Drinking water systems and definitions study guide
Subpart A of EPA's National Primary Drinking Water Regulations (40 CFR Part 141) sets the ground rules the rest of the part builds on: what counts as a public water system, which systems Part 141 covers, and what key terms such as community water system, GWUDI, MCL, MCLG and MRDL mean. It also states when variances and exemptions may not be granted, what a person must do before siting a new or expanded system, and when parts of the rule took effect. Thresholds like 15 service connections or 25 persons over 6 months decide which system type applies, so the exact words matter. Your State drinking water program may have its own requirements, so check them too. Every point links to the regulation and quotes its exact words.
Scope and coverage
Part 141 sets the national primary drinking water regulations under section 1412 of the Public Health Service Act, as amended by the Safe Drinking Water Act, and related regulations for public water systems.
“This part establishes primary drinking water regulations pursuant to section 1412 of the Public Health Service Act, as amended by the Safe Drinking Water Act (Pub. L. 93-523); and related regulations applicable to public water systems.”
A public water system provides water for human consumption to the public through pipes or, after August 5, 1998, other constructed conveyances, and has at least 15 service connections or regularly serves an average of at least 25 individuals daily at least 60 days out of the year.
“Public water system means a system for the provision to the public of water for human consumption through pipes or, after August 5, 1998, other constructed conveyances, if such system has at least fifteen service connections or regularly serves an average of at least twenty-five individuals daily at least 60 days out of the year.”
The system includes collection, treatment, storage and distribution facilities under the operator's control and used primarily with the system, and also collection or pretreatment storage facilities not under that control that are used primarily with the system.
“Such term includes: any collection, treatment, storage, and distribution facilities under control of the operator of such system and used primarily in connection with such system; and any collection or pretreatment storage facilities not under such control which are used primarily in connection with such system.”
A special irrigation district is not a public water system. Every public water system is either a community water system or a noncommunity water system.
“Such term does not include any "special irrigation district." A public water system is either a "community water system" or a "noncommunity water system."”
For a system that delivers water by a constructed conveyance other than a pipe, a connection does not count as a service connection if one of three listed conditions is met.
“Service connection, as used in the definition of public water system, does not include a connection to a system that delivers water by a constructed conveyance other than a pipe if:”
One of the options: the water is used only for purposes other than residential uses (drinking, bathing, cooking or similar uses).
“(1) The water is used exclusively for purposes other than residential uses (consisting of drinking, bathing, and cooking, or other similar uses);”
Another option: the State determines that alternative water giving the equivalent level of public health protection is provided for residential or similar uses for drinking and cooking.
“(2) The State determines that alternative water to achieve the equivalent level of public health protection provided by the applicable national primary drinking water regulation is provided for residential or similar uses for drinking and cooking; or”
The third option: the State determines that the water provided for residential or similar uses for drinking, cooking and bathing is centrally treated, or treated at the point of entry by the provider, a pass-through entity or the user, to the equivalent level of protection.
“(3) The State determines that the water provided for residential or similar uses for drinking, cooking, and bathing is centrally treated or treated at the point of entry by the provider, a pass-through entity, or the user to achieve the equivalent level of protection provided by the applicable national primary drinking water regulations.”
Part 141 applies to every public water system unless it meets all four conditions: only distribution and storage facilities (no collection or treatment), all water obtained from a regulated public water system that does not own or operate it, no water sold to any person, and not a carrier conveying passengers in interstate commerce.
“This part shall apply to each public water system, unless the public water system meets all of the following conditions: (a) Consists only of distribution and storage facilities (and does not have any collection and treatment facilities); (b) Obtains all of its water from, but is not owned or operated by, a public water system to which such regulations apply: (c) Does not sell water to any person; and (d) Is not a carrier which conveys passengers in interstate commerce.”
Not covered here
The full definition of special irrigation district, which refers to exclusion provisions in section 1401(4)(B) of the Act, is not summarized here. We only write what an official source states, so study these parts with your certifying program's materials.
System types and responsible parties
A community water system serves at least 15 service connections used by year-round residents or regularly serves at least 25 year-round residents.
“Community water system means a public water system which serves at least 15 service connections used by year-round residents or regularly serves at least 25 year-round residents.”
A non-community water system is any public water system that is not a community water system; it is either transient or non-transient.
