Module 6 — Shock Chlorination
Shock Chlorination
Shock chlorination is a relatively inexpensive and straightforward procedure used to
control bacteria in water wells. Many types of bacteria can contaminate wells, but the most
common are iron-related (IRB) and sulfate-reducing bacteria (SRB). Although not a cause of
health problems in humans, these nuisance bacteria may coat the inside of the well casing,
water piping and pumping equipment, creating problems such as:
Reduced well yield
Restricted water flow in distribution lines
Staining of plumbing fixtures and laundry
Plugging of water treatment equipment
Rotten egg” odour.
Increased corrosion of the metal parts of the well and distribution system
Wells can also be contaminated with harmful bacteria such as fecal coliforms or
E. coli. Shock chlorination is the most effective method to eliminate them. Bacteria may
be introduced during drilling of a well or when pumps are removed for repair and laid
on the ground. However, IRBs and SRBs (as well as other bacteria) can exist naturally
in groundwater.
A well creates a direct path for oxygen to travel into the ground where it would not
normally exist accelerating the growth of bacteria that require oxygen. When a well is
pumped, the water flowing in will also bring in nutrients that enhance bacterial growth.
Note: All iron staining problems are not necessarily caused by iron-related bacteria.
The iron naturally present in the water can be the cause. See Module 12 "Other
Resources" for more information.
For more information refer to
the Water Wells That Last video (Part III —
Shock Chlorination).
Water wells provide ideal conditions for
IRBs. To thrive, IRBs require 0.5-4 mg/L
of dissolved oxygen, as little as 0.01 mg/L
dissolved iron and a temperature range of 5
to 15°C. Some IRBs use dissolved iron in the
water as a food source.
Well maintenance is essential to ensure that
a well will last.