Bacteria in well water: shock chlorination, then retest

If a well tests positive for coliform bacteria, shock chlorinate it and retest. Virginia Cooperative Extension targets about 200 ppm of chlorine, left in the plumbing 12 to 24 hours, then a retest one to two weeks later. For a problem that keeps coming back, a UV light costs $895–$995 before installation, per supplier Mid Atlantic Water.

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Key figures

Item	Figure
Shock chlorination target	200 ppm¹
Contact time	12–24 hours¹
Wait before retesting	1–2 weeks¹
Coliform retest at a public lab	$10–$40^3,4
UV light, equipment	$895–$995²

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How much bleach

Virginia Cooperative Extension's table uses 5.25 percent household bleach and depends on the depth of water in the well and the casing diameter.

Bleach for shock chlorination, 5.25 percent, Virginia Cooperative Extension
Water in the well	Casing	Bleach
50 ft	4-inch	2 quarts
100 ft	6-inch	4 quarts
200 ft	8-inch	2.5 gallons

For springs and cisterns, Extension uses 3 pints of bleach or 4 ounces of calcium hypochlorite per 100 gallons.

Sources¹

The procedure

Leave the water standing in the plumbing for 12 to 24 hours with no use or flushing. Then flush until the chlorine odor is gone. Retest for total and fecal coliform after one to two weeks.

Sources¹

If it comes back

A repeat positive means contamination is still getting in, so have the well itself checked before adding equipment. A UV light then disinfects at the point of entry; Mid Atlantic Water prices one at $895–$995 and says installation takes about an hour.

Sources²

Common questions

What is the limit for coliform?

For public systems, EPA's Total Coliform Rule lists 5.0 percent of samples. For a private well, any positive is a reason to disinfect and retest.

When should I test for bacteria?

Every year, per CDC and EPA, and after any repair to the well system or a flood.

For the cost of a finished well in your state, see cost to drill a well.

Sources

Shock Chlorination: Disinfecting Private Household Water Supply Systems (Pub. 442-663) , Virginia Cooperative Extension, Virginia Tech (Benham, Ling). Accessed November 9, 2026. December 16, 2024
Whole house water filter system cost: what you'll actually pay in 2026 , Mid Atlantic Water (online supplier). Accessed November 9, 2026. Prices dated October 2, 2026; updated November 5, 2026
Well Water Testing , Indiana Department of Health Laboratories. Accessed November 8, 2026. Undated. $10.00 for bacteria; $10.00 for nitrate.
Drinking Water Testing , Penn State Agricultural Analytical Services Laboratory. Accessed November 8, 2026. Undated (2026 site). Bacteria (total coliform and E. coli) $40; nitrate-nitrogen $15; WD01 standard package $60; WD04 standard plus nitrate $70.
National Primary Drinking Water Regulations , U.S. EPA. Accessed November 9, 2026. Last updated August 31, 2026
Guidelines for Testing Well Water , CDC. Accessed November 9, 2026. July 1, 2024
Protect Your Home's Water (Private Wells) , U.S. EPA. Accessed November 9, 2026. Last updated February 26, 2026

How these figures are built: methodology. Updated November 2026.