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Here we describe the significance of different levels of bacteria found in water during a well test. This series of articles explains many common water contamination tests for bacteria and other contaminants in water samples. We describe what to do about contaminated water, listing common corrective measures when water test results are unsatisfactory. We include water testing and water correction measures warnings for home owners and especially for home buyers when certain conditions are encountered, with advice about what to do when these circumstances are encountered. If a well potability test "fails" and the well is considered contaminated with bacteria, be sure you have also reviewed Water Test Procedure Errors.

© 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Interpreting and Acting On the Significance of the Level of Bacteria Found in a Water Test

As we introduced at Water Test Procedure Errors If the source of the contamination is not due to an ongoing situation or a persistent source of well contamination, for example bad ground water source, and assuming that the well has not already been recently disinfected or "shocked" then a disinfection of the water system will solve the problem. See WELL CHLORINATION SHOCKING PROCEDURE.

Our photo (left) shows how easily a well might be contaminated by a simple error such as a loose well casing top or the placement of well piping on the ground during well repairs.

If we determine that the source of well water contamination is persistent (for example we've recently disinfected the well and that did not cure the problem) then water treatment may be the best and fastest "cure" for this problem.

Understanding the level of bacterial contamination in a well can help us interpret the meaning of a well water bacteria or potability test. So what was the level of well water contamination detected? This question is explored here.

Comparing Well Bacteria Test Types & Understanding What the Results Mean

We prefer the MFT/MPN test to the P/A test because getting an actual count or "number" of CFU/100ML is diagnostic (as we describe here) whereas a P/A test simply says OK or NOT OK with no indication of the level of contamination present.

For example, consider two wells with a persistent source of bacterial contamination. If a well that fails at 15 CFU/100mL is "shocked" the low starting count means it's easier to cover up the persistent problem source and more time may be needed for the bacteria to reappear. This would affect your follow-up testing strategy.

By contrast, if a second well with a persistent source of bacterial contamination fails at >10,000 CFU/ml, first it is very unlikely that any "well shock" treatment will be effective, and second, the level of contamination is so great that if a seller came up with a subsequent "pass" on a bacteria test without installing purification equipment or without telling me what problem was found and fixed, we would be very skeptical about the reliability of the follow-up test.

So while a property seller/owner may prefer to "shock" a well and re-test, that procedure, used alone, might or might not be acceptable. What to do after a seller/owner has "done something" to the water supply and re-testing has been done is discussed below.

When and How to Shock or Chlorinate a Well - Procedure for Shocking a Well to (temporarily or maybe longer) "Correct" Bacterial Contamination

At WELL CHLORINATION SHOCKING PROCEDURE we provide a description of a common procedure used to sterilize well water and water equipment. The purpose of shock disinfection of a well system is to destroy bacterial contamination present in the well system at the time of disinfection and is not intended to kill bacteria that might be introduced at a later time.

Therefore it is vital that the well be constructed so that no new contamination may enter the well following completion of the shock disinfection. In order to achieve a satisfactory disinfection of the system, the bacteria must be brought in contact with a chlorine solution of sufficient strength and remain in contact with that solution for a sufficient time to achieve a complete kill of all bacteria and other microorganisms.

If you are going to have a well disinfected after a failed bacteria or water potability test, follow the well shocking procedures and warnings that we provide at

WELL CHLORINATION SHOCKING PROCEDURE
  Chlorine in Wells - Safety Warnings
  Well Chlorination Procedure Details

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

WATER TEST INTERPRETATION
Common Water Tests for Bacteria
  PA - Coliform Bacteria Test
  M.F.T. - Coliform Bacteria Test
  MPN - Coliform Bacteria Test
  Interpreting Other Water Test Results
FAILED WATER TESTS - WHAT TO DO
  Water Test Procedure Errors
  Detecting Water Test Cheating
  Interpreting the Level of Bacteria
  WELL CHLORINATION SHOCKING PROCEDURE
FAILED WATER TESTS - WHEN to RE-TEST
  Wells that Pass a Second Water Test
  Wells that Fail a Second Water Test
  When to re-test a well
ODORS in WATER - TYPES, CAUSES
WATER TREATMENT EQUIPMENT CHOICES
  FILTERS for WATER
  FILTERS - SEDIMENT & IRON
  FILTERS - SULPHUR ODOR
  CHLORINATORS & CHARCOAL FILTERS
  REVERSE OSMOSIS WATER TREATMENT
  REVERSE OSMOSIS CONCENTRATE DISPOSAL
  UV -ULTRAVIOLET LIGHT TREATMENT
  WATER SOFTENERS
  WATER TREATMENT CHEMICAL SAFETY
EPA GUIDE to WATER QUALITY
FHA WATER TESTS REQUIRED
LEAD in WATER, ACTION GUIDE
ODORS IN WATER
SEWAGE CONTAMINATION
WATER CONTAMINANT LEVELS
WATER TEST INTERPRETATION
WATER TESTING GUIDE
WATER ODORS, CAUSE CURE
WATER PUMPS & WELLS
WATER QUALITY TESTS, CONTAMINANTS, TREATMENT
WATER QUANTITY IMPROVEMENT
WATER QUANTITY USAGE GUIDE
WATER SOFTENERS
WELL SHOCKING GUIDE
WELLS CISTERNS & SPRINGS
WELL CHLORINATION SHOCKING PROCEDURE
  Chlorine in Wells - Safety Warnings
  Well Chlorination Procedure Details

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• Drinking Water Testing Advice for home buyers home owners home inspectors
• Interpreting Drinking Water Test Results and Correcting Unsatisfactory Water
• Drinking Water Contaminant Levels - Maximum Allowed
• Drinking Water Contamination and Pollution How to Detect, Test, Correct, and Prevent Well Water Contamination - US EPA - Groundwater
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• Water testing for Pesticides: comprehensive and pesticides-example parameters
• Water Testing: background comments on classes of water contaminants, & links to home buyer advice about water testing, drinking water, water supply
• When and How to Shock or Chlorinate a Well - Procedure for Shocking a Well to (temporarily or maybe longer) "Correct" Bacterial Contamination
• Water Requirements, Home & Outdoor Living
• Typical Water Tests & Fees this water test fee schedule applies when testing is combined with other onsite building inspection services
• Drinking Water testing, contamination, correction - all topics
• General water testing and corrective measure advice: contact your local health department.

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