E. coli Positive Routine Sample at a Dead End

1. Repeat sampling within 24 hours of notification. Collect the three repeats the rule requires: the original site, one within five service connections upstream, and one downstream - per your state-approved plan - and take a chlorine residual at each. Get them on ice and to the lab promptly.
2. Notify the Illinois EPA of the E. coli-positive result as soon as possible. The clock matters: if any repeat comes back E. coli positive, you have an MCL violation and a Tier 1 public notice is due within 24 hours of that determination.
3. Do not wait for the repeats to act on the cause. Flush the 2,200 ft dead end, restore residual above 0.5 mg/L at the end of the main, and confirm with a residual reading at the hydrant and at the residence.
4. Investigate the hydrant installation - was the main properly disinfected and flushed after the tie-in, was a temporary cross-connection made, was the hydrant swabbed and chlorinated, was there any pressure loss during the work?
5. Increase routine monitoring. After a total coliform positive, a system collecting fewer than 40 per month must run at least five routine samples the following month.
6. Document everything - times, sites, residuals, who collected, chain of custody.

The result is valid on its face. Collected 3:40 p.m., received 9:00 p.m. (5 hours 20 minutes later), cooler at 6 degrees C - within the 0-10 degrees C requirement and not frozen - and analyzed within holding time. You cannot set aside a result because it is inconvenient; invalidation is limited to documented protocol failures (bottle leaked, insufficient volume, no thiosulfate, no ice, holding time exceeded, aerator left on the faucet, sample not first-draw), and even then you resample rather than erase data.

What the two clues tell you:

• A 0.05 mg/L residual at the end of 2,200 ft of 6-inch dead end is a stagnation signature. Long dead ends with no flushing lose residual first, and a dead end is exactly where regrowth and contamination persist.
• The new hydrant two weeks ago is a plausible introduction point - disturbed main, a tie-in that may not have been disinfected and flushed to standard, or debris and biofilm mobilized by the work.
• The presence/absence method tells you the organism was there; it does not tell you how much. Treat it as real.

Taken together, the working hypothesis is a localized dead-end/maintenance problem, not system-wide contamination - which should shape both the flushing plan and the repeat sampling layout.

• Any repeat is E. coli positive. This is an E. coli MCL violation. Tier 1 public notice within 24 hours, immediate notification to the Illinois EPA, and a Level 2 assessment conducted within 30 days by a state-approved party, plus corrective actions. Assume the water may be unsafe and act accordingly.
• Repeats are total coliform positive but E. coli absent. This is a treatment technique trigger, not an MCL violation. It requires an assessment (Level 1 unless a higher level already applies), increased monitoring, and corrective action - typically residual improvement and a flushing program at the affected dead ends.
• All repeats absent. No MCL violation, but the original positive still counts. Corrective action on the root cause, plus the five routine samples next month.

In every branch, fix the system that produced the result. The sanitary survey already flagged irregular dead-end flushing, and a 6-inch dead end 2,200 ft long is going to repeat this result until it has a scheduled flush program, a residual check point, and ideally a loop or an automatic flusher. Closing the sample out without closing the finding is how the same positive comes back next quarter.