Division of Environmental Health

Maine Center for Disease Control & Prevention

A Division of the Maine Department of Health and Human Services

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Water and Well Facts for Private Well Owners

Water and Well Facts for Private Well Owners

Recommended Minimum Flow Rate for Single Family Homes |
Disinfecting a Well | Coliform Bacteria | Water Facts Summary: Trace Metals

Recommended Minimum Flow Rate for Single Family Homes

These standards are based on a static water level of approximately 25 feet below ground surface. Every foot of a 6" well holds approximately 1 1/2 gallons of water.

Recommended Minimum Flow Rate for Single Family Homes
Well Depth (FT)
Recovery Rates (GPM)

Disinfecting a Well

  1. Remove the well cap. Pour the appropriate amount of chlorine bleach (Clorox, Dazzle, or other EPA/NSF approved bleach) from the table below into the well.
  2. Replace the well cap.
  3. Open all faucets, sill cocks and similar outlets individually until you smell chlorine in each outlet.
  4. Allow the mixture to stand in the system overnight, then flush the chlorine mixture from the system using an outside faucet and garden hose. DO NOT FLUSH THE MIXTURE INTO YOUR SEPTIC SYSTEM. You may resume using the toilet facilities as the septic system is designed to handle this, but the septic system cannot handle the large amount of water needed to flush the chlorine from the well. Since the chlorine will kill grass, be careful where you run the water outside.
  5. After disinfection, the water supply should be sampled and tested for coliform bacteria. Sample kits for coliform bacteria are available from private and state labs. Check the yellow pages or click here for a list of certified labs. Sampling should be done only after the odor of chlorine disappears. It takes about 3 or 4 days of normal water usage before all of the chlorine smell disappears.
Drilled Well
(PLEASE NOTE: A greater amount of chlorine may be needed to disinfect the water depending on the degree of contamination)
50 feet
2 1/2 cups
100 feet
1 1/2 quarts
150 feet
2 quarts
200 feet
2 1/2 quarts
250 feet
3 quarts
300 feet
3 1/2 quarts

Dug Well
(Approximate amount of water in the bottom of a well and not the total depth of the well. For a surface SPRING, use 2 gallons)
Dosage (Gal)
5 feet
1/2 gallon
10 feet
1 gallon
15 feet
1 1/2 gallons
20 feet
2 gallons

Coliform Bacteria

Coliform bacteria make up a large group of bacteria that are found in soils, on plants, and in surface water. Certain coliform bacteria live in the intestines of humans and animals. Coliforms are used as an ‘indicator’ in drinking water. They are not harmful themselves, but if they are present, it means that disease-causing microorganisms such as bacteria, viruses, and parasites could have gotten into the water supply by the same route as the coliforms. It would be difficult, if not impossible, to test for every disease-causing microorganism, so we test for coliforms. If coliforms are present, there is a chance that the others are too.

Normally as water from the surface seeps downward through the soils, microorganisms are filtered out. When a test for coliforms is positive (meaning bacteria are present), it tells us that the microorganisms did not get filtered out and they made it into the drinking water supply. The water should be disinfected by applying chlorine bleach to the well water. After the system has been disinfected, the water needs to be retested for bacteria. You should wait at least 3 or 4 days after the chlorine odor has disappeared before resampling.

If the test for E. coli or fecal coliforms is positive, the water is not safe to drink! It should not be consumed unless it has been boiled for at least one minute at a rolling boil or disinfected by some other means. E. coli and fecal coliforms are indicators that the water has been contaminated with fecal wastes and it is very likely that it contains disease-causing bacteria, viruses, or parasites. These can cause intestinal upset as well as diseases such as dysentery, hepatitis, and giardiasis.. IMPORTANT: If your water was positive for total coliforms and the water was not tested by the lab for E. coli or fecal coliforms, you cannot be sure they are not there and you should take the same precautions by boiling the water or disinfecting it as indicated above.

Most bacteria in wells or springs come from surface water contaminated by decayed animal waste or human activities. As surface water seeps downward through the soil towards the water table, these bacteria may be naturally removed by the soils. The extent of removal depends on the depth and character of the soil. In general, shallow wells and springs are more likely to contaminated than deep wells. Wells and springs must be properly located, constructed and maintained in order to prevent surface water from entering the well or spring. The presence of coliform bacteria and/or E. coli or fecal coliforms in wells or springs usually result from:

  • Well or spring covers that allow dust, rain, bird droppings, and other material to enter;
  • Wells or springs that are located in areas where surface water covers the well or spring during the wet periods of the year;
  • Defective steel well-casing seals;
  • Shallow wells or springs with rocked up sidewalls;
  • Recent changes or repairs to the well or spring, pumps, piping, etc; and,
  • Improper well location and/or construction.

If you have one or more of the problems described above, it should be immediately corrected. If the problem is not corrected, it may reoccur even after repeated disinfection.

Water Facts Summary: Trace Metals

Trace metals may occur naturally in ground water in very small amounts and may include arsenic, barium, cadmium, chromium, lead, mercury, selenium, silver and zinc. In small amounts these are harmless and in some cases even beneficial to health. Amounts over drinking water standards may have serious health effects.

Arsenic Drinking Water Standards and Health Risks:

The primary drinking water standard for arsenic is 0.01 milligrams per liter. Studies suggest that arsenic may be associated with several different forms of cancer, including bladder and skin cancers. Studies also indicate that arsenic in small amounts may be an essential element for normal human development.

Possible Source of Contamination:

Arsenic contamination of water may be the result of dissolved minerals from local bedrock or sediments which contain arsenic, or it may be caused by leachate from solid waste landfills or from the use of pesticides.

Barium Drinking Water Standards and Health Risks:

The primary drinking water standard for barium is 2.0 milligrams per liter. Small doses are not harmful. Large amounts can cause increased blood pressure, nerve damage or cardiovascular disease.

Possible Source of Contamination:

Barium pollution may come from natural sources or can enter water supplies through industrial waste discharges.

Cadmium Drinking Water Standards and Health Risks:

The primary drinking water standard for cadmium is 0.01 milligrams per liter. Cadmium in high concentrations can cause short-term intestinal illness. Since cadmium tends to accumulate in the body, long-term effects may occur, including intestinal, lung and kidney damage.

Possible Source of Contamination:

Cadmium contamination may be caused by disposal of waste from photographic, metal plating or pesticide manufacturing industries. The most common source of contamination is from the corrosion by acidic water of galvanized pipes or soldered joints in copper pipes.