The intended purpose of shock chlorination is to reduce the levels of microorganisms that can cause illnesses (pathogens) present in water using a concentrated liquid chlorine solution. There are two places where this disinfection treatment method is typically applied: down the water well itself and/ or inside water storage tanks. The practical benefits of shock chlorination are different for each application, as are the risks.
The purpose of this publication is to make well owners aware of the benefits and potential problems associated with water well shock chlorination, including when and who should do it
A reason often cited for shock chlorinating a well is that it has tested positive for total coliform bacteria (commonly found throughout the environment), which is not a health threat in itself, and/or positive for fecal coliform bacteria, i.e. E. coli bacteria, which can indicate a health threat since they are more frequently associated with fecal contamination (USEPA, 2013). These tests indicate the presence or absence of harmless coliform bacteria and only imply the likely presence of potentially harmful organisms (pathogens) in the water.
Natural waters contain materials such as salts, metals, and nutrients that provide an ideal medium for many types of organisms including algae and bacteria to grow. Many organisms found in water sources are not necessary harmful when ingested and form part of a complex network of naturally occurring organisms. Unfortunately, human or animal wastes can quickly degrade natural waters including groundwater, adding pathogens such as intestinal bacteria, viruses, and even parasites. The U.S. Environmental Protection Agency (USEPA, 2013) considers that there is no safe level of any pathogen in drinking water.
Shock chlorination typically uses a strong bleach solution, which can kill most microorganisms it comes into contact. Bleach can also react with well components and naturally occurring chemicals found in groundwater aquifers with unpredictable results. For this reason, private well owners should use caution when attempting to disinfect their water well using common household bleach for shock chlorination.