How to Decontaminate Your Water Well

Published:  January 12, 2017

Wells that have been inundated by floodwaters may pose a health risk.  Residents are advised not to use potentially contaminated well water for drinking, cooking or bathing until the water has been disinfected and tested and determined to be safe. Once the floodwaters have receded, the County recommends well disinfection using the following procedure: 

  1. Pump well water until water is relatively clear, and then stop the pump. 
    Warning:There is danger of electric shock and damage to the well or pump if they have been flooded.
  2. Add regular unscented household chlorine bleach (5.25%) to the well according to the following chart for each 100 feet of well depth:
Well Casing Diameter Amount of 5.25% (Household) Chlorine Bleach Needed
4 inches Two and one-half (2½) cups
6 inches Five (5) cups
8 inches Seven (7) cups
12 inches Twenty (20) cups OR One and one-quarter (1¼) gallons
  1. Open all faucets until water smells of chlorine at the tap farthest from the well. If there is no chlorine odor, add more chlorine bleach to the well until there is a chlorine odor.
  2. Close faucets and do not use any water from the system for 24 hours. Adequate chlorine contact time with bacteria is important for effectiveness of disinfection.
  3. After 24 hours, attach a garden hose to an outside tap and run water outside until the chlorine odor disappears. Chlorine can damage septic systems and may injure grass or plants, so select the water discharge method and area carefully.
  4. Have water tested for presence/absence of bacteria by a certified laboratory.

CAUTION: Wastewater from malfunctioning septic tanks or chemicals seeping into the ground can contaminate groundwater long after the flood. Test water quality regularly to ensure a safe water supply.

For more information go to http://sonomacounty.ca.gov/Health/Environmental-Health-and-Safety/, or contact Environmental Health at (707) 565-6565.

Laboratories Certified for Bacteriological and/or Chemical Analysis

Brelje and Race Laboratories, Inc
425 South E Street          
Santa Rosa, CA 95404
Telephone (707) 544-8807
(bacteriological and chemical testing)

CalTest Laboratory
1885 North Kelly Road
Napa, CA 94558
Telephone (707) 258-4000
(bacteriological and chemical testing)

Far West Laboratories Inc.
P O Box 355
Riverbank, CA 95367
Telephone (800) 750-9260
(209) 869-9260
(bacteriological and chemical testing)

Sonoma County Public Health Regional Laboratory
3313 Chanate Road
Santa Rosa, CA 95404
Telephone (707) 565-4711
(bacteriological testing)

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County of Sonoma
Business Hours
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8:00 AM – 4:00 PM
Office Location
625 5th Street
Santa Rosa, CA 95404
38.441823, -122.7137698

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