“Non-community water system means a public water system that is not a community water system. A noncommunity water system is either a "transient non-community water system (TWS)" or a "non-transient non-community water system (NTNCWS)."”
A non-transient non-community water system (NTNCWS) is not a community water system and regularly serves at least 25 of the same persons over 6 months per year.
“Non-transient non-community water system or NTNCWS means a public water system that is not a community water system and that regularly serves at least 25 of the same persons over 6 months per year.”
A transient non-community water system (TWS) is a non-community water system that does not regularly serve at least 25 of the same persons over six months per year.
“Transient non-community water system or TWS means a non-community water system that does not regularly serve at least 25 of the same persons over six months per year.”
A seasonal system is a non-community water system that is not operated as a public water system year-round and starts up and shuts down at the beginning and end of each operating season.
“Seasonal system is a non-community water system that is not operated as a public water system on a year-round basis and starts up and shuts down at the beginning and end of each operating season.”
A wholesale system treats source water as needed to produce finished water and delivers some or all of it to another public water system.
“Wholesale system is a public water system that treats source water as necessary to produce finished water and then delivers some or all of that finished water to another public water system.”
A consecutive system receives some or all of its finished water from one or more wholesale systems, through a direct connection or through the distribution system of other consecutive systems.
“Consecutive system is a public water system that receives some or all of its finished water from one or more wholesale systems. Delivery may be through a direct connection or through the distribution system of one or more consecutive systems.”
Subpart H systems are public water systems that use surface water or ground water under the direct influence of surface water as a source and are subject to subpart H.
“Subpart H systems means public water systems using surface water or ground water under the direct influence of surface water as a source that are subject to the requirements of subpart H of this part.”
A supplier of water is any person who owns or operates a public water system.
“Supplier of water means any person who owns or operates a public water system.”
"State" means the State or Tribal agency with jurisdiction over public water systems; while a State or Tribe lacks primary enforcement responsibility, it means the EPA Regional Administrator.
“State means the agency of the State or Tribal government which has jurisdiction over public water systems. During any period when a State or Tribal government does not have primary enforcement responsibility pursuant to section 1413 of the Act, the term "State" means the Regional Administrator, U.S. Environmental Protection Agency.”
Not covered here
The size categories small, medium and large water system are defined in 141.2 for subpart I only and belong with the lead and copper rule. Your State drinking water program may have its own requirements, so check them too. We only write what an official source states, so study these parts with your certifying program's materials.
Water sources
Surface water is all water open to the atmosphere and subject to surface runoff.
“Surface water means all water which is open to the atmosphere and subject to surface runoff.”
Ground water under the direct influence of surface water (GWUDI) is water beneath the ground surface with significant occurrence of insects or other macroorganisms, algae, or large-diameter pathogens such as Giardia lamblia or Cryptosporidium, or with significant and relatively rapid shifts in turbidity, temperature, conductivity or pH that closely track climatological or surface water conditions.
“Ground water under the direct influence of surface water (GWUDI) means any water beneath the surface of the ground with significant occurrence of insects or other macroorganisms, algae, or large-diameter pathogens such as Giardia lamblia or Cryptosporidium, or significant and relatively rapid shifts in water characteristics such as turbidity, temperature, conductivity, or pH which closely correlate to climatological or surface water conditions.”
Direct influence is decided source by source using criteria set by the State, which may base its determination on site-specific water quality measurements and/or well construction and geology documentation with field evaluation.
“Direct influence must be determined for individual sources in accordance with criteria established by the State. The State determination of direct influence may be based on site-specific measurements of water quality and/or documentation of well construction characteristics and geology with field evaluation.”
Finished water is water introduced into the distribution system and intended for distribution and consumption without further treatment, except treatment needed to maintain water quality in the distribution system, such as booster disinfection or corrosion control chemicals.
“Finished water is water that is introduced into the distribution system of a public water system and is intended for distribution and consumption without further treatment, except as treatment necessary to maintain water quality in the distribution system (e.g., booster disinfection, addition of corrosion control chemicals).”
A flowing stream is a course of running water flowing in a definite channel.
“Flowing stream is a course of running water flowing in a definite channel.”
A lake/reservoir is a natural or man-made basin where water collects or is stored, which may or may not have a current or single direction of flow.
“Lake/reservoir refers to a natural or man made basin or hollow on the Earth's surface in which water collects or is stored that may or may not have a current or single direction of flow.”
