Shock Chlorination of Wells and Springs

A Common Problem

Bacterial contamination is one of the most common water quality problems in private water wells and springs. A recent survey of 450 private wells in Pennsylvania found that about 35 percent contained coliform bacteria. Past studies have shown that springs are even more susceptible to bacterial contamination. These bacteria are a potential problem because they may cause serious gastrointestinal illnesses. For more detailed information on coliform bacteria consult Coliform Bacteria in Drinking Water.

Homeowners often assume that a positive test for coliform bacteria in their well or spring indicates a contamination problem that must be treated continuously with a disinfection treatment system such as ultraviolet light or chlorination. However, some positive bacteria tests are the result of a less serious, one-time contamination incident. For example, bacteria may be introduced when a new submersible pump is installed in a well or from surface runoff during an unusually heavy rainfall. Shock chlorination is a simple and inexpensive process that can be used to disinfect water supplies that have been contaminated as a result of these one-time contamination incidents.

When done properly, shock chlorination will kill all the bacteria existing in a well. A recent Penn State study of wells contaminated by coliform bacteria found that shock chlorination and installation of a sanitary well cap successfully removed the bacteria for one year in 15 percent of the wells. This procedure was most successful in wells that had small numbers of coliform bacteria (fewer than 10 colonies per 100 mL) and no E. coli bacteria.

When to Shock Chlorinate Your Well or Spring

• After construction of a new well (many well drillers do this as a standard practice)
• After working on an existing well or installing a new submersible pump
• After receiving a positive water test report for coliform bacteria

How to Disinfect Your Well or Spring

For specific instructions on how to disinfect a well or spring, consult the Pennsylvania Department of Environmental Protection (DEP) publication Disinfection of Home Wells and Springs and the National Ground Water Association (NGWA) site on Water Well Disinfection. The NGWA site also has a contractor search engine to find a professional contractor who can disinfect your well or spring for you.

Some water treatment equipment can be damaged or exhausted by high chlorine concentrations in water. Contact your water treatment company or equipment manuals to determine if your equipment should be bypassed during shock chlorination.

Retesting Your Water

After disinfection of a well, you should retest your water for total coliform bacteria approximately 10 to 14 days later. If no coliform bacteria are present, wait an additional two to three months and have the water tested again. If the bacteria return in either of these subsequent tests, a continuous disinfection treatment system will be necessary.

Disinfection of Springs

Shock chlorination of springs is difficult and rarely successful because the water often runs through the spring box too quickly to provide adequate contact with the chlorine to kill bacteria. Disinfection of the spring box should not be attempted if the spring overflow (the water that does not enter the house) enters a stream, pond, or wetland area where high chlorine water may cause environmental damage especially a fish kill.

More Information

For more information on private water system management including continuous disinfection of bacteria problems, consult the Penn State Extension Water Quality website.

Prepared by Bryan R. Swistock, extension associate; Stephanie Clemens, research assistant; and William Sharpe, professor of forest hydrology.