Skip to main content
King County logo
A shallow well is a hole which has been dug, bored, driven or drilled into the ground for the purpose of extracting water is a well. A well is considered to be shallow if it is less than 50 feet deep. The source of a well is an aquifer. An aquifer is an underground layer of permeable soil (such as sand or gravel) that contains water and allows the passage of water.
Aquifers are replenished as rainfall seeps down through the soil. Ground water travels through permeable soil on top of hard or impermeable layers. Shallow wells usually are only deep enough to intercept the uppermost (or most easily reached) perched water table.

Water table wells 

  • Water table wells penetrate into aquifers in which the water is not confined by an overlying impermeable layer. The level at which the soil is saturated is the water table. Pumping the well lowers the water table near it. These wells are particularly sensitive to seasonal changes and may dwindle during dry periods.

Artesian wells 

  • Artesian wells penetrate into ground water having confining layers above and below the aquifer. Rainfall enters into the aquifer through permeable layers at high elevations causing the ground water to be under pressure at lower elevations. Because of this pressure, the water level in the well is higher than the aquifer. A well that yields water by artesian pressure at the ground surface is a "flowing" artesian well.

Since shallow wells penetrate into aquifers that are near the ground surface, they can become contaminated by barnyards, pastures, sewers, chemicals, or septic tank systems. Both rainfall and surface water runoff can carry pollutants down into shallow aquifers and well water. Since a hole penetrating an aquifer provides a direct route for contamination, wells must be designed to prevent pollution from entering and contaminating ground water.

A poorly protected well carries the following characteristics:

  • Is located within 100 feet of pollution sources, and is not sloped to divert surface water runoff away.
  • Does not have a sanitary well seal.
  • The annular space around the well casing is not properly sealed with cement grout or bentonite clay.
  • Does not have a pump house to protect the well-head, storage tank, and other equipment.
  • Has a well pit to house the pumping equipment or to permit access to the top of the well.
  • The wellhead protection area is not under the control of the operator or purveyor.

A properly protected well carries the following characteristics:

  • Has no sources of pollution within a 100 foot radius and is on high ground, sloped away in all directions from the well casing to divert surface water runoff.
  • Has an overlapping, tight-fitting cover or sanitary seal at the top of the casing or pipe sleeve.
  • The annular space outside the well casing is sealed with cement grout or bentonite clay at least 2 inches thick to a minimum depth of 18 feet.
  • Has a pump house to protect equipment, storage tank, and piping.
  • Has a pitiless adapter instead of a well pit.
  • Has a well-head protection area under the control of the operator or protective covenants.

Wells can be constructed in a number of ways. The simplest and least expensive of all wells is the driven well. It is constructed by driving a well-drive point into the ground that is fitted to the end of a series of pipe sections. A driven well is usually 2 inches in diameter.

Most wells, however, are drilled by either cable tool, percussion or hydraulic rotary, creating a 4 - 8 inch diameter hole. A casing or pipe sleeve is extended into the hole to prevent the sides from caving in. The annular space outside of the casing is sealed with cement grout or puddling clay.

Water can be pumped to the surface by a variety of methods. Very shallow wells (less than 20 feet) can use a suction pump at the ground surface. Deeper wells must use a submersible pump to push the water to the surface. Ground water is allowed to enter the casing by either an open end pipe, perforated pipe or a well screen, depending on the size of the aquifer soil particles. If the holes in the pipe or screen are too large, then the well may pump sand with the water. If the holes are too small, they may become plugged and reduce the overall yield of the well.

Dug wells

In the past, holes or pits were dug by hand or machines into the ground to tap the water table. Dug wells are usually 3 to 10 feet in diameter, 10 to 40 feet deep and lined with brick, stone, tile, wood cribbing or steel rings to prevent the walls from caving in. They depend entirely on the natural seepage from the penetrated portions of water table aquifers.

Dug wells have disadvantages to driven or drilled wells. They are more difficult to protect from contamination, and their yields are also very low because they do not penetrate into the reliable, productive water table aquifer. A dug well can be made much safer and more productive by driving a well point with a screen into the water-bearing formation, thus converting it into a driven well.

Getting water from the well to the house is done through a storage tank and distribution system. The distribution system includes the network of pipes, valves, connections, and other fixtures in between the storage tank and the houses being served. A pressure of at least 30 psi should be maintained throughout the system. (If fire flow is provided, a pressure of at least 20 psi should be maintained.)

Two types of storage facilities commonly used are:

  • A pressurized storage tank - protects the pump, so the pump does not have to turn on every time someone uses the water. Compressed air in the tank maintains water pressure throughout the distribution system. Pressure can be kept between desired limits by using electrical switches. Typically, only 10 to 40 percent of pressure tank volume is usable for storage. For this reason, pressure tanks are only designed for peak water demands.

  • An elevated storage tank - uses gravity to maintain pressure in the distribution system. These tanks should have a capacity of at least 2 days of average consumption.

Routine operation and maintenance procedures

  • Water testing
    After an initial analysis, the operator of a well must submit a water sample for coliform analysis (bacteria) at least once every 12 months and one sample for nitrate analysis at least once every 36 months. The coliform sample may be required more often, depending on the source, protection and construction.

  • Disinfection
    Wells should be disinfected:
    1. prior to use
    2. after construction or repai
    3. when coliform tests are unsatisfactory
    4. yearly for dug wells

Disinfection procedure for drilled, driven, or bored wells

After making sure that all sources of contamination are removed and your water system is clean, proceed as follows:

  1. Determine the depth of the water in the well when the pump is running. This can be done by subtracting the static water level from the total well depth. This information can be found on the well log or well driller's report. You can also use a disinfected steel tape or well sounder (from a well driller) to measure the water depth.

