A modern drilled well is more than just a hole in the ground. To deliver an adequate supply of clean water to your tap, the well must be properly lined and sealed to prevent contaminated surface water from entering. In loose soils, the well must also have special screens at the bottom to maintain a good flow and filter out silt that could clog the well. Finally, a pump must be installed along with underground plumbing to a pressurized storage tank in the house.
If you buy a piece of land with a “well installed,” you will still need to pay for the pump, trenching, plumbing lines, and pressure tank – which may cost as much as the drilling itself.
The casing is a steel or PVC pipe, typically 6 inches in diameter, that extends from about a foot above grade to the bottom of the well or to the point where it hits solid rock. PVC is now commonly used for the casing as it is inexpensive and highly resistant to corrosion. However, steel is still common in some regions.
The casing keeps soil and rock fragments from collapsing into the well shaft, and provides protection for the wiring and water piping connected to the submersible pump near the bottom of the well. State codes often require a minimum of 18 feet of casing in unconsolidated soil (i.e., not solid rock).
The space between the casing and the borehole, called the annulus, is filled to prevent contaminated surface water from polluting the well. The filling or “grout” may be sand, gravel, cement or bentonite clay depending on the soil layers. Special porous sand is often used at the bottom around the screen and dense clay or cement is used in the upper section. At the point where the casing meets bedrock, it is sealed or “seated” into the rock using either cement, bentonite clay, or other techniques.
At the top of the casing is the visible well head, which is tightly capped to prevent contaminants from entering the well casing. The wiring for the submersible pump is protected at the well head by PVC piping. The well head should be inspected periodically for damage.
If the bottom of the well is in solid bedrock, pure water reaches it though narrow cracks and fissures in the rock. However, if the bottom of the well is in sand and gravel, it needs special filters to prevent sediment from entering the well, degrading water quality, and harming the pump.
Wells that terminate in loose material use a filtering device called a well screen, installed at the bottom of the well casing, to keep soil particles from entering the well. There are many types of screens available and designed with openings sized and shaped for the specific type of soil. Given the cost and difficultly of replacing the screen, it’s worth spending a little more for a high-quality screen that will resist damage during installation and corrosion during service.
In addition, a gravel pack several inches thick is usually placed around and above the well screen, unless the soil is already coarse sand and gravel. The gravel pack provides a prefilter to keep fine sand and sediment from reaching the screen and well. This allows the use of a screen with larger openings, improving the flow rate. The packing gravel must be carefully sized or it can get clogged with sediment and block the water flow.
Before the well is fully functional, it must first be “developed.” Web development removes the fine particles of clay, silt, and sand leftover from the drilling process, especially around the area of the well screen. This improves the quality and flow of water from the aquifer into the well, and helps protect the pump from being damaged by sand and silt intrusion.
A number of different methods are used, all involving forcing water or compressed air into the well at high speed, often with reversal of the flow, to effectively flush out the fine particles. Common techniques include overpumping, backwashing, and surging (with compressed air, bailer, or surge block). Depending on the well characteristics this can take from one to several hours.
If all is working properly and the well yield is acceptable, the “annular” space between the casing and borehole is “grouted” with cement to or bentonite clay to prevents contaminated surface water from draining into the aquifer. The well is then disinfected and capped at the top to provide sanitary conditions until the well is put into use.
Once the well is drilled, but before it is connected to the house, a yield test is done to determine how much water it can deliver continuously over an extended period of time. The test is typically done over 48 hours or more to see how quickly the well can be replenished from the aquifer and provide a “sustainable yield”.
How much water is needed depends on the size of the household and any special equipment or activities that use a lot of water. For an average household, a figure of 110 gallons per bedroom per day is often used. In addition to the total daily load, there is also the issue of peak load: can the supply meet the simultaneous demands of, say, the dishwasher running while someone is showering.
The minimum acceptable level for most single-family homes is 5 gallons per minute, the rate required by many mortgage companies. With some storage, this allows occupants to run two average fixtures or appliances at the same time, meeting typical peak demand. Rates above 10 gallons per minute will meet most household demands with no storage. However, wells with rates as low as 2 to 3 gallons per minute can provide adequate pressure and flow by adding a larger than normal pressure tank, a storage tank that used in most modern well systems.
If the well yield is still unacceptable, options include drilling deeper in the hope that you will get a better yield, drilling a second well on the property and either combining the yields of both or decommissioning the first well. Be aware that some municipalities stipulate a minimum well yield in order to sell a property
For wells drilled into bedrock, hydrofracking is a process that may increase the yield significantly. The technique, originally developed for oil and gas wells, injects water into the well under high pressure in order to enlarge or create new water-bearing fractures and fissures in the bedrock surrounding the well. Hydrofracking equipment is now pretty common in areas with a lot of bedrock wells.
The process is expensive, but usually less than drilling deeper or drilling a new well. If it works, the benefits of hydrofracking should be permanent. However, as with all well drilling, there is no guarantee that hydrofracking will work. Still, some well drillers now offer a written guarantee that hydrofracking will increase your well’s yield, subject to the small print of course, but certainly better than no guarantee at all.
If you buy a site with a well already drilled, it’s just a hole in the ground with a casing and cap. To complete the well, you will need a submersible pump, a pipe run from the well to the house, and a pressure tank installed in the home. Don’t skim on the well pump as it is an expensive item to replace and a top quality pump does not cost much more than a mediocre pump. The size of the pump is determined by the yield of the well, the daily use of water, and the amount of storage.
One of the leading causes of pump failure is lighting strikes as a well provides a good grounding source for lightning. Since replacing a pump can cost $1,500 or more, it may be worth considering a lightning protection system for the pump. This usually involves installing a grounding rod close to the well and other measures. Make sure the work is done by a professional electrician with experience in lightning protection.
Typical storage or “pressure” tanks for private residences range from 20 to 40 gallons, but may need to be larger for low-yield wells. The pressure tank provides pressurized water to the household between pump cycles, helps meet the household demand beyond the pump capacity, and also above helps prolong the life of the pump (by preventing rapid cycling).
The total storage volume is determined by the both the storage in the well casing itself, plus the storage in the tank. The deeper the well and the greater its diameter, the more water is stored in the well. The State of New York Dept. of Health recommends the following level of storage (combined pressure tank and well storage):
Recommended Minimum Water Storage
Number of Bedrooms
|≥ 5 gpm|
|3-5 gpm||100 gal.||150 gal.||200 gal.||250 gal.|
|1-3 gpm||150 gal.||200 gal.||250 gal.||300 gal.|
|0.5-1 gpm||200 gal.||250 gal.||Not recommended for use|
|≤ 0.5 gpm||Not recommended for us|
Source: Bureau of Water Supply Protection, New York State Department of Health