Well and Cistern Disinfection

What are coliform bacteria?

Coliform bacteria occur naturally in the intestinal tracts of warm-blooded animals and are found throughout the natural environment. They are typically harmless, but their presence in water may signal possible fecal contamination (human or animal wastes) and hence, the potential of pathogens (disease-causing organisms) in the form of fecal coliform or E-coli. If fecal contamination of your water has occurred, your water supply must be disinfected to reduce the risk of illness.

When and Why?

Construction or maintenance work can temporarily contaminate a well or cistern. All modified wells or cisterns should be assumed to be contaminated by microorganisms. Before use, each well or cistern must be thoroughly cleaned and disinfected to ensure all pathogens have been eliminated.

Care should be taken to ensure that all areas of the well or cistern come into contact with a solution containing enough available chlorine (disinfectant) to completely destroy all harmful bacteria. Household laundry bleach containing sodium hypochlorite, such as Clorox or Purex will work fine for this use.

How much do I use?

To shock chlorinate a well or cistern, a disinfecting solution of bleach/water must have a concentration of approximately 100 parts per million (ppm) or Mg/L. This equates to about 2 cups household bleach (5 ¼ %) to every 100 gallons of water in the well or cistern.

To calculate how much water is in your well, use the table and formula below:

Cups of bleach(2) = gallons per foot x depth of water in well* per 100 gallons

*Depth of water = depth of well minus the static water level
 

Add the bleach to a couple gallons of water and pour into the well through a vent plug in the top of the casing.
 

After adding the bleach, turn on each faucet in the distribution system (outside hose faucets also) until you can detect a chlorine odor. Then, turn everything off and do not use any water for 12 hours. After 12 hours, turn fixtures on again until no chlorine odor persists. (try to flush most of the water to the outside of the house and NOT into the septic tank)
 

The steps involved in disinfecting a cistern are the same, but use the following formula to calculate volume of your cistern:

1. Volume in gallons = Length x Width x Height x 7.5

2. Then divide this number by 100 to get # of cups need to disinfect.
   *To verify that your water source is free of Coliform bacteria & for your own assurance, Deschutes County recommends having the water sampled by a certified laboratory. Deschutes County does not sample or analyze water.