WTO Quality Monitoring 2 — Questions and Answers
Question 1: What is the Maximum Contaminant Level (MCL) for total trihalomethanes (TTHMs) in drinking water under the Stage 2 Disinfectants and Disinfection Byproducts Rule?
- 0.040 mg/L
- 0.080 mg/L (Correct answer)
- 0.060 mg/L
- 0.100 mg/L
Correct answer: 0.080 mg/L
The MCL for TTHMs under the Stage 2 DBPR is 0.080 mg/L, calculated as a locational running annual average.
Question 2: A water system collects a bacteriological sample that tests positive for total coliform. What must the operator do next according to the Total Coliform Rule?
- Immediately issue a boil-water advisory
- Collect repeat samples within 24 hours
- Notify the state within 24 hours and collect repeat samples (Correct answer)
- Shut down the distribution system
Correct answer: Notify the state within 24 hours and collect repeat samples
A positive total coliform result requires the operator to notify the state within 24 hours and collect repeat samples to determine if E. coli is present.
Question 3: Which parameter is measured using a turbidimeter and is critical for evaluating filter performance?
- pH
- Turbidity (Correct answer)
- Hardness
- Alkalinity
Correct answer: Turbidity
Turbidity, measured in NTU using a turbidimeter, indicates the cloudiness of water and is a key indicator of filter performance and microbial risk.
Question 4: Under the Surface Water Treatment Rule, what is the maximum turbidity limit for combined filter effluent (CFE) that cannot be exceeded more than 5% of the time in any month?
- 0.3 NTU
- 1 NTU (Correct answer)
- 5 NTU
- 0.5 NTU
Correct answer: 1 NTU
The SWTR allows CFE turbidity to exceed 1 NTU no more than 5% of measurements in any month, with an absolute limit of 5 NTU.
Question 5: What does a chlorine residual of zero in the distribution system most likely indicate?
- Over-chlorination at the treatment plant
- Inadequate disinfection or excessive demand in the system (Correct answer)
- The system uses chloramine instead of free chlorine
- The meter needs recalibration
Correct answer: Inadequate disinfection or excessive demand in the system
A zero chlorine residual suggests either insufficient disinfectant was applied or high chlorine demand from biofilm, sediment, or contaminants is consuming it.
Question 6: Which method is the EPA-approved reference method for measuring free chlorine residual in drinking water?
- Colorimetric method using OTO
- DPD colorimetric method (Correct answer)
- Iodometric titration
- Starch-iodide method
Correct answer: DPD colorimetric method
The DPD (N,N-diethyl-p-phenylenediamine) colorimetric method is the EPA-approved reference method for measuring free chlorine residual.
Question 7: A water system monitoring for lead under the Lead and Copper Rule collects first-draw samples from household taps. How long must water stagnate in the plumbing before sample collection?
- At least 2 hours
- At least 6 hours (Correct answer)
- At least 30 minutes
- At least 8 hours
Correct answer: At least 6 hours
First-draw lead samples must be collected after water has stood motionless in the plumbing for at least 6 hours to maximize leaching from lead-containing materials.
WTO Quality Monitoring 2
What is the Maximum Contaminant Level (MCL) for total trihalomethanes (TTHMs) in drinking water under the Stage 2 Disinfectants and Disinfection Byproducts Rule?