WOH Well Disinfection Guide

Disinfect a well after new construction, pump work, casing or screen repairs, positive bacteriological samples, floods, surface water intrusion, or extended non-use (over 30 days). Some state programs also require periodic maintenance disinfection. Remember that shock chlorination kills microorganisms present in the well at the time of treatment but does not fix a contaminated aquifer or repair defective casing—if positive results recur, investigate the source of contamination with your state agency and a licensed well driller.

Chlorine is hazardous. Wear chemical-resistant gloves, safety glasses, and old clothing. Work in well-ventilated areas and never mix different chlorine products or combine chlorine with ammonia, acids, or other chemicals. Keep children and animals away during disinfection. Discharge flushed chlorinated water to a sanitary sewer or grassed area where it infiltrates soil—never to surface water.

Three chlorine sources work for well disinfection: household liquid bleach (5.25–8.25% available chlorine, best for small wells), granular calcium hypochlorite (65–70%, suits deep wells), and bulk sodium hypochlorite solution (10–12.5%, for large systems). Always use products labeled safe for potable water. The goal is to achieve at least 200 mg/L free chlorine throughout the well. Calculate well volume using the formula: gallons = (casing diameter in inches)² × 0.0408 × standing water depth in feet. The standing water depth is measured from the water surface to the bottom of the well or pump intake, not total well depth.