Well Chlorination: The Complete Guide
for Maryland Homeowners
When to shock your well, how the process works, what chlorination treats (and what it doesn't), and when to call a licensed professional. Written by Terry Foster, MD Master Plumber & Gas Fitter #98433.
What Is Well Chlorination (Shock Treatment)?
Well chlorination — also called shock chlorination or well disinfection — is the process of introducing a high concentration of chlorine solution directly into the well to kill bacteria, viruses, and other microorganisms present in the well casing, water column, and connected plumbing.
The key distinction: well chlorination is a one-time or periodic disinfection procedure, not a permanent treatment for ongoing bacterial contamination. It cleans the well and system at a point in time. If the source of contamination is not identified and addressed, bacteria will return.
Think of it like cleaning a wound — chlorination disinfects the existing contamination, but it doesn't prevent future exposure. For homes with recurring bacterial issues, a UV disinfection system installed after chlorination provides continuous protection at the point of use.
Important: Chlorination disinfects the well and water system. It does not remove dissolved minerals, iron, hardness, or chemical contaminants. A water test is always the first step to determine what treatment is actually needed.
When to Chlorinate Your Well
Several situations call for well chlorination. Some are reactive (responding to a known contamination event), others are precautionary (reducing risk after work or extended non-use).
After Flooding
Any flooding event that may have introduced surface water into the well casing or surrounding soil. Surface water carries coliform bacteria, E. coli, and other pathogens.
After Well Work or Pump Replacement
Any time the well is opened for pump replacement, casing repair, or other work, the well should be chlorinated before returning to service.
Positive Bacteria Test
A positive result for total coliform or E. coli requires immediate chlorination followed by a confirmation test 2 weeks later.
New Home Purchase
Always test and chlorinate a well when purchasing a home. Previous owners may not have maintained the well or addressed contamination issues.
Sulfur or Musty Odor
A sulfur or musty smell that appears suddenly — especially after rain or flooding — may indicate bacterial activity in the well or casing.
Iron Bacteria Infestation
Orange or rust-colored slime in toilet tanks, fixtures, or the well casing indicates iron bacteria — a biofilm-forming organism that requires chlorination of the entire system.
After Extended Non-Use
Wells that have been unused for months (vacation homes, seasonal properties) should be chlorinated before resuming regular use.
What Chlorination Treats — and What It Doesn't
What Chlorination Treats
- Coliform bacteria (total coliform)
- E. coli and other fecal bacteria
- Sulfur bacteria (causing rotten egg odor)
- Iron bacteria (biofilm in well casing and pipes)
- Viruses and protozoa in the water column
- Musty or earthy odors from biological activity
What Chlorination Does NOT Treat
- Dissolved hydrogen sulfide (H₂S) from the aquifer — geological source
- Chemical contaminants (nitrates, arsenic, VOCs)
- Iron in solution (dissolved ferrous iron)
- Hardness (calcium and magnesium)
- Sediment or turbidity
- Recurring contamination if the source is not addressed
The Chlorination Process — Step by Step
Professional well chlorination follows a precise sequence. Each step matters — skipping or rushing any phase reduces effectiveness or risks equipment damage.
Calculate Chlorine Dose
Based on the well's depth, diameter, and water volume, we calculate the exact amount of chlorine needed to achieve 50–200 ppm concentration at the well head. Household bleach (5.25–8.25% sodium hypochlorite) is the standard disinfectant.
Mix and Introduce Chlorine Solution
The chlorine solution is mixed and poured directly into the well casing. For deep wells, a hose is used to direct the solution to the bottom of the well, ensuring the entire water column is treated.
Circulate Through the System
The pump is run to circulate the chlorinated water through all household plumbing, fixtures, and water treatment equipment. Each faucet and fixture is opened until chlorine odor is detected, then closed.
Contact Time (12–24 Hours)
The chlorinated water sits in the well and plumbing for a minimum of 12 hours — 24 hours is preferred for iron bacteria or heavy contamination. During this time, the water is not used.
Flush the System
After contact time, the chlorine is flushed from the system by running water to an outdoor location (not a septic system — high chlorine concentrations kill septic bacteria). Flushing continues until chlorine odor is no longer detectable.
Post-Treatment Water Test
A water sample is collected 2 weeks after chlorination for certified lab analysis. This confirms successful disinfection and that chlorine has fully flushed from the system before the results are interpreted.
Chlorine Concentration — Getting It Right
Chlorine concentration is the most critical variable in shock chlorination. The target range is 50–200 ppm at the well head. Outside this range, the treatment either fails or causes damage.
