Introducting a prepared chlorine solution into a well.

Bulk displacement is the recommended method of introducing a prepared chlorine solution into a well. It involves preparing a predetermined volume of disinfectant solution and pouring or pumping the solution into the well. The chlorine solution displaces water in the casing, screen/borehole and aquifer, replacing it with the chlorinated solution. This is the most practical method to assure that chlorine is uniformly distributed throughout the well and reaches the aquifer surrounding the well intake.

Chlorine Solution Preparation

Determine the volume of the chlorine solution that needs to be prepared. The volume of the chlorine solution should be a minimum of five times the combined volume of the casing and screen/borehole. This volume helps ensure that there is enough disinfectant to fill the casing/borehole and flow out into the aquifer around the screen or rock borehole.

For example, a 125-foot deep well with 5-inch diameter casing, with a static water level of 25 feet below ground, has a volume of 100 gallons. Therefore, the volume of the stock solution should be 500 gallons. For fractured limestone and other highly porous formations, the volume of the chlorine solution should be increased to assure chlorine contact with all contaminants that may have been introduced into the formation.

For use in determining casing and screen/borehole volumes, use the following formulas:

  • 2-inch well = 0.16 gallons per foot

  • 4-inch well = 0.65 gallons per foot

  • 5-inch well = 1.0 gallons per foot

  • 6-inch well = 1.5 gallons per foot

Adjust the pH of the water in the bulk tank to 4.5 before adding chlorine (optional, but recommended). Controlling the pH of a chlorine solution is recommended, especially when previous disinfection attempts have been unsuccessful.

Use a well cleaning acid (carefully following manufacturers' instructions) to lower the pH of the chlorine solution being prepared in the bulk tank to 4.0 to 4.5. After the acid is added to the water in the bulk tank, the water must be agitated to assure uniform distribution of the acid. An air line, a recirculation pump, paddles or other agitation methods should be used. Use pH test strips to check the pH level. Make sure the pH has stabilized by testing several times.

Caution: At a pH of 5 or below, some chlorine gas is formed. More gas evolves if the pH drops below 4. Fortunately, the chemical reaction forming the chlorine gas is slow at or near a pH of 4 and the hypochlorite solution quickly begins to neutralize the acid as soon as it is added, raising the pH to a safer level. Above a pH of 5, no chlorine gas is produced. Add the acid to the source water only in a well-ventilated area, and avoid exposure to the fumes from the acid container. Do not use source water that has hydrogen sulfide present unless the water has been aerated enough for the hydrogen sulfide to dissipate. Adding an acid to water containing hydrogen sulfide will cause the hydrogen sulfide gas to immediately be released from the water. Exposure to elevated levels of hydrogen sulfide is a serious health risk.

Recommended Acids

Use only NSF-certified acids. There are several commercial well cleaning products on the market that have been developed to lower the pH of the source water.

Acetic acid - This is a good biocide and biofilm dispersing acid. Acetic acid usually is sold as glacial acetic acid, which is at least 95 percent (except for vinegar). In this form, it is extremely dangerous and very corrosive to skin and lungs. Vinegar is a weak form of acetic acid, and can successfully lower the pH of source water. There are several types of vinegar available, but the white distilled vinegar is the recommended type. Vinegar is safe to use, readily available and inexpensive. Sources of water to be used for the chlorine solution may differ in quality and adding the same amount of a specific acid may produce slightly different results from case to case. A general rule is to add 1 gallon of vinegar to each 100 gallons of water to lower the pH to the desired starting point of 4 to 4.5 (remember to always check with suitable pH test strips for that range).

Sulfamic acid - Relatively effective against carbonate scales, but is not effective against biofouling. Sulfamic acid is relatively safe to handle. It may form insoluble solids in a water well. It produces the sulfamate ion, which is very soluble for up to 6 hours to 12 hours. After that, it hydrolyzes to sulfate and then produces the insolubles.

Hydroxyacetic acid - This product has good biocidal properties.

Acids Not Recommended

Oxalic acid - This product is fairly dangerous to skin and eyes and produces oxalates, which are poisonous.

Citric acid - Citric acid is a food source for bacteria and is difficult to get out of a well.

Muriatic (Hydrochloric) acid - Contractors should avoid using muriatic acid due to its hazardous properties. Several deaths and injuries have occurred that were associated with use of muriatic acid for water well rehabilitation. Also, it is relatively ineffective against biofouling.

Phosphoric acid - One of the milder mineral acids, phosphoric acid leaves behind phosphate residue, which can stimulate bacterial growth and magnify biofouling problems. Use of this acid must be followed by aggressive agitation of the aquifer to remove any of the phosphate residue that may have been deposited.

Adding the Chlorine

Add chlorine to the source water and agitate to assure proper chlorine distribution. An air line, a recirculation pump, paddles or other means of agitation should be used. Add enough chlorine to obtain a concentration of approximately 200-ppm chlorine.

If calcium hypochlorite is used as the source of chlorine, it should be dissolved in a pail of water before being added to the prepared source water. After being added to the bulk tank and thoroughly mixed, the calcium carbonate impurities should be allowed to settle to the bottom of the tank. The clear solution above the sediment then is drawn off for discharge into the water well. Use sodium hypochlorite if the well water has more than 100 ppm dissolved calcium (calcium carbonate). The addition of chlorine to the source water should take place in a well-ventilated area; hypochlorite fumes or chlorine gas can become concentrated in a confined area and can cause severe respiratory problems.

