NASCO Water Chemistry & Quality Control 5 — Questions and Answers
Question 1: After a fecal accident in a pool, the CDC recommends raising free chlorine to what level (at pH 7.5) to inactivate non-Crypto pathogens?
- 1 ppm for 30 minutes
- 2 ppm for 30 minutes (Correct answer)
- 5 ppm for 30 minutes
- 10 ppm for 30 minutes
Correct answer: 2 ppm for 30 minutes
For a formed stool fecal accident, CDC recommends raising free chlorine to 2 ppm at pH 7.5 and maintaining it for at least 25–30 minutes before reopening.
Question 2: What does a high ORP (Oxidation-Reduction Potential) reading indicate about pool water?
- Low chlorine demand
- Strong sanitizing ability of the water (Correct answer)
- High combined chlorine levels
- Poor water circulation
Correct answer: Strong sanitizing ability of the water
A high ORP (ideally 650–750 mV) indicates that the water has a strong oxidizing capacity and is effective at killing pathogens.
Question 3: Muriatic acid (hydrochloric acid) is added to pool water to accomplish which goal?
- Raise pH and alkalinity
- Lower pH and total alkalinity (Correct answer)
- Increase calcium hardness
- Eliminate phosphates
Correct answer: Lower pH and total alkalinity
Muriatic acid lowers both pH and total alkalinity, making it the primary chemical for reducing these levels when they are too high.
Question 4: Sodium thiosulfate is used in aquatic facility management primarily to:
- Raise chlorine levels quickly
- Neutralize excess chlorine (Correct answer)
- Lower cyanuric acid levels
- Increase total alkalinity
Correct answer: Neutralize excess chlorine
Sodium thiosulfate is a chlorine neutralizer used to quickly reduce dangerously high chlorine levels in pool water.
Question 5: What is the MOST accurate description of 'superchlorination'?
- Adding 1–2 ppm of chlorine daily
- Adding a large dose of chlorine (typically 10 ppm or more) to oxidize contaminants (Correct answer)
- Adding cyanuric acid to protect chlorine
- Adjusting pH to maximize chlorine efficiency
Correct answer: Adding a large dose of chlorine (typically 10 ppm or more) to oxidize contaminants
Superchlorination involves adding a large dose of chlorine (10 ppm or more) to oxidize organic contaminants, chloramines, and algae.
Question 6: Which test should be performed FIRST when troubleshooting cloudy pool water?
- Calcium hardness
- Cyanuric acid level
- Free chlorine and pH (Correct answer)
- Total dissolved solids (TDS)
Correct answer: Free chlorine and pH
Free chlorine and pH are tested first because low chlorine or improper pH are the most common causes of cloudy water and affect all other chemistry.
Question 7: Phosphates in pool water are a concern because they:
- Lower pH dramatically
- Serve as nutrients that promote algae growth (Correct answer)
- React with chlorine to form chloramines
- Reduce calcium hardness
Correct answer: Serve as nutrients that promote algae growth
Phosphates act as fertilizers for algae, so elevated phosphate levels can fuel algae blooms even when chlorine is adequate.
NASCO Water Chemistry & Quality Control 5
After a fecal accident in a pool, the CDC recommends raising free chlorine to what level (at pH 7.5) to inactivate non-Crypto pathogens?