Water Stewardship

Guidelines for Minimum Standards in Water Well Construction, Province of British Columbia — June 1982

Part 2 — Technical Information Appendices

APPENDIX 6: WELL DISINFECTION

1. General

Well disinfection standards must take into account the following:

1. Prior inspection and removal of foreign matter from the well.

2. Scheduling of the disinfection times.

3. Contact time and amounts of disinfection agent to be used.

4. Procedures and equipment required for disinfecting.

All wells should be disinfected by the drilling contractor following completion. Existing Ministry of Health regulations require all new and repaired wells for water works systems to be disinfected before use according to the AWWA standards provided for such disinfection. Also existing wells for water works systems are to be chlorinated as deemed necessary by the Ministry of Health. The guidelines recommended here follow the AWWA recommendations and are given below.

2. Cleaning the Well

All oil, grease and scum and any other material which could harbour and protect bacteria from disinfectants should be thoroughly cleared from the well following completion of construction. The casing pipe should be thoroughly swabbed using alkalis if required, to remove oil, grease and joint dope, etc.

3. Disinfectants

Chlorine compounds are considered to be the most suitable disinfectants. The dry form of chlorine compounds should not have been stored for more than one year and the storage of liquid compounds not more than 60 days. During storage, compounds should not have been exposed to the atmosphere or to direct sunlight. A minimum concentration of 50 ppm free available chlorine is required in solution when mixed with the total volume of water in the well. See the table below for effective well disinfection.

4. Time of Disinfection

According to the AWWA standard the time for disinfecting a well is when construction is considered complete and the well is to be tested for yield. Where test pumping equipment is to be utilized, such equipment should be installed prior to or during disinfection and be thoroughly cleaned of foreign material. The well should however, be disinfected before the collection of any samples for determination of bacteriological quality. It is also desirable to disinfect the well after a permanent pump has been installed or when a pump is replaced after repair.

5. Disinfection of Non-flowing Wells

According to the AWWA standard the disinfectant should remain in the well for a period of at least 2 hours.* Following disinfection the well is to be pumped to clear out the disinfecting agent. To protect aquatic life or vegetation, the disposal point for the waste water from the well must be carefully selected.

Method 1: "Where practical, the chlorine solutions of 50 ppm standard concentrations used to disinfect the well should be prepared on the surface in containers having a volume equal to at least twice the volume of water contained in the well. This prepared solution should then be rapidly discharged into the well, care bring taken to flush the walls above the water level."

Method 2: A stock chlorine solution of 15,000 ppm (11/2% solution) is added to a continuous flow of water into the well to provide the standard 50 ppm standard chlorine concentration.

Method 3: A stock solution sufficient to produce a 50 ppm standard chlorine concentration in the well water is added to the well at different intervals from top to bottom and then agitated to distribute it evenly throughout the well.

Method 4: A perforated pipe container capped at both ends, containing a granular chlorine compound (Perchloron, etc.) is mixed up and down in the well by means of a weighted cable. The amount of compound applied should be such as to provide the standard concentration of 50 ppm available chlorine.

6. Disinfection of Flowing Artesian Wells

Flowing wells discharging at the surface generally require no disinfection, however a bacteriological analysis should be made. Should the well prove to be unsafe for use the following disinfection procedures can be used:

Dry chlorine compounds: A capped perforated container filled with a granular chlorine compound is placed at or below the top of the well screen. This process is to be repeated as often as necessary to achieve and maintain the standard 50 ppm concentrations for a period of not less than one hour.

Controlled flow disinfections: The well cap for this method is to be equipped with a suitable one inch valve. A stock chlorine solution is injected under pressure by means of a drop pipe to the bottom of the well. After injection is complete, air is injected for agitation while simultaneously the valve is opened in the cap permitting the chlorine solutions to be dispersed to the surface. The valve is then closed and the flow stopped for six hours while maintaining the chlorine solution at 50 ppm. An alternative procedure is to introduce a stock chlorine solution by a drop pipe or hose into the well screen. The rate of application to be such that a standard 50 ppm concentration is achieved and maintained during the application period.

Chlorine Compound Required to Treat 100 Ft. of Water-filled Well at 50 PM**

*EPA 570/9-75-001 however, stipulates a 12 hour period.

**EPA recommends a minimum concentration of 100 ppm available chlorine. To obtain this concentration DOUBLE the amounts indicated in the above table.

***Where a dry chemical is used, it should be mixed with water to form a chlorine solution prior to placing it into the well.

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