Dead-End Pressure Loss, Falling Chlorine, and an Unverified Valve
Your 1.2 MGD pressure zone is fed by a 500,000-gallon elevated tank. For three nights the tank has dropped 9 feet between midnight and 6 a.m. while metered demand stayed near 0.6 MGD. An 8-inch dead-end main serving 40 homes reads 34 psi at 4 a.m. and 21 psi at 10 a.m.; the low-pressure threshold in your operating plan is 20 psi. Free chlorine at that dead end has fallen from 0.8 to 0.15 mg/L in eight days, and three customers report rusty water. A new 12-inch main was tied in six weeks ago and a 10-inch valve on the old loop was reportedly reopened, but nobody has verified it. SCADA shows no sudden pressure spike or step change at any time in the last week.
1. What are the plausible explanations for the pressure loss, and in what order would you investigate? Name the specific field checks and what each result would tell you.
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Rank by how well each explanation fits the evidence, cheapest decisive check first:
- A closed or partly closed valve is the leading candidate. Pressure sags with demand, there is no step change or spike on SCADA, and the 10-inch valve was never verified after the tie-in. Check position and exercise it; confirm with a pressure test at the dead end plus a nearby hydrant flow test. A closed valve produces a steep gradient that does not change when you operate valves on the other side; a capacity problem produces a gradient that moves with demand everywhere in the zone.
- An undetected leak on the new main or the old loop. Compare zone night flow to the pre-tie-in baseline; the tank is losing water overnight that the meter does not see. Listen at the tie-in and along the dead end, and check the tank outlet for continuous flow at 3 a.m.
- Tank or altitude-valve trouble. Never trust a 9-foot drop until you verify the level: cross-check the transmitter against a hand gauge or sight tube, and confirm the altitude valve and tank check valve are seating.
- Real demand growth on an undersized 8-inch dead end. Least likely to appear this fast, but worth checking new irrigation or a private booster after the valve and leak checks come back clean.
Sequence: valve status, tank level verification, night flow, then leak survey.
2. What do you do about the 0.15 mg/L chlorine residual and the discolored water while the investigation is open? Is a precautionary boil water notice required?
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Free chlorine fell from 0.8 to 0.15 mg/L at a dead end. Florida's distribution minimum is on the order of 0.2 mg/L free chlorine, so 0.15 is at or below the floor and requires action, not monitoring.
- Flush the dead end unidirectionally to a dechlorinated discharge point, never straight to a storm drain, and record flush volume and residual before and after. Flush until turbidity clears, then re-chlorinate through normal system flow and re-sample.
- Sample for bacteria and chlorine at the dead end, at the nearest upstream sites, and at the tie-in. Rusty water plus residual loss can mean sediment, biofilm, or a cross-connection; walk the construction area for a temporary hose, an unapproved tie, or an irrigation system on the new main.
- Notify the 40 affected customers through your public information process.
- Boil water notice: it is mandatory when pressure in the distribution system drops below 20 psi or when contamination is confirmed, coordinated with FDEP. Your low is 21 psi, so a precautionary notice is not automatic. But you are one bad hour from 20 psi: document the pressure record, watch the trend, and be ready to issue. If any bacteriological sample comes back positive, issue the notice immediately.
- Escalate the pressure problem as an operational emergency rather than a chlorine problem, because the low residual is almost certainly a symptom of poor flow and stagnation, not a chlorination failure.