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Free WDO Practice Questions

10 free, exam-style Water Distribution Operator Exam (WDO) practice questions with answers and explanations. No signup required. Work through them below, then take the full free WDO practice test to study every exam domain.

These 10 free WDO questions are organized by exam domain, so you can see how each part of the Water Distribution Operator Exam blueprint is tested. Reveal the answer and explanation under each question.

Domain 1: Distribution System Components 35% of exam

Question 1

A cylindrical storage tank is 24 ft (7.3 m) tall. The current water level is 18 ft (5.5 m). What is the approximate water pressure at the base of the tank?

  1. 7.8 psi (53.8 kPa)
  2. 10.4 psi (71.7 kPa)
  3. 41.6 psi (286.8 kPa)
  4. 3.4 psi (23.4 kPa)
Show answer & explanation

Correct answer: A - 7.8 psi (53.8 kPa)

Question 2

A large commercial nursery has a direct hose connection from the potable water system to a chemical fertilizer injection tank. Which backflow prevention assembly is REQUIRED at this connection?

  1. Pressure Vacuum Breaker (PVB)
  2. Atmospheric Vacuum Breaker (AVB)
  3. Double Check Valve Assembly (DCVA)
  4. Reduced Pressure Principle (RP) assembly
Show answer & explanation

Correct answer: D - Reduced Pressure Principle (RP) assembly

Question 3

A SCADA system alarm shows that pressure in Distribution Zone 2 has dropped from 65 psi to 38 psi over the past 30 minutes. No maintenance work is scheduled in that zone. What is the FIRST action the operator should take?

  1. Increase pump speed to restore zone pressure immediately
  2. Open the inter-zone isolation valve to equalize pressure
  3. Investigate for a main break or unusually high demand event in Zone 2
  4. Contact the state drinking water program to report a possible pressure violation
Show answer & explanation

Correct answer: C - Investigate for a main break or unusually high demand event in Zone 2

Domain 2: Equipment Installation, Operation, Maintenance, and Repair 30% of exam

Question 4

A pump delivers 650 gpm (2,460 L/min) against a total dynamic head of 95 ft (29 m). The pump efficiency is 72%. What is the required brake horsepower (BHP)?

  1. 15.6 hp (11.6 kW)
  2. 21.7 hp (16.2 kW)
  3. 11.2 hp (8.4 kW)
  4. 23.8 hp (17.7 kW)
Show answer & explanation

Correct answer: B - 21.7 hp (16.2 kW)

Question 5

According to AWWA C651, when disinfecting a newly installed water main using the continuous-feed method, what is the MINIMUM chlorine concentration that must be maintained, and for how long?

  1. 10 mg/L for at least 24 hours
  2. 25 mg/L for at least 12 hours
  3. 50 mg/L for at least 1 hour
  4. 25 mg/L for at least 24 hours
Show answer & explanation

Correct answer: D - 25 mg/L for at least 24 hours

Domain 3: Water Quality Monitoring, Evaluation, Adjustment, and Laboratory Analysis/Interpretation 15% of exam

Question 6

A water system treats 2.5 MGD (9,463 m³/day). The target chlorine dose is 1.8 mg/L. The sodium hypochlorite solution being used is 12.5% available chlorine. How many pounds (kg) of hypochlorite SOLUTION must be fed per day?

  1. 37.5 lb/day (17.0 kg/day)
  2. 300 lb/day (136.1 kg/day)
  3. 150 lb/day (68.0 kg/day)
  4. 469 lb/day (212.7 kg/day)
Show answer & explanation

Correct answer: B - 300 lb/day (136.1 kg/day)

Question 7

A community water system's 90th percentile lead tap sample result is 18 µg/L, exceeding the Lead and Copper Rule action level of 15 µg/L. What is the FIRST required action the system must take?

  1. Replace all identified lead service lines within 30 days
  2. Collect confirmation samples at all tier-one sites within 48 hours
  3. Notify affected customers of the exceedance within 24 hours
  4. Submit a corrosion control treatment study to the state within 6 months
Show answer & explanation

Correct answer: C - Notify affected customers of the exceedance within 24 hours

Under the Lead and Copper Rule, exceeding the action level triggers Tier 1 public notification. Per 40 CFR 141.85 (Public Education) and 40 CFR 141 Subpart Q (Public Notification Rule), a Tier 1 notification must be made within 24 hours.

Question 8

A distribution system water quality report shows a Langelier Saturation Index (LSI) of −1.2. What does this indicate about the water?

  1. The water is over-saturated and scale-forming, likely depositing CaCO₃ on pipe walls
  2. The water is under-saturated and corrosive, likely leaching lead and copper from pipes
  3. The water is chemically balanced and requires no corrosion control treatment
  4. The water has elevated dissolved oxygen, accelerating galvanic corrosion at metal joints
Show answer & explanation

Correct answer: B - The water is under-saturated and corrosive, likely leaching lead and copper from pipes

See CSUS Water Distribution System O&M, 7th Ed., Ch. 11, and AWWA WSO Water Distribution Grade 3-4. A negative LSI (such as −1.2) indicates the water is under-saturated and corrosive, likely leaching lead and copper from pipes, whereas a positive LSI indicates over-saturated, scale-forming water.

Domain 4: Security, Safety, Administrative Procedures, and Public Interactions 20% of exam

Question 9

Before entering a below-grade valve vault, an operator's multi-gas meter reads: O₂ = 20.6%, LEL = 12%, H₂S = 4 ppm. What is the correct action?

  1. Enter the space - oxygen and hydrogen sulfide levels are both within acceptable limits
  2. Enter with SCBA - the combined readings are acceptable with respiratory protection
  3. Do not enter; ventilate the space until the LEL drops below 10% before proceeding
  4. Retest in 15 minutes to confirm the readings before making an entry decision
Show answer & explanation

Correct answer: C - Do not enter; ventilate the space until the LEL drops below 10% before proceeding

Under 29 CFR 1910.146 (Permit-Required Confined Spaces), an acceptable atmosphere requires the LEL to be below 10%. With LEL = 12%, the space must not be entered; ventilate until the LEL drops below 10% before proceeding, even though the O₂ and H₂S readings are acceptable.

Question 10

An operator has shut down a pump, opened the electrical disconnect, applied a personal lockout device, and closed the upstream isolation valve. What is the NEXT required step before beginning maintenance work?

  1. Attempt to start the pump to verify it will not energize
  2. Release or restrain all stored energy - bleed residual pressure from the discharge line and verify capacitor discharge
  3. Notify the maintenance supervisor that lockout is complete and work is beginning
  4. Apply a danger tag to the pump that is visible to all other operators in the area
Show answer & explanation

Correct answer: B - Release or restrain all stored energy - bleed residual pressure from the discharge line and verify capacitor discharge

Per 29 CFR 1910.147 (The Control of Hazardous Energy), Section (d)(4), release of stored energy precedes energy verification. All stored energy must be released or restrained - bleed residual pressure from the discharge line and verify capacitor discharge - before beginning maintenance work.

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