It depends on what contaminant you are measuring, but the testing frequency can vary from "every several years" to "continuously monitored and sets off a SCADA alarm if it exceeds a given threshold." The biggies--IIRC, turbidity, pH, and dosages of coagulant and treatment chemicals--are logged every 15 minutes, with more tests happening on hourly 6-hourly, and daily frequencies, followed by yet more contaminants happening largely on monthly or quarterly assessment bases. The issue in question would have shown up in a pH measurement, so there's no reason it shouldn't have been caught within minutes.
Indeed, you'll see that if a water test comes back positive, there will be multiple retests and a much greater rate of testing until the problem is abated, at least at my local drinking water board.
This isn't remotely true.
E.g., Seattle explicitly states:
> We monitor your water 24 hours a day, 365 days a year. We test samples from the region between 10 and 100 times per day.
https://www.seattle.gov/Documents/Departments/SPU/Services/W... (PDF)
> To ensure the safety of our drinking water, SPU's water quality laboratory analyzes over 20,000 microbiological samples each year (more than 50 a day) and conducts chemical and physical monitoring daily, 365 days per year.
https://www.seattle.gov/utilities/your-services/water/water-...
It is my understanding that most municipal water utilities only test water quality every 3 months. A problem can come and go between testing cycles.
Even with weekly testing, I’d expect the same risk (there’s still a window between tests). Basically you’re only going to know about a problem when it’s too late.