The Anatomy Of A Drilled Well

anatomy of a drilled well - diagram of side view undergroundThe amount of water that can be obtained from a well depends on the nature of the water bearing layers drilled into. For instance, in terms of the anatomy of a drilled well, if you drill a well into an area composed of shale where there are many voids between the layers and millions of fractures to hold water, the chances are that the well will be a good producer.

The amount of water that can be obtained from a well depends on the nature of the water bearing layers drilled into. For instance, in terms of the anatomy of a drilled well, if you drill a well into an area composed of shale where there are many voids between the layers and millions of fractures to hold water, the chances are that the well will be a good producer.

To understand why this is, imagine a fish tank packed with flat rocks to the top like the one in the illustration above. Then, in your mind’s eye, fill the tank 2/3rds full of water. Then bring in a little drilling rig and drill a hole down through one of the rocks. Water will begin to rush into the hole when you get to the first space below the water level. The water level in the hole will rise until it is the same depth as the water in the tank.

If you stop drilling here and install a pump in your well, you may not get enough water for your needs. You will have only one crack feeding the well and if the water level in the tank drops a few inches your well will be dry. If you continue to drill you will go through additional layers, each providing an additional connection to the water supply and you will have more and more water stored in the hole as you drill deeper. When you finally begin to pump water from the well, the more cracks you have feeding it the more gallons per minute you will be able to take out.

As you can see, odds are that you would end up with a good well no matter where you drill in this type of rock formation.

A Low Yield Well

There are many areas of the world where people live over dense, sparsely fractured rock. Once again, in your mind’s eye, dump everything out of the fish tank because we need to start over to understand this.

This time we will put two wedge shaped rocks into the tank. They are shaped like a 90 degree triangle and are the same height and width as the tank. Put the rocks in so that the 90 degree corner of one is at the lower left end of the tank and the second so that the 90 degree angle is at the upper right side of the tank. The two wedges now fill the tank. Take a hammer and give the wedge shaped rocks a few good wacks to generate a some random cracks then fill the tank 2/3rds with water again. Note that it takes a lot less water to fill the tank this time.

In this type of rock formation, it does matter where you drill your well. See the illustration above. As you can see, well #1 was drilled all the way to the bottom of the tank but is a “dry hole”. The only fracture encountered here is above the water table. Well #2 found water, but the fracture feeding it is not very far below the water table. If the water level drops, this well too will be dry. Well #3 encountered water near the bottom of the hole. The water level in your fish tank (or aquifer) will have to drop quite low before this well goes dry.

Even the best of these three wells has only one fracture feeding it, so compared to the well in shale it will yield far fewer gallons per minute. That is why they are referred to as low yield wells.

Why a Pump and Pressure Tank System Perform so Poorly on a Low Yield Well

A modern water system consists of a well, a pump and a storage tank similar to the arrangement pictured below. In most systems, the pump is controlled by a switch activated by water pressure. As you use water from the storage tank the pressure goes down. When the pressure drops to a preset cut-in point, the switch activates the well pump which then replaces the water you used from the tank. The pump is turned off when the tank pressure rises to a preset cut-out pressure. The normal difference between cut-in and cut-out pressures is 20 pounds. Though system pressures can be set higher, a common set up would start the pump when the pressure drops to 20 lbs. and turn it off when it rises to 40 lbs.

If your home system has one of the new bladder type pressure tanks, you should be able to use between 5 gallons and 46 gallons of water, depending on whether you have a small or a very large tank, before the well pump comes on to refill it.

When you step into the shower in the morning, you are using water from the storage tank for the first few minutes. What happens after the contents of the tank are used depends on how many gallons per minute your well can provide and how many gallons per minute your pump can deliver.

For the sake of simplicity, let’s assume you have a 10 gallon per minute well and a 10 gallon per minute pump. Let us assume further that your shower head uses 5 gallons per minute. (most shower heads use closer to 3 gpm but the math is simpler this way)

With this pump and well combination, the water level in the well will remain constant because the pump is incapable of taking more than the well produces and since the pump is delivering twice the water necessary to supply the shower, the excess will go into the storage tank which will refill and turn the pump off when the pressure reaches 40 lbs. In this example the pressure in the shower will fluctuate from 40 to 20 and back to 40 psi as the water in the storage tank is depleted on the one end of the cycle and refilled by the well pump on the other. If someone else were showering at the same time, the entire output of the pump would be used to supply the two 5 GPM shower heads so the pressure tank would not begin to refill until one of the showers were shut off.

To someone with a low yield well, the previous paragraphs represent something others have and they can only dream about. There are places where a 1 GPM well is more common than one that provides 10 GPM. When the owners of such wells step into the shower they will get reasonable performance from their shower until the contents of the storage tank is depleted. Once they have to depend on the flow coming directly from the well pump everything changes.

Let’s assume that we have the same 10 GPM pump hooked to a 1 GPM well. Because the pump is taking more water than the well can provide, the water level in the well begins to drop. Each foot of height in a 6″ diameter well represents 1.5 gallons of stored water. If the pump is taking 10 GPM and the well is producing 1GPM then the well is loosing 9 gallons or 6′ of water for each minute the pump runs. If the pump were to run for 10 minutes, the water level in the well would drop 60′!

At this point, there is something you need to know about well pumps. The lower the water level is in the well, the harder the pump must work to lift it and the fewer gallons per minute the pump can deliver. In the first example where the well level remained fairly constant, the pump’s performance also remained fairly constant. In the second example, the 1GPM well, the pump’s performance gets poorer and poorer the longer it pumps because the water level is dropping rapidly.

If you are standing in the shower connected to this system, you are getting less water and poorer pressure as time goes on. Even though you have a 3 1/2 bath home you won’t be able to use two showers at the same time because there is not going to be enough water volume or pressure to operate both.

Eventually the water in the low yield well will drop so low that the pump runs out of lift (has insufficient power to lift the water) or it simply empties the well of water. Either of these situations mean the owner is out of water for the time being. Since you cannot see how high the water is in the well when you start, a person with a low yield well is never sure how much water they will be able to use before they run out.

Can you imagine standing in the shower covered with soap on a cold winter morning when the water just stops coming out of the shower head? If you have a low yield well you may have had this experience. A Well Manager® can put an end to this and many other problems owners of low yield wells thought they were doomed to endure.