Drinking water maximum contaminant levels practice questions
20 free practice questions from 40 CFR Part 141 Subpart G. Each answer is explained and linked to the rule it comes from.
Drinking water maximum contaminant levels
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Effective dates and who the limits cover
1. What effective date does 141.60(a)(4) set for the PFAS entries in 141.61(c)(2)(i) through (vii)?
- A. January 17, 1994
- B. April 26, 2029
- C. January 23, 2006
- D. December 8, 2003
Show answer
Answer: B. April 26, 2029
141.60(a)(4): "The effective date for § 141.61(c)(2)(i) through (vii) is April 26, 2029."
Source: eCFR: 40 CFR 141.60, Effective dates (as of Nov 3, 2026)
2. Under 141.62(b), which systems does the fluoride MCL in paragraph (b)(1) apply to?
- A. Community and non-transient, non-community water systems
- B. Community, non-transient non-community and transient non-community water systems
- C. Transient non-community water systems only
- D. Community water systems only
Show answer
Answer: D. Community water systems only
141.62(b) states that the MCL "specified in paragraph (b)(1) of this section only applies to community water systems."
Source: eCFR: 40 CFR 141.62, Maximum contaminant levels for inorganic contaminants (as of Nov 3, 2026)
Organic contaminants and PFAS
3. Which technologies does 141.61(b) identify as the best technology for the organic MCLs in 141.61(a) and (c) other than PFAS?
- A. Granular activated carbon, packed tower aeration or oxidation, as shown in table 2
- B. Anion exchange, reverse osmosis and nanofiltration
- C. Lime softening and electrodialysis
- D. Alkaline chlorination and corrosion control
Show answer
Answer: A. Granular activated carbon, packed tower aeration or oxidation, as shown in table 2
141.61(b) identifies "granular activated carbon (GAC), packed tower aeration (PTA), or oxidation (OX)" for the organic contaminants in (a) and (c), "except for per- and polyfluoroalkyl substances (PFAS)."
Source: eCFR: 40 CFR 141.61, Maximum contaminant levels for organic contaminants (as of Nov 3, 2026)
4. How does the 141.61(c)(2) table (effective April 26, 2029 under 141.60(a)(4)) state the MCL for the Hazard Index PFAS (HFPO-DA, PFBS, PFHxS and PFNA)?
- A. 0.0000040 mg/l
- B. 10 ng/l
- C. 1 (unitless)
- D. 2000 ng/l
Show answer
Answer: C. 1 (unitless)
The MCL row for "Hazard Index PFAS (HFPO-DA, PFBS, PFHxS, and PFNA)" reads "1 (unitless)"; the Hazard Index is "the sum of component hazard quotients (HQs)".
Source: eCFR: 40 CFR 141.61, Maximum contaminant levels for organic contaminants (as of Nov 3, 2026)
5. What does the MCL column of the 141.61(c)(2) PFAS table (effective April 26, 2029 under 141.60(a)(4)) show for PFBS?
- A. No individual MCL
- B. 0.0000040 mg/l
- C. 0.00001 mg/l
- D. 0.0002 mg/l
Show answer
Answer: A. No individual MCL
The PFBS row reads "No individual MCL"; its entry in the column headed "HBWC (mg/l) for Hazard Index calculation" is 0.002.
Source: eCFR: 40 CFR 141.61, Maximum contaminant levels for organic contaminants (as of Nov 3, 2026)
6. Under the Hazard Index footnote in 141.61(c)(2), how is each component hazard quotient calculated?
- A. By multiplying the PFAS concentration by 4.0
- B. By averaging the four PFAS results over a calendar year
- C. By dividing the measured PFAS concentration in water by its health-based water concentration in the same units
- D. By subtracting the PFOA result from the PFOS result
Show answer
Answer: C. By dividing the measured PFAS concentration in water by its health-based water concentration in the same units
Hazard quotients "are calculated by dividing the measured component PFAS concentration in water by the relevant health-based water concentration when expressed in the same units".
Source: eCFR: 40 CFR 141.61, Maximum contaminant levels for organic contaminants (as of Nov 3, 2026)
Inorganic contaminants
7. Which of these 141.62(b) MCLs applies to transient non-community water systems as well as to community and non-transient, non-community water systems?