Bank filtration uses a well to recover surface water that has naturally infiltrated into ground water through a river bed or bank(s).
“Bank filtration is a water treatment process that uses a well to recover surface water that has naturally infiltrated into ground water through a river bed or bank(s).”
Standards and goals
A maximum contaminant level (MCL) is the maximum permissible level of a contaminant in water delivered to any user of a public water system.
“Maximum contaminant level means the maximum permissible level of a contaminant in water which is delivered to any user of a public water system.”
A maximum contaminant level goal (MCLG) is the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur, and which allows an adequate margin of safety. MCLGs are nonenforceable health goals.
“Maximum contaminant level goal or MCLG means the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur, and which allows an adequate margin of safety. Maximum contaminant level goals are nonenforceable health goals.”
A maximum residual disinfectant level (MRDL) is a level of a disinfectant added for water treatment that may not be exceeded at the consumer's tap without an unacceptable possibility of adverse health effects.
“Maximum residual disinfectant level (MRDL) means a level of a disinfectant added for water treatment that may not be exceeded at the consumer's tap without an unacceptable possibility of adverse health effects.”
For chlorine and chloramines, a system complies with the MRDL when the running annual average of monthly averages of distribution system samples, computed quarterly, is at or below the MRDL.
“For chlorine and chloramines, a PWS is in compliance with the MRDL when the running annual average of monthly averages of samples taken in the distribution system, computed quarterly, is less than or equal to the MRDL.”
For chlorine dioxide, a system complies with the MRDL when daily samples are taken at the entrance to the distribution system and no two consecutive daily samples exceed the MRDL.
“For chlorine dioxide, a PWS is in compliance with the MRDL when daily samples are taken at the entrance to the distribution system and no two consecutive daily samples exceed the MRDL.”
Notwithstanding the MRDLs in 141.65, operators may raise chlorine or chloramine residuals (but not chlorine dioxide) in the distribution system, to a level and for a time necessary to protect public health, to address specific microbiological contamination problems such as line breaks, storm runoff events, source water contamination or cross-connections.
“Notwithstanding the MRDLs listed in § 141.65, operators may increase residual disinfectant levels of chlorine or chloramines (but not chlorine dioxide) in the distribution system to a level and for a time necessary to protect public health to address specific microbiological contamination problems caused by circumstances such as distribution line breaks, storm runoff events, source water contamination, or cross-connections.”
A maximum residual disinfectant level goal (MRDLG) is a nonenforceable health goal that does not reflect the benefit of adding the chemical for microbial control.
“MRDLGs are nonenforceable health goals and do not reflect the benefit of the addition of the chemical for control of waterborne microbial contaminants.”
"Action level" is defined for subpart I only: the lead or copper concentrations in 141.80(c) that determine what subpart I requires. The definition as it now reads sets a lead action level of 0.010 mg/L and a copper action level of 1.3 mg/L. That wording belongs to the 2024-revised subpart I; see the last point of the dates topic below for its November 1, 2027 compliance date.
“Action level, for the purpose of subpart I of this part only, means the concentrations of lead or copper in water as specified in § 141.80(c) which determines requirements under subpart I of this part. The lead action level is 0.010 mg/L and the copper action level is 1.3 mg/L.”
Best available technology (BAT) is what the Administrator finds available after examination for efficacy under field conditions, not only in the laboratory, taking cost into consideration. For MCLs for synthetic organic chemicals, any BAT must be at least as effective as granular activated carbon.
“Best available technology or BAT means the best technology, treatment techniques, or other means which the Administrator finds, after examination for efficacy under field conditions and not solely under laboratory conditions, are available (taking cost into consideration). For the purposes of setting MCLs for synthetic organic chemicals, any BAT must be at least as effective as granular activated carbon.”
For PFAS, a mixture Hazard Index greater than 1 (unitless) is an exceedance of the MCL. The PFAS compliance dates are in the dates topic below.
“For PFAS, a mixture Hazard Index greater than 1 (unitless) is an exceedance of the MCL.”
Not covered here
141.2 has no general definition of "treatment technique". The individual MCL, MRDL and MCLG values are set in subpart G and are not listed here. We only write what an official source states, so study these parts with your certifying program's materials.
