Stain Identification and Treatment Standard
Purpose
Nobody can name a pool stain by looking at it, and the shops that try spend a season treating iron with an oxidizer and copper with an acid wash. A metal stain and an organic stain want opposite chemistry: one is removed by pulling the sanitizer to nothing, the other by driving it up. Guess wrong and you set the stain harder or you strip a surface that was never stained. This procedure guarantees the stain is identified by two spot tests before a single pound of anything enters the pool, the source that put the metal in the water is found before the stain is treated, and the window in which the pool holds no sanitizer is planned, posted and closed out rather than discovered on a Saturday.
Scope
Covers surface staining on cementitious interior finishes, tile and vinyl in residential and small semi-commercial pools: the spot tests, source elimination, organic oxidation, whole-pool ascorbic treatment for metals, and the sanitizer restoration afterwards.
It does not cover the differential between cobalt, iron and copper discoloration, which the PoolService stain troubleshooting tree owns, nor plaster defects that look like stains and are not, such as mottling and calcium nodules, which the plaster surface diagnostics reference owns. Acid washing a stain out of the surface is the drain and acid wash standard.
Roles and responsibilities
| Role | Owns | Hands off |
|---|---|---|
| Route technician | Both spot tests, photographs, the fill-water and equipment source check | Hands the test results and the suspected source to the service manager BEFORE any treatment is scheduled |
| Lead technician | The treatment day, the sanitizer window, closure posting, sanitizer restoration | Hands the pool back only on a measured sanitizer reading, never on elapsed time |
| Service manager | Whether treatment proceeds without source removal, and the disclosure that goes with it | Hands the customer the source finding in writing, because an untreated source restains the pool |
Procedure
Confirm it is a stain before treating it as one. Run a gloved thumb across the mark under water. Accept it as a stain when the surface under the discoloration is smooth and continuous with the surface around it. Wrong is a mark you can feel as a pit, a rough patch or a raised nodule: that is surface damage or a finish defect, not a deposit, and no chemical removes it. Stop and route it to the plaster diagnostics reference rather than selling a treatment that cannot work. Hazard is reaching into the water near a running outlet, so the pump is off and its disconnect locked under 29 CFR 1910.147 before any hand goes below the surface.
Run the ascorbic spot test first, then the oxidizer spot test, and gate both on the surface. Hold a vitamin C tablet against the stain for up to 2 minutes. Accept a metal identification when the stain visibly lightens under the tablet and the area around it does not. If nothing changes, hold a trichlor tab against a different part of the same stain for no more than 60 seconds, and accept an organic identification when that area lightens. Wrong is leaving either tablet longer to force a result: a trichlor tab left on plaster bleaches a halo you now own, and it is never used on a vinyl liner, which it damages outright. Neither test moving it means scale or embedded debris, which routes to the tile line standard. Glove the trichlor tab, keep it away from calcium hypochlorite on the truck because that pairing reacts, and do not breathe dust off a broken tab.
Find the source before you treat, because treatment does not remove the metal from the water. Test the fill water separately from the pool for iron and copper, and check the plumbing history: a well or a corroded copper service on the fill side, a heat exchanger running at a pH that eats it, a copper-based algaecide, or a mineral ioniser. Accept when the fill water result and the equipment finding are both written on the ticket. Wrong is treating a pool fed by untreated well water and calling the job done, because the stain returns with the next few inches of makeup water. Where the source is a heat exchanger, that is a heater fault and goes to the heater service standard rather than to a chemical. Hazard at this step is a hose bib sample at a hot fill line, so run it to temperature before filling the vial.
For an organic stain, oxidize and brush rather than reaching for a metal treatment. Brush the stained area first to open the deposit, then dose to the calculated shock level and brush again once circulation has distributed it. Accept when the stain is gone or visibly reduced at 24 hours with combined chlorine at or below 0.2 ppm. Wrong is a stain that does not move under a correct shock: reclassify it, because it was probably never organic. The dose itself is not computed here; the shock dosing standard owns it. Brush from the deck with a pole rather than in the water while free chlorine is elevated, and keep the oxidizer away from acid on the same deck.
For a metal stain, prepare the pool before any ascorbic acid opens: post it closed, pull free chlorine to the gate, and isolate the heater. Ascorbic acid destroys free chlorine on contact, so the pool holds no sanitizer for a day or more, and swimming in unsanitized water is a recreational water illness exposure by ingestion, not a cosmetic issue. Accept when a closure notice is posted at every gate, free chlorine reads 0.5 ppm or below, pH is 7.0 to 7.2, and the heater is off with the salt cell switched out of service. Wrong is starting with free chlorine at 3 ppm, which wastes the dose: stop and wait rather than doubling the ascorbic. On a public pool the health department's closure and reopening rules govern this window and are not a shop decision.
Broadcast ascorbic acid at a starting rate, add sequestrant, and repeat rather than doubling. A common field starting rate is about 0.5 lb per 10,000 gal with the pump running and the filter on recirculate; the product label governs and it is binding where the two disagree. Follow with a sequestrant at the label rate to hold the released metal in solution. Accept when the stain lifts within about 30 minutes and the water may go faintly cloudy or tinted, which is the metal coming off the surface. Wrong is no change at 30 minutes: add a second measured dose rather than a doubled first one, and if two doses do nothing, the identification was wrong. Broadcast from upwind with goggles and gloves, because ascorbic dust in the eyes and airway is an irritant route the granular form makes easy to catch.