- A. Arsenic
- B. Nitrite
- C. Fluoride
- D. Barium
Show answer
Answer: B. Nitrite
The MCLs "specified in (b)(7), (b)(8), and (b)(9) of this section apply to community water systems; non-transient, non-community water systems; and transient non-community water systems." Nitrite is "(8) Nitrite"; barium and arsenic apply to community and non-transient, non-community systems, and fluoride "only applies to community water systems."
Source: eCFR: 40 CFR 141.62, Maximum contaminant levels for inorganic contaminants (as of Nov 3, 2026)
8. Which systems and which MCL does the small system compliance technology table in 141.62(d) address?
- A. Systems serving more than 10,000 persons, for all inorganic MCLs
- B. Systems with fewer than 500 service connections, for the fluoride MCL
- C. Transient non-community water systems, for the nitrate MCL
- D. Systems serving 10,000 persons or fewer, for the arsenic MCL
Show answer
Answer: D. Systems serving 10,000 persons or fewer, for the arsenic MCL
141.62(d) identifies "the affordable technology, treatment technique, or other means available to systems serving 10,000 persons or fewer for achieving compliance with the maximum contaminant level for arsenic".
Source: eCFR: 40 CFR 141.62, Maximum contaminant levels for inorganic contaminants (as of Nov 3, 2026)
9. 141.62(c) identifies best available technologies for the inorganic contaminants in 141.62(b) with one exception. Which contaminant is left out?
- A. Arsenic
- B. Nitrate
- C. Barium
- D. Fluoride
Show answer
Answer: D. Fluoride
141.62(c) covers the MCLs "for inorganic contaminants identified in paragraph (b) of this section, except fluoride".
Source: eCFR: 40 CFR 141.62, Maximum contaminant levels for inorganic contaminants (as of Nov 3, 2026)
10. According to the key to the BAT table in 141.62(c), for which systems is coagulation/filtration not a BAT?
- A. Systems with fewer than 500 service connections
- B. Systems serving 10,000 or more persons
- C. Systems using only ground water
- D. Transient non-community water systems
Show answer
Answer: A. Systems with fewer than 500 service connections
The key reads "2 = Coagulation/Filtration (not BAT for systems <500 service connections)".
Source: eCFR: 40 CFR 141.62, Maximum contaminant levels for inorganic contaminants (as of Nov 3, 2026)
Microbiological contaminants
11. Beginning April 1, 2016, which of these events is one of the conditions in 141.63(c) that puts a system out of compliance with the E. coli MCL?
- A. A single routine sample is total coliform-positive and all repeat samples are negative
- B. The system collects more routine samples than required
- C. The system fails to test for E. coli when any repeat sample tests positive for total coliform
- D. A routine sample is collected on a holiday
Show answer
Answer: C. The system fails to test for E. coli when any repeat sample tests positive for total coliform
Condition (c)(4) is: "The system fails to test for E. coli when any repeat sample tests positive for total coliform."
12. Beginning April 1, 2016, for what period must a public water system determine compliance with the E. coli MCL under 141.63(d)?
- A. As a running annual average of quarterly results
- B. For each month in which it is required to monitor for total coliforms
- C. Once per calendar year
- D. As a locational running annual average at each sample site
Show answer
Answer: B. For each month in which it is required to monitor for total coliforms
A system "must determine compliance with the MCL for E. coli in paragraph (c) of this section for each month in which it is required to monitor for total coliforms."
13. For the public notification requirements in subpart Q, how does 141.63(c) describe a violation of the E. coli MCL?
- A. It may pose an acute risk to health
- B. It is a monitoring and reporting violation only
- C. It needs no public notice
- D. It is treated as a secondary standard exceedance
Show answer
Answer: A. It may pose an acute risk to health
"For purposes of the public notification requirements in subpart Q of this part, violation of the MCL may pose an acute risk to health."
Disinfection byproducts
14. For the subpart V TTHM and HAA5 MCLs in 141.64(b)(2), what best available technology does 141.64(b)(2)(iii) list for consecutive systems serving fewer than 10,000, for the disinfected water they buy or otherwise receive?
- A. Control of ozone treatment process
- B. Packed tower aeration
- C. Coagulation/filtration followed by lime softening
- D. Improved distribution system and storage tank management to reduce residence time
Show answer
Answer: D. Improved distribution system and storage tank management to reduce residence time
The (b)(2)(iii) table, which "applies only to the disinfected water that consecutive systems buy or otherwise receive", lists "Systems serving <10,000: Improved distribution system and storage tank management to reduce residence time".