Treatment and monitoring terms
A compliance cycle is the nine-year calendar year cycle during which public water systems must monitor, made up of three three-year compliance periods. The first three cycles ran 1993–2001, 2002–2010 and 2011–2019 (each from January 1 to December 31).
“Compliance cycle means the nine-year calendar year cycle during which public water systems must monitor. Each compliance cycle consists of three three-year compliance periods. The first calendar year cycle begins January 1, 1993 and ends December 31, 2001; the second begins January 1, 2002 and ends December 31, 2010; the third begins January 1, 2011 and ends December 31, 2019.”
A locational running annual average (LRAA) is the average of results for samples taken at one monitoring location during the previous four calendar quarters.
“Locational running annual average (LRAA) is the average of sample analytical results for samples taken at a particular monitoring location during the previous four calendar quarters.”
"Near the first service connection" means one of the 20 percent of all service connections nearest the treatment facility, measured by water transport time in the distribution system.
“Near the first service connection means at one of the 20 percent of all service connections in the entire system that are nearest the water supply treatment facility, as measured by water transport time within the distribution system.”
A sanitary survey is an onsite review of the water source, facilities, equipment, operation and maintenance of a public water system to evaluate their adequacy for producing and distributing safe drinking water.
“Sanitary survey means an onsite review of the water source, facilities, equipment, operation and maintenance of a public water system for the purpose of evaluating the adequacy of such source, facilities, equipment, operation and maintenance for producing and distributing safe drinking water.”
Conventional filtration treatment is coagulation, flocculation, sedimentation and filtration resulting in substantial particulate removal.
“Conventional filtration treatment means a series of processes including coagulation, flocculation, sedimentation, and filtration resulting in substantial particulate removal.”
Direct filtration includes coagulation and filtration but excludes sedimentation.
“Direct filtration means a series of processes including coagulation and filtration but excluding sedimentation resulting in substantial particulate removal.”
Slow sand filtration passes raw water through a bed of sand at low velocity (generally less than 0.4 m/h), removing particulates by physical and biological mechanisms.
“Slow sand filtration means a process involving passage of raw water through a bed of sand at low velocity (generally less than 0.4 m/h) resulting in substantial particulate removal by physical and biological mechanisms.”
A Level 1 assessment is an evaluation to identify the possible presence of sanitary defects, defects in distribution system coliform monitoring practices, and (when possible) the likely reason the system triggered the assessment. It is conducted by the system operator or owner.
“Level 1 assessment is an evaluation to identify the possible presence of sanitary defects, defects in distribution system coliform monitoring practices, and (when possible) the likely reason that the system triggered the assessment. It is conducted by the system operator or owner.”
A Level 2 assessment provides a more detailed examination of the system (including its monitoring and operational practices) than a Level 1 assessment. It is conducted by an individual approved by the State, which may include the system operator.
“A Level 2 assessment provides a more detailed examination of the system (including the system's monitoring and operational practices) than does a Level 1 assessment through the use of more comprehensive investigation and review of available information, additional internal and external resources, and other relevant practices. It is conducted by an individual approved by the State, which may include the system operator.”
Not covered here
CT and disinfectant contact time, the membrane, bag and cartridge filter definitions, the PFAS chemical identities, and the subpart I sampling and service line terms are not summarized here. We only write what an official source states, so study these parts with your certifying program's materials.
Variances, siting and effective dates
Variances or exemptions from certain provisions may be granted by the entity with primary enforcement responsibility under sections 1415 and 1416 of the Act and subpart K of part 142 (small system variances), except that variances or exemptions from the total coliform and E. coli MCLs, and variances from subpart H treatment technique requirements, may not be granted.
“Variances or exemptions from certain provisions of these regulations may be granted pursuant to sections 1415 and 1416 of the Act and subpart K of part 142 of this chapter (for small system variances) by the entity with primary enforcement responsibility, except that variances or exemptions from the MCLs for total coliforms and E. coli and variances from any of the treatment technique requirements of subpart H of this part may not be granted.”
A note to 141.4(a) says that, as provided in 142.304(a), small system variances are not available for rules addressing microbial contaminants, which would include subparts H, P, S, T, W and Y.
“As provided in § 142.304(a), small system variances are not available for rules addressing microbial contaminants, which would include subparts H, P, S, T, W, and Y of this part.”