  2. Use Chart A below to determine the amount of household bleach required for the well diameter and depth of the water in the well. A chlorine concentration of 100 mg/L (100 ppm) is desired.

  3. Turn on the well pump.

  4. Add the proper dose of chlorine for the entire volume of water in the well to 5 gallons of water and pour this solution into the well by either:
    1. Remove the vent plug in the top of the well head. Using a small hose and funnel, pour the disinfecting solution in the well. (Try to move the hose or tube around as the solution is added to the well so as to wash the inside of the well casing) or;
    2. Raise the top of the casing, then add the disinfecting solution as in step A. (Note: Only raise the top of the casing a few inches so as not to lift the pump above the water level at the bottom of the well).

  5. If there is a faucet between the well and the pressure tank, and a valve between the faucet and the tank, you may connect one end of a potable water hose to the faucet (sample tap) and place the other end into the top of the well. Shut the valve between the faucet and the tank. Start the pump motor. Then, introduce the disinfectant into the well. This allows the water to circulate up the well, through the hose, and back into the well. In this way, you can wash down the casing with chlorinated water while assuring that the chlorine is thoroughly mixed in the well.

  6. After 15 minutes, shut off the pump and remove the hose.

  7. Open all faucets on the water system and monitor each one until you can detect a chlorine odor in the water.

  8. Close all faucets and allow the chlorine solution to remain in the well for at least 24 hours.

  9. After 24 hours or more have elapsed, pump the well to remove all remaining traces of chlorine. DO NOT DRAIN into a septic tank, stream, wetland or lake. Any water being discharged to the ground should NOT have any chlorine in it. Chlorine test strips are available from swimming pool or spa dealers. Use a test strip to check chlorine content before pumping the well or storage tank. Also, there should be no chlorine in the well when taking a coliform sample.

  10. Collect a water sample for coliform testing. If results are unsatisfactory, repeat the disinfection process until coliform tests are acceptable.

Disinfection procedure for dug wells

  1. Use a clean stiff broom or brush to wash the interior wall of the casing or lining with a chlorine solution of 100 mg/L (100 ppm). Be sure to have adequate ventilation. Do not enter the well without another person, who stays on the surface, and is connected by a safety line.

  2. Place the cover over the well and pour the required amount of chlorine solution into the well through the manhole or pipe sleeve opening. Distribute the chlorine solution over as much of the surface area as possible to get the best distribution of chlorine in the water.

  3. Follow steps 5 through 10 above.

Coliform testing

  • Coliforms are a large group of bacteria which commonly live in the intestinal tracts of warm blooded animals and humans.
  • Most coliforms are harmless. They are tested for as an indication of water contamination by sewage or animal waste. Coliforms are used as indicator organisms because:
    • They are often more numerous, longer living and easier to find than disease causing organisms or pathogens.
    • They are typically less susceptible to treatment and disinfection than pathogens. Therefore, the absence of coliforms in drinking water is a good indication that disease causing organisms are not present.
    • Sewage and animal waste contain many pathogens such as protozoa, bacteria and viruses.
    • Specific examples include cholera, typhoid, hepatitis A, giardiasis and epidemic dysentery.
    • Common symptoms of water borne disease include diarrhea, cramps, nausea and possibly jaundice.

However, coliforms are not a perfect indicator of the actual or potential presence of pathogens. Protozoans such as Giardia, Cryptosporidium and Cyclospora are more resistant to treatment and disinfection which remove coliforms. These protozoa are commonly found in surface water and may be present in improperly protected or developed wells. Symptoms of these diseases include diarrhea, stomach cramps, vomiting, weakness and slight fever.

Nitrate testing

Sources of nitrogen include septic systems, animal feedlots, manure storage facilities and fertilizers. Once in the soil, micro-organisms convert nitrogen to nitrate. What nitrate is not absorbed by plants is carried down to ground water by rainfall.

Health effects

Nitrate, the chemically active form of nitrogen, causes Methemoglobinemia or Blue Baby Syndrome. Methemoglobinemia is a life threatening condition in infants. Nitrite reacts with hemoglobin (the oxygen carrier in the blood) and changes it to methemoglobin. Methemoglobin cannot carry oxygen; this leads to oxygen starvation and in extreme cases, suffocation.

Nitrite can also be converted to other compounds that have been known to cause cancer in laboratory animals. It is, therefore, assumed that these compounds could increase the risk of human cancer, although there is no direct evidence for this.

For these reasons nitrate sampling and analysis is required at least once every 36 months. Laboratories report nitrate test results either as nitrate (NO3) or nitrate-nitrogen (NO3 - N). The acceptable levels are 45 milligrams/liter for nitrate and 10 milligrams/liter of nitrate-nitrogen.

C = Cup

Q = Quart

G = Gallon

Depth of water in feet Well diameter in inches
  2" 3" 4" 6" 8" 10" 12" 16" 20" 24" 36" 48"
5' 1C 1C 1C 1C 1C 1C 1C 2C 4C 1Q 3Q 5Q
10' 1C 1C 1C 1C 1C 2C 2C 1Q 2Q 3Q 6Q 2.5G
15' 1C 1C 1C 1C 2C 3C 4C 2Q 2.5Q 4Q 2G 4G
20' 1C 1C 1C 1C 2C 4C 1Q 2.5Q 3.5Q      
30' 1C 1C 1C 2C 4C 1.5Q 2Q 4Q 5Q      
40' 1C 1C 1C 2C 1Q 2Q 2.4Q 4.5Q 7Q      
60' 1C 1C 2C 4C 2Q 3Q 4Q          
80' 1C 1C 2C 1Q 2Q 3.5Q 5Q          
100' 1C 2C 3C 1.5Q 2.5Q 4Q 6Q          
150' 2C 2C 4C 2.5C 4Q 6Q 2.5G