Find where contamination enters a water system by walking it from source to tap while testing at progressively closer points: the entry lies between the last clean sample and the first contaminated one. With 16 stretches of pipe, 15 junction samples tested together find the stretch in a single 24-48 hour round, while testing one point at a time by halving takes 4 rounds, so the choice trades vials against days, and the chlorine residual and turbidity, which read at once, narrow the search before any bacteria result arrives.
Part of Community Water Testing
Source investigation is the systematic search for the point at which contamination enters a water supply, combining a walk-through survey of the system with sampling at chosen points along the flow path.
Contamination Enters At A Point, And The Pattern Of Results Shows Which
The community water testing page gives the method in outline: walk the entire system from source to taps looking for obvious contamination points; check for animal access to the source, broken spring box seals and cracked pipe joints; check uphill of the source for new latrines, animal pens or disturbed soil; and test at progressively closer points to the source to isolate where contamination enters.
Water moves in one direction. Anything contaminated at a point is contaminated everywhere downstream, unless treatment or dilution cleans it. So along a single flow path the results should be clean up to the entry and contaminated after it. The entry is at the boundary.
What You Need
- A map of the system showing every junction, valve, tank and tap
- Test kits and vials for the bacteria test, with controls (H2S Test Kit)
- A chlorine test kit and a turbidity tube, which read at once
- A person who knows the system and one who keeps the log
- Gloves and boots
Method: Survey, Then Bracket
Step 1: Keep the evidence and make the water safe first (Immediate Actions).
Step 2: Survey the source and its surroundings. Use the protections set for a spring (Spring Capture) as the checklist: a fence enclosing at least a 10 m radius, no livestock inside it, no latrines or waste disposal uphill within 100 m, a diversion ditch uphill to intercept runoff, and a sound lid and seals.
Step 3: Survey the tank: lid, vent screen, overflow, cracks, animal access, sludge.
Step 4: Survey the pipes and taps: leaks and wet spots, which can let dirty water in whenever the pressure falls; cross-connections to troughs, irrigation or hoses; damaged taps.
Step 5: Read the instant tests along the path. Turbidity and chlorine residual from source to tap show where something changes.
Step 6: Choose the bacteria sampling strategy (below) and run it with controls.
Step 7: Confirm the finding by fixing the cause and showing a clean result downstream, with the tests after the fix.
Table: Common Causes And Their Evidence
| Cause | Evidence | Where it shows |
|---|---|---|
| Animal or dead body at the source | Tracks, bones, smell, a positive at the source | Source and everywhere downstream |
| Latrine, pen or disturbed soil uphill | Positive after rain; visible runoff path | Source, worse after rain |
| Broken spring box seal or lid | Runoff or insects in the box | Source |
| Tank lid, vent or overflow open | Birds, insects, dirt in the tank | Tank outlet |
| Cracked pipe or leaking joint | Wet ground; positive after pressure loss | Downstream of the crack, intermittently |
| Cross-connection | Hose, trough or irrigation pipe joined to the drinking line | Local, appears when pressure drops |
| Dirty tap or container | Positive at one tap only | That tap |
Worked Example: Searching Sixteen Stretches
Suppose the system from tank to the farthest tap has 16 stretches separated by 15 junctions, and a tank-outlet sample was clean while the farthest tap was positive.
- All at once: take 15 junction samples together. Vials: 15, plus a blank and a positive control, 17. Time: one round, 24-48 hours. The first positive in the flow direction marks the entry stretch.
- By halving: test the middle junction. If it is clean, the entry lies in the downstream half (8 stretches); if positive, upstream. Test the middle again: 4, then 2, then 1 stretch. That is 4 rounds, because 2⁴ = 16. Vials: 4 plus controls. Time: 4 × 24 = 96 hours at the earliest, and up to 4 × 48 = 192 hours if each round waits for the 48 hour read.
So the parallel method uses about four times the vials and finishes in about a quarter of the time or less. When contamination is present and people are drinking, time costs more than vials, and the choice is the parallel test. If vials are scarce, narrow the search first with the readings that take minutes.
Narrowing with chlorine. Chlorine residual is read in minutes. If the tank outlet reads 0.6 mg/L, the first four taps read 0.5, 0.4, 0.4 and 0.3, and the fifth reads 0.0, then something at or before the fifth tap used up the chlorine: contaminated water entering a leak, a cross-connection, or a dirty branch. Start there. The turbidity tube can do the same for cloudy inflow.
Reading An Untidy Pattern
Real results are untidy. Contamination can be intermittent, so a stretch that is clean today may be positive tomorrow. There can be more than one entry. A chlorinated stretch can look clean below a contaminated one. A positive result can be a dirty sample bottle. Repeat any result that does not fit. If no entry is found, say so plainly, keep the advisory, increase the testing and re-survey after the next rain.
Common Mistakes
- Cleaning or flushing before sampling. The trail is erased.
- Looking only at the source. Tanks, pipes and taps are common entry points.
- Stopping at the first cause found. There may be a second.
- Calling it fixed without retesting downstream.
- Ignoring pressure loss. A leak that leaks out under pressure can leak in when pressure drops.
Safety
The search can put people in danger as well as find the cause
Do not touch or open an animal that died suddenly of unknown cause; some diseases of livestock can infect people through the carcass, so use gloves, keep children away and bury or burn it as the community’s health lead advises. Do not enter flooded ground, tanks, wells or pits without ventilation, a harness on a line held by a person outside, and a 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 repair.
What’s Next
- Learn next: Spring Capture — the protections that stop entry at the source
- Combine with: Leak Detection — the pipe-walking half of the search