Restore sanitizer slowly, verify it holds, and put the heater and cell back with a check. Bring free chlorine up in small increments over hours rather than one shock dose, because a fast return re-oxidizes metal still in the water and restains the surface within a day. Accept reopening only on a measured free chlorine inside the operating band for the pool's stabilizer level, holding across a second reading at least 4 hours later, with the notice removed by the technician who took it. Restore the heater and salt cell and confirm each runs on its own controls, because a cell left switched off is a call next week. Wrong is reopening on elapsed time with a chlorine demand still running. Then schedule the dilution, because the metal is still dissolved and a sequestrant only postpones it.
Why the source matters more than the treatment
An ascorbic treatment takes metal off the surface and puts it back into solution. It does not leave the property. The sequestrant that follows is a holding action: it keeps the metal chelated so it stays dissolved rather than plating out again, and it degrades over weeks, which is why sequestered pools get a maintenance dose on the route rather than one treatment.
That is why step 3 sits before step 5. A pool fed by iron-bearing well water can be cleaned perfectly on Tuesday and be staining again a month later after enough makeup water goes in, and the customer reads that as a failed treatment rather than an untreated source. Treat the source first where it can be treated, usually a hose-end pre-filter on the fill or a correction to the chemistry dissolving the copper, and plan a partial dilution where the dissolved load is already high. Where the customer declines, the service manager puts the expected return in writing before the treatment is booked, not after the callback.
The record this produces
The stain record, attached to the property, because the same stain recurs on the same pools and the second technician needs the first one's tests.
Fields: photographs of the stain at a fixed spot and angle, with a scale object in frame; the surface feel result from step 1; the ascorbic test result with the hold time; the oxidizer test result with the hold time and the surface it was run on; fill water iron and copper results, sampled and dated; the equipment source finding; closure posted and removed times; free chlorine and pH at the gate before treatment; ascorbic product, rate and number of doses; sequestrant product and rate; free chlorine at each restoration reading with times; and the dilution or source action scheduled.
The service manager reads the source finding before quoting a repeat. The next technician reads the spot test results so the identification is not re-run from scratch. The office keeps the closure times, because that is the record that answers a health inspector or a customer asking when the pool was safe to use.
Worked pass: brown staining on a plaster pool fed by a well
A 20,000 gal plaster pool, brown-orange staining across the deep-end floor and up the shady wall, worse since the pool was topped up after a dry month.
Step 1: thumb across the stain reads smooth and continuous with the surface either side. Pump was locked out at 08:30 before the hand went in. Gate passes: this is a deposit, not damage.
Step 2: vitamin C tablet held on the deep-end floor stain lightens it clearly at about 45 seconds, well inside the 2 minute limit. Metal identified, so the trichlor test is not run at all and no bleached halo is created.
Step 3: fill water sampled at the hose bib after running to temperature reads iron at 0.4 ppm; the property is on a well. Heater is a heat pump with a titanium exchanger, so copper is not in play. Source is the fill water, written on the ticket, and a hose-end pre-filter is quoted with the treatment.
Step 4 does not apply, recorded as not applicable rather than skipped.
Step 5 FAILS on the first attempt. Treatment was booked for Thursday morning. Free chlorine at 08:10 reads 2.8 ppm against a gate of 0.5 ppm or below, because the customer added tabs mid-week. The stop rule is taken: the treatment does not start and the ascorbic is not increased to burn through it. The closure notice stays up, the feeder is switched off, and the tech returns Friday. Friday free chlorine reads 0.3 ppm, pH brought from 7.6 to 7.1 with acid, heater off and salt cell switched out. The gate now passes.
Step 6: at 0.5 lb per 10,000 gal, a 20,000 gal pool takes 1.0 lb of ascorbic acid, broadcast upwind with the pump on recirculate. At 30 minutes the deep-end stain is gone and the water reads faintly tinted, which is the expected sign. A second dose is not needed. Sequestrant added at the label rate.
Step 7: free chlorine brought up in three small increments across the afternoon. Reading at 16:00 is 2.2 ppm; second reading at 20:15, more than 4 hours later, is 1.9 ppm, so it is holding rather than being consumed. Closure notice removed by the technician who took the 20:15 reading. Heater and salt cell restored and each confirmed running on its own controls. Dilution scheduled for the following month, because the water still carries the iron the surface gave up.
References
- The printed label directions for the ascorbic acid product and the sequestrant in use; label rate and sequence are binding where they differ from any field rule of thumb.
- 29 CFR 1910.147 for lockout of the circulation pump before any hand or body goes below the water surface during testing.
- CDC guidance on recreational water illness and the local health department's closure and reopening requirements for the period a treated pool holds no sanitizer; the health department's rules bind, a model code only where adopted.
- See related: the PoolService stain troubleshooting tree for the cobalt, iron and copper differential, the plaster surface diagnostics reference for defects that are not stains, the shock dosing standard for organic stains, and the partial drain and dilution decision for the dissolved metal left behind.