Too Little (<50 ppm)
- Insufficient to kill all bacteria
- Biofilm in iron bacteria cases survives
- False sense of security — well appears treated but isn't
- Positive test result at 2-week follow-up
Correct (50–200 ppm)
- Effective disinfection of bacteria and viruses
- Penetrates biofilm in iron bacteria cases
- Minimizes equipment exposure risk
- Flushes cleanly within 2–6 hours
Too Much (>200 ppm)
- Degrades rubber pump seals and O-rings
- Damages check valves and pressure tank bladder
- Attacks water softener resin
- Difficult to flush — extended downtime
After Chlorination — What to Expect
After flushing, most homeowners can resume normal water use. However, there are a few things to expect in the days and weeks following treatment:
Chlorine Smell for 1–3 Days
A faint chlorine odor in the water is normal for 1–3 days after flushing. This is residual chlorine in the pipes and is not harmful at these low concentrations. Run cold water for 30–60 seconds before use if the odor is noticeable.
Follow-Up Water Test at 2 Weeks
Do not skip the follow-up test. Testing too soon (within a few days) may show residual chlorine masking a positive bacteria result. Two weeks gives the chlorine time to fully flush and provides an accurate picture of the well's bacterial status.
Possible Recurrence if Source Not Addressed
If the source of contamination — a cracked well casing, nearby septic issue, surface water intrusion, or iron bacteria biofilm in treatment equipment — is not identified and corrected, bacteria will return. Chlorination treats the symptom; finding and fixing the source is the permanent solution.
Temporary Discoloration
In wells with iron bacteria, chlorination may temporarily dislodge rust-colored biofilm, causing brief discoloration in the water. This clears within a few hours of flushing and is a sign the treatment is working.
Iron Bacteria — Why Chlorination Alone May Not Be Enough
Iron bacteria are a group of naturally occurring organisms that oxidize dissolved iron in well water, forming a rust-colored, slimy biofilm in the well casing, pipes, fixtures, and water treatment equipment. If you see orange or rust-colored slime in your toilet tank, you likely have iron bacteria.
Iron bacteria are notoriously difficult to eliminate because they form a protective biofilm that chlorine has difficulty penetrating at standard concentrations. A single chlorination treatment may reduce the population significantly, but complete elimination often requires:
Higher chlorine concentration (100–200 ppm) with extended contact time (24 hours)
Physical cleaning of the well casing and pump components before chlorination
Chlorination of all water treatment equipment — iron filters, softeners, UV housings
Repeat chlorination 30 days later to catch any surviving organisms
UV system installation after chlorination to prevent recolonization at the tap
Iron bacteria and dissolved iron are different problems. Iron bacteria is a biological organism that forms biofilm. Dissolved iron is a chemical contaminant removed by an iron filter. Many Southern Maryland wells have both — and both require separate treatment approaches.
Professional vs. DIY Chlorination
When DIY Is Appropriate
- Shallow well (less than 100 feet) with accessible well head
- Precautionary chlorination after minor well work
- Routine annual disinfection with no known contamination
- Homeowner is comfortable calculating chlorine dose by well volume
- No iron bacteria — simple bacterial disinfection only
When to Call a Professional
- Deep wells (200+ feet) — chlorine dose calculation is critical
- Positive E. coli result — health risk requires confirmed resolution
- Iron bacteria infestation — requires system-wide treatment protocol
- Post-flood contamination — source identification is essential
- Unknown source of contamination — testing must precede treatment
- Previous DIY chlorination failed — recurrence indicates a deeper issue
The Deep Dive Approach to Well Chlorination
We don't just pour chlorine in the well and leave. Our chlorination process is systematic, documented, and followed up with lab-confirmed results.
Pre-Treatment Assessment
We review your water test results, well log, and system configuration before determining the correct chlorine dose and treatment protocol. Iron bacteria cases get a different protocol than simple bacterial contamination.
System-Wide Treatment
We chlorinate the well, all household plumbing, and every piece of water treatment equipment. A well that's been chlorinated but has untreated iron bacteria in the softener will recontaminate within days.
Equipment Protection
We use the minimum effective concentration and flush promptly to protect pump seals, pressure tank bladders, and softener resin. For iron bacteria cases, we assess pump condition before and after treatment.
Post-Treatment Testing
We collect a water sample 2 weeks after treatment and send it to a certified Maryland lab. You receive a written report with the results and our follow-up recommendations.
Follow-Up Recommendations
If the follow-up test is positive, we identify the source of recontamination and recommend the appropriate fix — whether that's a UV system, well casing repair, or repeat chlorination with a modified protocol.
UV Installation When Indicated
For homes with recurring bacterial contamination or iron bacteria, we recommend UV disinfection after chlorination. UV provides continuous protection at the point of use — the last line of defense before the tap.
Well Chlorination FAQ
What is well shock chlorination?
When should I chlorinate my well?
Will chlorination fix my sulfur smell?
How long does well chlorination take?
Is it safe to use my water during chlorination?
How much chlorine do I use to shock a well?
Will chlorination damage my well pump?
How do I know if my well chlorination worked?
Can I chlorinate my well myself?
What is the difference between well chlorination and a UV system?
Need Your Well Chlorinated?
Deep Dive Plumbing Maryland performs professional well chlorination throughout Calvert County and Anne Arundel County. We test first, treat correctly, and confirm results with a certified lab follow-up.