Source: eCFR: 40 CFR 141.64, Maximum contaminant levels for disinfection byproducts (as of Nov 3, 2026)
15. What does 141.64(a)(2) identify as the best available technology for bromate?
- A. Enhanced coagulation plus GAC10
- B. Control of ozone treatment process to reduce production of bromate
- C. Use of chloramines for disinfectant residual maintenance
- D. Packed tower aeration
Show answer
Answer: B. Control of ozone treatment process to reduce production of bromate
The BAT table lists for bromate: "Control of ozone treatment process to reduce production of bromate".
Source: eCFR: 40 CFR 141.64, Maximum contaminant levels for disinfection byproducts (as of Nov 3, 2026)
16. For community and non-transient, non-community water systems, what compliance date does 141.64(a)(1) set for the bromate and chlorite MCLs for a subpart H system serving fewer than 10,000 persons?
- A. January 1, 2002
- B. December 8, 2003
- C. January 1, 2004
- D. April 1, 2016
Show answer
Answer: C. January 1, 2004
"Subpart H systems serving fewer than 10,000 persons and systems using only ground water not under the direct influence of surface water must comply with this paragraph (a) beginning January 1, 2004."
Source: eCFR: 40 CFR 141.64, Maximum contaminant levels for disinfection byproducts (as of Nov 3, 2026)
Maximum residual disinfectant levels
17. What does 141.65(c) identify as the best technology, treatment techniques, or other means available for achieving compliance with the MRDLs?
- A. Control of treatment processes to reduce disinfectant demand and control of disinfection treatment processes to reduce disinfectant levels
- B. Enhanced coagulation or enhanced softening, plus GAC10
- C. Control of ozone treatment process to reduce production of bromate
- D. Packed tower aeration
Show answer
Answer: A. Control of treatment processes to reduce disinfectant demand and control of disinfection treatment processes to reduce disinfectant levels
141.65(c) identifies "control of treatment processes to reduce disinfectant demand and control of disinfection treatment processes to reduce disinfectant levels."
Source: eCFR: 40 CFR 141.65, Maximum residual disinfectant levels (as of Nov 3, 2026)
18. Under 141.65(b)(2), which transient non-community water systems must comply with the chlorine dioxide MRDL?
- A. All transient non-community systems, whatever disinfectant they use
- B. Only those using chloramines
- C. Only those serving 10,000 or more persons that use chlorine
- D. Those using chlorine dioxide as a disinfectant or oxidant
Show answer
Answer: D. Those using chlorine dioxide as a disinfectant or oxidant
Each compliance date in 141.65(b)(2) covers systems "using chlorine dioxide as a disinfectant or oxidant", which "must comply with the chlorine dioxide MRDL".
Source: eCFR: 40 CFR 141.65, Maximum residual disinfectant levels (as of Nov 3, 2026)
Radionuclides
19. Under 141.66(b), how is the combined radium-226 and radium-228 value determined?
- A. By taking the higher of the two results
- B. By averaging the two results
- C. By adding the results of the radium-226 analysis and the radium-228 analysis
- D. By using the radium-226 result alone
Show answer
Answer: C. By adding the results of the radium-226 analysis and the radium-228 analysis
"The combined radium-226 and radium-228 value is determined by the addition of the results of the analysis for radium-226 and the analysis for radium-228."
Source: eCFR: 40 CFR 141.66, Maximum contaminant levels for radionuclides (as of Nov 3, 2026)
20. Table A in 141.66(d) gives average annual concentrations assumed to produce a total body or organ dose of 4 mrem/yr. Which radionuclides does it list?
- A. Radium-226 and radium-228
- B. Tritium and strontium-90
- C. Uranium and radon
- D. Gross alpha emitters and radium-226
Show answer
Answer: B. Tritium and strontium-90
Table A, "Average annual concentrations assumed to produce: a total body or organ dose of 4 mrem/yr", has two rows: "Tritium Total body 20,000" and "Strontium-90 Bone Marrow 8" (pCi per liter). For other man-made radionuclides the concentration is calculated under 141.66(d)(2).
Source: eCFR: 40 CFR 141.66, Maximum contaminant levels for radionuclides (as of Nov 3, 2026)