Verify the final pH and chlorine concentrations. For maximum bactericidal efficiency, the final pH of the chlorine solution should be between 6 and 7. Use a chlorine test kit or meter to verify proper levels of chlorine and a pH test kit to verify proper levels of pH. Adjust as necessary. Note that the use of pH test strips may not be practical after the chlorine has been added to the stock solution because of the bleaching effect of the chlorine on the strips.

Chlorine Solution Application

Use the following procedures to apply the chlorine solution to the well:

1. Pump to waste. Turbidity in the water can reduce the effectiveness of the chlorine. If the well water is not clear, pump it to waste until it clears up before starting the chlorine treatment.

2. Bypass treatment units. Bypass water treatment units, such as cartridge filters, water softeners, reverse osmosis (RO) systems and iron-removal systems. Following manufacturer's recommendations, disinfect the treatment units with chlorine. Then, leave the units in by-pass until the chlorine has been completely flushed (high concentrations of chlorine can damage softener resin and RO membranes, so pay attention to manufacturers' instructions).

3. Application of chlorine solution into the well using bulk displacement. After the chlorine solution has been prepared (see previous section), apply the chlorine solution into the well. Delivering the chlorine solution with sufficient hydraulic energy to effectively penetrate all areas of the well is crucial.

Assure that the wellhead area is well ventilated, especially if the well is in a confined space such as a well house, well pit or basement offset. Use appropriate safety precautions and follow state laws when working in confined spaces.

For screened wells, the chlorine solution may be discharged directly into the top of the well. The solution will displace the water in the casing and screen, and force the chlorine solution out into the formation.

For screened wells in extremely coarse formations or where screen lengths more than 10 feet are used, a jetting tool extending into the screen to apply the chlorine solution is recommended. The jetting tool will force the solution farther out into the formation, and the jetting action will help break loose cuttings and drilling fluids that have been trapped in the geologic materials outside the well.

For rock wells, pump the chlorine solution through a tremie pipe extending to the bottom of the borehole and slowly withdraw the tremie pipe as the solution is added. This helps uniformly distribute the chlorine solution throughout the uncased portion of the borehole. A jetting tool or other means of agitation is recommended during or after application of the chlorine.

4. Agitation. Agitation of the well is recommended when disinfecting existing wells where biofilms are likely, or where repeated chlorinations have not been successful. Agitation of the chlorine in the well will help distribute the chlorine into the aquifer, and will enhance the penetration of the chlorine into slimes, scale or other bacteria-containing material. The recommended method of agitation is by surge block or jetting tools.

5. Circulation. After the chlorine has been applied to the well, turn on the pump and circulate the chlorinated water through the service lines and plumbing. Use a garden hose attached to an outside tap to run the water to waste (discharge onto the ground surface) until a chlorine smell can be detected in the water and the water is clear.

6. Recirculation. Use the garden hose to recirculate the chlorinated water back into the top of the well and wash down the inside of the casing for at least 30 minutes (the longer the better) with the chlorinated water. Do not start the recirculation until water from the hose is clear. Do not discharge turbid water from the hose back into the well. During this recirculation process, water circulates from the pump, through the drop pipe, service line, pressure tank, distribution piping and hose back into the top of the well. Recirculation enhances the chlorination process by exposing these surfaces to chlorinated water and cleaning water-contact surfaces within the well.

Some water well drilling contractors have extended the recirculation time to several hours. Extended recirculation has been shown to increase the effectiveness of the disinfection process. During extended recirculation, a seal needs to be placed on the wellhead to keep contaminants from entering the well.

Where practical, the pump intake should be located as close to the bottom of the well as possible during this period of recirculation. After completing the recirculation, install the permanent well cap.

7. Open taps. Open each tap within the home, one at a time, starting closest to the pressure tank, and run water until a strong chlorine smell is present. Check with a chlorine test strip, and then close the tap. Do not forget to flush each hot water tap.

8. Contact time. Allow the chlorine to remain in the water supply for 4 hours to 12 hours, or overnight. Contact time is an important part of the chlorine treatment process. The longer the chlorine remains in the water supply system, the better the chance that the chlorine will contact and kill microorganisms that may be present. Water use during the contact time should be minimized to assure that residual chlorine remains in the well. The well drilling contractor should notify building occupants that the water is being disinfected and warned not to consume or bathe in the water until the disinfection is complete.

9. Flush. After the contact period, pump to waste to remove the chlorine. Flushing should be continued for at least one hour after a chlorine smell can no longer be detected. This helps assure that all traces of chlorine have been removed from the system. Flushing the well for an extended period of time after treatment with chlorine also will help clean the system because of the scouring action of the water.

While pumping to waste, use a hose to discharge into the yard, roadside ditch or to another point where a nuisance will not be created. Avoid pumping strong chlorine solutions onto lawns or landscape plants. Do not run the water into the household plumbing, and subsequently into the septic tank/tile field, during this pumping period. This may overload the drain field. Do not run the water into a lake, stream or other body of water.

10. Reactivate treatment systems. When all traces of chlorine are gone, place the already disinfected water treatment units back on-line.

11. Sample. Before collecting a water sample for bacteriological analysis, check for residual chlorine using a chlorine (swimming pool) test kit or a chlorine meter. If chlorine still is present, continue to flush. If a test kit or meter is not available, continue to flush for at least 2 hours after there is no chlorine smell, before collecting the bacteriological water sample.


Water in a water supply system that has been treated with chlorine can be turbid due to the effect of the chlorine on contaminants in the water (such as iron and biofouling). Extended continuous pumping normally clears the water of turbidity, unless the turbidity is the result of a well construction defect.