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APPENDIX C SUGGESTED PROCEDURES FOR DISINFECTING WELLS Disinfection of all wells is recommended to eliminate pathogenic organisms as well as organisms that can grow in wells and thereby cause clogging and effect the quality of water produced. Disinfection of the well is the final act of well construction or repair before it is placed in service. Wells should also be disinfected following repair or replacement of the pump and/or well maintenance. The procedures described in this appendix are recommended for disinfecting wells; however, other methods may be used provided it can be demonstrated that they will yield comparable results. For new wells, disinfection should take place following development (this will assure that the well is purged of drilling mud, dirt and other debris that reduces the effectiveness of the disinfection), testing for yield, and installation of the pump. When there is a delay in pump installation, interim •or partial disinfection should be undertaken. Disinfection involves seven steps: 1. A chlorine solution containing at least 50 mg/1 (or parts per million) available chlorine, is added to the well. Table 6 lists quantities of various chlorine compounds required to dose 100 feet (30 metres) of water filled casing at 50 mg/l for diameters ranging from 2 to 24 inches (50 to 600 millimetres). For wells that have been repaired or when the pump has been repaired or replaced and; bringing the well back into service quickly is desired, the solution should contain at least 100 mg/1 available chlorine. To obtain this concentration, double the amounts shown in Table 6. 2. The pump column or drop pipe shall be washed with the chlorine solution as it is lowered into the well. 3. After it has been placed into position, the pump shall be turned on and off several times (i.e., "surged") so as to thoroughly mix the disinfectant with the water in the well. Pump until the water discharged has the odor of chlorine. Repeat this procedure several. times at one-hour intervals. 4. The well shall be allowed to stand without pumping for 24 hours.