Test the water immediately, and do not wait for the monthly date, after heavy rain or flooding, an illness outbreak (especially diarrhoea), any repair work on the system, a dead animal found near the source, a visible change in colour, smell or taste, or an earthquake or landslide near the source. Because the bacteria test takes 24-48 hours to read, treat the water as suspect until it clears, and rely meanwhile on the tests that read at once, turbidity and chlorine residual.
Part of Community Water Testing
Event-triggered testing is the unscheduled sampling of a water supply immediately after an event that may have contaminated it, as distinct from the routine tests on the calendar.
An Event Can Contaminate Water Between Two Scheduled Tests
The community water testing page lists six occasions to test at once: heavy rainfall or flooding; an illness outbreak in the community, especially diarrhoea; any repair work on the system; a dead animal found near the source; a visible change in water colour, smell or taste; and an earthquake or landslide near the source. It adds that after changing the chlorine dose or switching filters the test comes within 24 hours.
What each event does to the water, and what to check first:
| Event | What may have happened | Check first |
|---|---|---|
| Heavy rain or flood | Runoff, sewage and animal waste washed in; intake or tank flooded | Turbidity at source and tank; residual; bacteria; look at the intake and covers |
| Diarrhoea outbreak | The water may be the cause, or the cause may be elsewhere | Bacteria and residual at several points; look for a breach; tell the health lead (Disease Surveillance) |
| Repair work | Soil and dirty tools in the open pipe; pressure loss drew dirt in | Flush, then residual, turbidity and bacteria at the repaired section (Leak Detection) |
| Dead animal near source | Decaying body upstream or in the catchment | Locate and remove it; bacteria at source; consider the source unsafe until tested |
| Visible change | Something has entered or failed | Find where: source, tank, branch (Smell and Taste) |
| Earthquake or landslide | Cracked box, pipe or tank; sediment | Inspect the structures; turbidity; bacteria |
| Treatment change | New dose or filter performing differently | Residual at once; bacteria within 24 hours |
What You Need
- A standing test kit, ready to use, with vials in date
- A named person on call and a deputy
- A written event list posted where the committee meets
- A plan for what the community does while results are pending
Method: Sample First, Then Act
Step 1: Take samples before you change anything. Flushing, chlorinating or cleaning the tank first destroys the evidence of what the water was like.
Step 2: Measure what reads at once: turbidity with the tube and the chlorine residual at the tank and at a far tap.
Step 3: Set up the bacteria vials at the source, the tank outlet and the affected points, with controls (H2S Test Kit).
Step 4: Decide the interim precaution. Advise the community to boil drinking water, or confirm a good residual, until the 48 hour reading. A committee should decide in advance whether an outbreak or a flood triggers an advisory at once, so that it need not argue while people drink.
Step 5: Look for the cause in parallel (Source Investigation).
Step 6: Log everything, including events where nothing was found.
Worked Example: The Clock After A Flood
Heavy rain begins on Monday at 06:00. Samples are taken at 08:00 and vials set up at 09:00.
- Turbidity and residual: read at 08:30, within half an hour. Both can be acted on the same morning.
- Bacteria, first read at 24 hours: Tuesday 09:00.
- Bacteria, second read at 48 hours: Wednesday 09:00.
- From the start of the rain to the final read: Monday 06:00 to Wednesday 09:00 is 51 hours.
For that whole time the supply is a suspect. If the turbidity is bad and the residual is low at 08:30 on Monday, there is no reason to wait for Tuesday to advise boiling. If both look good, a black vial on Tuesday still means action then. A clear vial on Wednesday makes the water more credible, not certain.
After a flood a tank, spring box or well that has been under water should be treated as contaminated until cleaned and disinfected. The disinfection of flooded structures uses a much stronger chlorine solution than the routine dose, held for hours; follow a written procedure for it and do not improvise from the routine dose here.
What A Clear Result After An Event Does And Does Not License
It says the sample showed no faecal bacteria at that moment. It cannot see viruses, parasites or chemicals, it may miss contamination at low levels, and contamination from a flood or a body may arrive in pulses after the sample was taken.
Common Mistakes
- Waiting for the calendar. The event is the reason to test.
- Cleaning before sampling. The evidence is gone.
- Assuming the water caused the outbreak, or assuming it did not. Test, and tell the health lead.
- Lifting the precaution at 24 hours because one vial is clear.
- Using a routine dose as the cure for a flooded tank.
Safety
Floods and slides bring physical danger as well as germs
Do not inspect a flooded site until it is safe to approach, and do not enter a flooded or cracked tank, well or pit without ventilation, a harness on a line held by a person outside, and the rule that the person outside never goes in after a collapsed worker. Do not test the air with a flame. Never mix chlorine with acids, vinegar or ammonia products when disinfecting after a flood.
What’s Next
- Learn next: Responding to Failures — what to do when a test comes back bad
- Combine with: Immediate Actions — protecting the community while you find out