Groundwater, by nature, is bacterially pure. The presence of total and faecal coliform in well water is evidence that pollution has occurred in the well. These contaminants cannot be detected by taste or visual inspection, making them a hidden hazard.
Bacterial contaminants are common to the hands, tools and materials used in well construction and maintenance, requiring disinfection immediately after a well is worked on. A coliform contamination problem that persists after disinfection is indicative of poor well construction, degrading construction materials, or nearby land/water use activities.
From a well owners point of view, a bacteriological analysis is the most important water test to be done on drinking water. Well water may have a poor taste because of high concentrations of some parameters, such as iron and manganese, however, the physical and chemical aspects of groundwater quality rarely result in well water being unsafe for drinking.
Testing for bacterial contamination in a well and water supply system is recommended as follows:
The purpose of disinfecting a dug or drilled well is to destroy all disease causing microorganisms (pathogens) that may have been introduced into the well during construction, hookup, maintenance, or as a result of faulty well construction.
Disinfection is also recommended for well systems that experience problems with iron bacteria or sulphate-reducing bacteria. Iron bacteria create deposits of stringy, rusty coloured jelly-like slime on the inside of the well, pumping system, and in the water filled pore spaces in the ground. Sulphate-reducing bacteria feed off sulphate to produce hydrogen sulphide gas (characterized by a "rotten egg" odour) and scale deposits. Both problems can severely reduce well yield and the efficiency of the water supply system.
Chlorine compounds are the most popular disinfectant for drinking water because of their potency and effectiveness on a wide range of germs. Chlorine is also easy to use, relatively inexpensive, and persists in a well long enough to kill harmful organisms.
Of the chlorine compounds, calcium hypochlorite is the preferred source of chlorine used to disinfect wells. Calcium hypochlorite remains fairly stable when dry retaining 90% of its chlorine content after a one year storage period. It can be purchased in granular and tablet form containing 65-70% available chlorine by weight. This means for every 1 kg of calcium hypochlorite, 0.65 to 0.70 kg (650 to 700 grams) of chlorine is available for disinfection. Care must be taken when handling calcium hypochlorite because deadly chlorine gas can be released when the substance is wet.
Sodium hypochlorite may be used in the absence of calcium hypochlorite. This chemical is available only in liquid form in strengths up to 12-15% available chlorine. This means for every 1 litre used, 0.12 to 0.15 litres of chlorine is available for disinfection, the rest is water. The unstable nature of sodium hypochlorite solutions may result in the labeled chlorine concentration not being available for disinfection at the time of use. A common form of sodium hypochlorite is household laundry bleach that has, at best, approximately 5% available chlorine.
Storage of any chlorine compound is difficult, therefore, it is best to have on hand only the amount needed at any one time. Dry chlorine compounds should not be stored for more than one year. Liquid chlorine compounds should not be stored for more than 60 days. Compounds should not be exposed to the atmosphere or direct sunlight during storage. It must be remembered that storage under some conditions can be dangerous. Contact can damage eyes and skin, therefore, care must be taken when handling all the chlorine compounds.
Shock chlorination refers to the placement of a strong chlorine solution in a well and/or water supply and distribution system to kill nuisance and disease causing organisms. Shock chlorination of a dug or drilled well and water supply system is recommended immediately after:
Note: Shock chlorination may not completely eliminate iron bacteria or sulphate-reducing bacteria from a well and water supply system, but should hold it in check. Shock chlorination may have to be repeated periodically in order to control these problems.
To properly disinfect a well and water supply system, the minimum concentration of chlorine in the well should be 100 to 300 parts per million (ppm), depending on the need for disinfection. For minor repairs outside the well, a minimum chlorine concentration of 100 ppm is recommended. To disinfect a newly constructed well, a well contaminated with coliform bacteria, or following repairs to a well or water supply system, a minimum chlorine concentration of 200 ppm is recommended. For nuisance problems, such as iron bacteria and sulphate reducing bacteria, a minimum chlorine concentration of 300 ppm is recommended. Over chlorination of a well typically does not present a problem. Therefore, it is better to add an excessive amount of chlorine solution rather than not add enough.
The following procedure is recommended for thorough disinfection of a dug or drilled well and water supply system.
|Amount of Chlorine Source to be used Per Foot of Well Depth
(source - calcium hypochlorite or sodium hypochlorite)
|100 ppm||200 ppm||300 ppm|
|Well||% Chlorine Available in Source||% Chlorine Available in Source||% Chlorine Available in Source|
To chlorinate a newly drilled well 6 inches in diameter and 220 feet deep. Using the minimum recommended chlorine concentration of 200 ppm for a newly constructed well, and selecting to use calcium hypochlorite (65% available chlorine) as the source of chlorine, the amount of source to be used is calculated as follows:
220 feet x 0.0017 kilograms per foot = 0.374 kilograms = 374 grams
To chlorinate a dug well because of coliform bacteria contamination. The well is 60 inches in diameter and 14 feet deep with a water depth of 4 feet. Using the minimum recommended chlorine concentration of 200 ppm for a well contaminated by coliform bacteria, and selecting to use sodium hypochlorite at 5% available chlorine (laundry bleach) as the source of chlorine, the amount of source to be used is:
4 feet x 2.224 litres per foot = 8.9 litres
Note: For chlorinating drilled wells, normally the well depth to the water line is negligible and therefore does not require consideration.
Caution: A rubber air-water separator in the pressure tank may be damaged by too strong a chlorine solution.
Caution: Before disinfecting the distribution system, temporarily remove or bypass any carbon filter in the system.
Caution: Chlorinated water should NEVERbe allowed to drain into the septic system.
Flowing artesian wells (wells that are naturally overflowing at the surface) are disinfected by lowering a perforated container filled with an adequate quantity of dry calcium hypochlorite to the bottom of the well. The perforated container can be constructed from a short length of tubing capped at both ends. The natural upflow of water in the well will distribute the dissolved chlorine throughout the full depth of the well. The flow at the top of the well can be partially or completely restricted to reduce the loss of chlorine. The water supply system can be disinfected in the manner previously outlined.
After the well has been thoroughly pumped to flush the system of chlorine, use the water supply (except for drinking) for approximately 10 days. Then sample for the presence of coliform bacteria. Do not rely on one sample. Two or three consecutive safe tests are needed to guarantee there is no coliform bacteria present in the well and water supply system. Samples should be collected by pumping the well water to waste for a minimum of 15 minutes and duplicate bacteriological samples collected not less than 30 minutes apart.
The well may be used as a drinking water supply only if none of the samples collected indicate the presence of coliform bacteria. The well water should be retested if any samples do indicate coliform bacteria. If this fails, subject the well to corrective action determined by a qualified professional.
It is recommended that the Government Services Centre collect water samples for coliform bacteria testing. If the owner must do so, use only the special sterile bottle and the instructions provided by the Department of Health and Community Services.
The following safeguards are helpful in preventing contamination problems in a well due to bacteria and other pollutants. A well owner should make an inspection of his or her well and take the necessary steps to ensure these safeguards are met.
Note: Failure to provide adequate ventilation to a drilled well will likely result in damage to the formation seal due to a vacuum created inside the well during ‘drawdown’ of the water level. A damaged formation seal will permit surface water and/or shallow groundwater to enter a well carrying pollutants into the water supply.