and the thickness of water standing in the well, more calculations can estimate the potential yield of the well.

For example, if a well has 50 feet of static water in the well (100 feet total depth and water table at 50 feet), and the pump produced 30 gallons per minute with 30 feet of draw down, the well yield is one gallon per minute per foot of draw down. This well does not have much more potential than is being produced. While if the 30 gallon per minute yield only pulled the water table down three feet, the well yield would be 10 gallons per minute per foot of draw down, which is an excellent well. As a rule of thumb, drawing down 75% of the static water produces 90% of the available water.

Where one has much at risk dependent upon a well, performing one of these simple tests could provide the insight to increase concern or put your mind at rest. In reality, the test is determining recharge of the aquifer from which you are pumping. Some aquifers have excellent recharge characteristics, while others are only perched water tables with poor recharge, entirely dependent upon rainfall. Well depth has very little to do with its dependability. Shallow wells with good recharge are often as dependable as deep wells.

If you have additional questions about testing a well, please don't hesitate to call.

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