17.2 Distribution Sampling Programs & Field Analyses
Key Takeaways
- A written sample siting plan identifies representative routine sampling locations and the repeat sample locations upstream and downstream of each.
- Under the Revised Total Coliform Rule a total coliform positive triggers three repeat samples within 24 hours: one at the original site and one each within five service connections upstream and downstream.
- Bacteriological sample bottles contain sodium thiosulfate to neutralize chlorine residual so that disinfection does not continue inside the bottle.
- DPD is the standard field method for chlorine residual, and free chlorine is read immediately while total chlorine requires the full reaction time.
- Sampling taps must be smooth-nosed, free of aerators, leaks, and hoses, and flushed until the line is cleared before a bacteriological sample is drawn.
17.2 Distribution Sampling Programs & Field Analyses
Distribution sampling produces the data that determines compliance. A technically perfect analysis of an improperly collected sample is worthless, so technique matters as much as the instrument.
The Sample Siting Plan
Every system must have a written sample siting plan identifying:
- Routine sampling locations, distributed to be representative of the system throughout — not clustered at the plant or at convenient office locations
- Repeat sampling locations for each routine site: one within five service connections upstream and one within five service connections downstream
- The sampling frequency, which is based on population served
- The rotation schedule if sites rotate
[!IMPORTANT] "Representative" is a regulatory term with teeth. Sampling only where the water is best — near the plant, on large mains with high turnover — misrepresents the system and is a compliance finding on sanitary surveys. A defensible plan includes extremities, dead ends, areas served by storage, and each pressure zone.
Revised Total Coliform Rule Sampling
The Revised Total Coliform Rule (RTCR) replaced the older acute-and-monthly MCL structure with a find-and-fix framework.
| Element | Requirement |
|---|---|
| Routine samples | Per the siting plan, at a frequency set by population |
| Total coliform positive | Triggers three repeat samples within 24 hours: at the original site, within five connections upstream, and within five connections downstream |
| E. coli positive | An E. coli MCL violation if confirmed; triggers Tier 1 public notification within 24 hours |
| Level 1 assessment | Triggered by exceeding the coliform trigger (for example, more than 5.0 percent of samples positive in a month for systems taking 40 or more samples) |
| Level 2 assessment | Triggered by an E. coli MCL violation or a second Level 1 trigger within a rolling 12 months |
| Seasonal systems | Must complete start-up procedures before serving water |
Total coliform is an indicator, not a pathogen. Its presence means a pathway may exist — a cross-connection, loss of pressure, a repair, an unprotected storage vent — and the assessment requirement is the rule's way of forcing the utility to find the sanitary defect and correct it rather than simply resample until a clean result appears.
Bacteriological Sampling Technique
- Select an approved smooth-nosed tap on a line in active service. Not a hose bibb with a hose attached, not a leaking faucet, not a swivel or mixing faucet, and not a tap with an aerator or screen unless it is removed.
- Remove aerators and screens. Some protocols call for disinfecting the tap; where used, flame or a hypochlorite wipe is applied, but never on a plastic fixture that could melt.
- Flush at moderate flow until the service line and main are cleared — commonly 2 to 5 minutes — and until temperature stabilizes.
- Reduce to a gentle, non-splashing stream.
- Use a sterile bottle containing sodium thiosulfate. Do not rinse it — rinsing removes the thiosulfate.
- Remove the cap without touching the inside of the cap or the bottle neck, and do not set the cap down.
- Fill to the 100 mL fill line, leaving headspace for mixing.
- Cap immediately, label with location, date, time, and collector, and complete the chain of custody.
- Hold at less than 10°C and deliver within the 30-hour maximum holding time for compliance samples.
[!WARNING] The sodium thiosulfate in the bottle is not optional and must not be rinsed out. It neutralizes the chlorine residual at the moment of collection. Without it, chlorine continues disinfecting the sample during transport, and organisms present in the distribution system are killed in the bottle, producing a falsely satisfactory result. This defeats the entire purpose of the sample.
Chlorine Residual: The DPD Method
DPD (N,N-diethyl-p-phenylenediamine) turns pink in the presence of chlorine, with intensity proportional to concentration. It is read on a colorimeter or spectrophotometer, or by visual comparator for rough field work.
| Reading | Procedure |
|---|---|
| Free chlorine | Add DPD Free reagent and read immediately — within about a minute |
| Total chlorine | Add DPD Total reagent (or a potassium iodide reagent) and allow the full reaction time, typically about 3 minutes |
| Combined chlorine | Total minus free |
Common errors:
- Waiting too long to read free chlorine. Combined chlorine slowly develops color in the free test, so a delayed reading over-reports free chlorine.
- Dirty or scratched sample cells, or cells indexed inconsistently.
- Not zeroing the instrument with an untreated sample of the same water.
- Exceeding the method range without diluting; very high chlorine can bleach the DPD color and read low, which is a dangerous failure mode because an overfed line can read as though it has no residual.
- Sampling from a stagnant line rather than flushing first.
Amperometric titration is the reference method, more accurate at very low concentrations and unaffected by color and turbidity, and it is used where DPD interference is a problem.
Field pH and Temperature
pH is measured with a portable meter calibrated with at least two buffers bracketing the expected value, with automatic temperature compensation enabled. Distribution pH matters for corrosion control compliance, for chlorine effectiveness (hypochlorous acid, the more effective form, predominates at lower pH), and as a general indicator of a source change.
Temperature is measured in the flowing stream, not in a collected sample that has warmed. It is required for pH correction, drives chlorine decay and nitrification risk, and is part of the water quality parameter set monitored under corrosion control requirements.
Other Distribution Sampling Programs
| Program | Key requirements |
|---|---|
| Disinfection byproducts (Stage 2 D/DBPR) | Sampled at locations identified by an initial distribution system evaluation; compliance is a locational running annual average at each site, so a single bad location can cause a violation even if the system average is fine |
| Lead and copper | First-draw, 1 liter, after at least 6 hours stagnation, from Tier 1 high-risk sites; do not remove the aerator or pre-flush |
| Water quality parameters | pH, alkalinity, calcium, conductivity, temperature, and orthophosphate at entry points and in the distribution system for corrosion control |
| Nitrite and nitrate in chloraminated systems | Nitrification monitoring, especially at storage tanks in warm months |
| Informational sampling | Complaint investigation, flushing effectiveness, new main clearance, tank turnover verification |
[!NOTE] Note the direct contradiction between bacteriological and lead sampling technique. For bacteriological samples you remove the aerator and flush thoroughly; for lead and copper you leave the aerator in place and do not flush, because the objective is to capture the water that has been sitting in contact with the plumbing. Applying the wrong protocol invalidates the sample, and this contrast is a favorite exam question.
A routine distribution sample returns total coliform positive. What must the system do within 24 hours?
An operator collecting a bacteriological sample rinses the sterile bottle three times with the sample water before filling it. What is the consequence?
How does proper technique for a lead and copper compliance sample differ from that for a bacteriological sample at the same tap?