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Shock Chlorination: When and How to Disinfect Your Well

When to shock chlorinate a well, step-by-step process, safety precautions, and when to hire a professional for well disinfection.

Key Takeaways

  • Do after positive coliform/E. coli, well repair, flooding, or new construction
  • Use unscented bleach; calculate dose from well depth/diameter; bypass treatment systems
  • Circulate, sit 12–24 hr, flush, retest in 1–2 weeks
  • Hire C-57 if well is deep or you're unsure

Shock chlorination—adding a high dose of chlorine to the well and plumbing—is a standard way to disinfect a well after bacterial contamination or when coliform tests come back positive. Here's when and how to do it safely, and when to call a professional.

When to Shock Chlorinate

  • Positive coliform or E. coli test — Bacteria in the water may indicate surface water contamination or biofilm in the well
  • After well repair or deepening — Contractors often disinfect as part of the job; if not, you should
  • After flooding — Surface water may have entered the well
  • New well construction — Before first use, typically required
  • Recurring bacterial issues — Periodic disinfection may help if the source (e.g., faulty well cap) is also addressed

Step-by-Step Process (Overview)

  1. Test first — Confirm coliform or E. coli with a certified lab; know what you're treating
  2. Choose the right chlorine — Unscented liquid bleach (5.25% or 6% sodium hypochlorite) is common; avoid splashless or scented products
  3. Calculate the amount — Based on well depth, diameter, and water volume. County health departments or extension offices often provide charts
  4. Bypass treatment systems — Softeners, filters, and RO units can be damaged by high chlorine; bypass or turn off during treatment
  5. Add chlorine to the well — Pour the calculated amount into the well (or use a funnel down the casing)
  6. Circulate — Run water from each faucet until you smell chlorine, then return to the well to recirculate for 30–60 minutes
  7. Let it sit — Wait 12–24 hours; avoid using the water during this time
  8. Flush — Run all faucets until chlorine is gone (no smell); may take several hours
  9. Retest — Wait 1–2 weeks, then test again. Do not drink until test results confirm the water is safe

Safety Precautions

  • Ventilation — Chlorine gas can build up; use fans, open doors, avoid confined spaces
  • Skin and eyes — Wear gloves and eye protection; chlorine is corrosive
  • Pets and plants — Avoid flushing chlorinated water onto lawns or plants; it can harm them
  • Septic systems — Large amounts of chlorine can affect septic bacteria; flush to outdoor drains or spread flushing over time if possible

When to Hire a Professional

Consider hiring a C-57 well driller or water treatment professional when:

  • The well is deep or inaccessible — Proper circulation and dosing can be tricky
  • You're not comfortable with the process — Mistakes can damage equipment or leave unsafe chlorine levels
  • Bacterial contamination persists — The source may be a faulty well cap, cracked casing, or surface water intrusion; a professional can inspect and fix
  • After well repair or construction — Your contractor may include disinfection as part of the job

A licensed well driller can also inspect the well for contamination sources that shock chlorination alone won't fix.

After Shock Chlorination

  • Retest — Wait 1–2 weeks, then test for coliform and E. coli again
  • Do not drink until results confirm the water is safe
  • If contamination returns — The problem may be ongoing (e.g., poor well seal, surface contamination). A professional inspection is recommended

Find a Licensed Driller

For well inspection, repair, or professional disinfection, use our California Well Driller Directory to find C-57 licensed contractors in your area. For well water testing guidance, see our well water testing guide.