Coliform Positives and Residual Loss in a Stagnant Distribution System
A community system serving 9,200 people collects 10 routine coliform samples per month. It buys treated surface water at about 1.1 MGD and distributes it through 60 miles of main, three elevated tanks, and a long dead-end loop serving a hospital and a school. Summer data: free chlorine at the plant is 1.05 mg/L; at six dead-end sites residuals are 0.12 to 0.25 mg/L; water temperature is 25 degrees C; total chlorine reads higher than free chlorine at two of those sites; HPC at one dead-end site is 380 CFU/mL; ammonia in the purchased water is 0.06 mg/L as N. In the first two weeks of July, two routine total coliform samples are positive (both E. coli negative) at different dead-end sites three miles apart. All repeat samples collected within 24 hours are total coliform negative.
1. State your RTCR compliance position given the two positives and the clean repeats.
Read the worked answer
This is a total coliform MCL violation, not a clean bill of health. For a system collecting fewer than 40 samples per month, the MCL is exceeded when more than one routine or repeat sample in a month is total coliform positive. You had two routine positives in July. The negative repeats do not erase the second routine positive; they only establish that no E. coli or fecal trigger fired and that the repeat obligation was met.
Consequences and follow-through:
- Tier 2 public notice within 30 days, using the required health-effects language.
- A Level 1 assessment completed and submitted within 30 days of the trigger; fix any sanitary defects found and document them.
- Confirm the repeat set was complete: three repeats per positive (original tap, one within five service connections upstream or downstream, one at another representative location), collected within 24 hours.
- Maintain the increased monitoring EGLE directs while the assessment findings are addressed.
If any repeat had been E. coli positive, this becomes Tier 1 notice within 24 hours and a Level 2 assessment - which is why the field crews must know the distinction before they sample.
2. Diagnose the loss of residual and the regrowth signal, and list the corrective actions you would sequence over the next two weeks.
Read the worked answer
The plant is delivering 1.05 mg/L and the ends of the system are at 0.12-0.25 mg/L at 25 degrees C, with total chlorine reading above free chlorine where ammonia (0.06 mg/L as N) is present. That is chlorine demand plus chloramine/organic chloramine interference plus high water age in a dead-end loop - a distribution chemistry problem, not a plant chlorine feed problem. HPC at 380 CFU/mL confirms biofilm/regrowth.
Sequence.
- Unidirectional flushing of the dead-end loop and the problem tank zones, measuring residual and turbidity until stable at baseline - this removes sediment, biofilm and old water.
- Restore circulation: open or loop valves, cycle the elevated tanks so each turns over, and consider a recirculation pump or an altitude-valve change to stop the loop from stagnating.
- Verify a measurable free residual at every dead end and adjust chlorination for it, while checking that TTHM/HAA5 LRAA stays in bounds - do not solve one rule by breaking another.
- Track ammonia, nitrite and nitrate across the loop to rule nitrification in or out, and revisit whether chloramination is involved.
- Resample the affected sites frequently until two clean months are banked, and carry the two failing sites in the siting plan rotation.
3. What changes belong in the coliform sample siting plan, and what would you refuse to do?
Read the worked answer
Revise the plan so the sites that failed are permanently in the rotation, along with the hospital, the school, the far ends of the dead-end loop, and storage tank outlets - the locations most likely to show a problem. Use more sites than the 10 required per month and rotate through them so coverage is genuinely representative of the whole distribution rather than a fixed handful of convenient taps near the plant.
Document the rationale for each site (map location, service connections, why it is representative or high-risk) and submit the revised plan to EGLE for review, since siting plans are subject to state approval.
What to refuse: do not drop the two failing dead ends from the plan because they keep coming back positive; do not sample only at cool, high-flow sites to get clean results; do not accept samples from the same friendly customer every month. The positives are the most valuable data in the plan - they locate the problem the assessment is required to fix.