EPA stayed the effective date of 141.4 for the total coliform MCL of 141.63(a) only for systems that demonstrate to the State that the violation is due to persistent growth of total coliforms in the distribution system rather than fecal or pathogenic contamination, a treatment lapse or deficiency, or an operation or maintenance problem. The stay ran until March 31, 2016, when the total coliform MCL stopped being effective.
“EPA has stayed the effective date of this section relating to the total coliform MCL of § 141.63(a) for systems that demonstrate to the State that the violation of the total coliform MCL is due to a persistent growth of total coliforms in the distribution system rather than fecal or pathogenic contamination, a treatment lapse or deficiency, or a problem in the operation or maintenance of the distribution system. This is stayed until March 31, 2016, at which time the total coliform MCL is no longer effective.”
Before a person enters into a financial commitment for or starts construction of a new public water system, or increases an existing system's capacity, the person must notify the State and, to the extent practicable, avoid sites with the listed risks.
“Before a person may enter into a financial commitment for or initiate construction of a new public water system or increase the capacity of an existing public water system, he shall notify the State and, to the extent practicable, avoid locating part or all of the new or expanded facility at a site which:”
One listed risk: a site subject to a significant risk from earthquakes, floods, fires or other disasters that could cause a breakdown of the system or part of it.
“(a) Is subject to a significant risk from earthquakes, floods, fires or other disasters which could cause a breakdown of the public water system or a portion thereof; or”
The other listed risk: except for intake structures, a site within the floodplain of a 100-year flood or lower than any recorded high tide where records exist. EPA will not seek to override State or local land use decisions on siting.
“(b) Except for intake structures, is within the floodplain of a 100-year flood or is lower than any recorded high tide where appropriate records exist. The U.S. Environmental Protection Agency will not seek to override land use decisions affecting public water systems siting which are made at the State or local government levels.”
Except as provided in 141.6(b) through (l), the Part 141 regulations took effect on June 24, 1977.
“Except as provided in paragraphs (b) through (l) of this section the regulations set forth in this part take effect on June 24, 1977.”
The arsenic MCL in 141.62 became effective for the purpose of compliance on January 23, 2006.
“The arsenic maximum contaminant levels (MCL) listed in § 141.62 is effective for the purpose of compliance on January 23, 2006.”
The PFAS regulations in subpart Z are effective June 25, 2024; reporting under the consumer confidence and public notification rules is required April 26, 2027 (except 141.203 notices of MCL violations); and the compliance date for the PFAS MCLs and their 141.203 violation notices is April 26, 2029.
“The regulations pertaining to the per- and polyfluoroalkyl substances (PFAS) chemicals set forth in subpart Z of this part are effective June 25, 2024. See § 141.900 for the compliance dates for provisions under subpart Z. Compliance with reporting requirements under subpart Z, in accordance with subparts O (the consumer confidence rule) and Q (the public notification rule) of this part are required on April 26, 2027, except for notification requirements in § 141.203 related to violations of the MCLs. The compliance date for the PFAS MCLs in § 141.61, as specified in § 141.60, and for § 141.203 notifications of violations of the PFAS MCLs is April 26, 2029.”
Lead and copper (subpart I): under the 2024-revised text, community and non-transient non-community water systems must comply with subpart I no later than November 1, 2027, except where 141.81, 141.84, 141.85, 141.86 or 141.90 say otherwise or where the Administrator has issued an exemption under 40 CFR part 142, subpart C or F.
“(3) Community water systems and non-transient non-community water systems must comply with the requirements of this subpart no later than November 1, 2027, except where otherwise specified in §§ 141.81, 141.84, 141.85, 141.86, and 141.90, or where an exemption in accordance with 40 CFR part 142, subpart C or F, has been issued by the Administrator.”
Not covered here
The older effective dates in 141.6(b) through (h) and (k), and the variance and exemption procedures in Part 142, are not summarized here. What 141.80(a)(4) requires of systems until November 1, 2027 is described in the lead and copper study guide; the July 1, 2020 text it points to is not in our snapshot. We only write what an official source states, so study these parts with your certifying program's materials.
Sources
- eCFR: 40 CFR 141.1, Applicability (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 40 CFR 141.2, Definitions (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 40 CFR 141.3, Coverage (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 40 CFR 141.4, Variances and exemptions (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 40 CFR 141.5, Siting requirements (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 40 CFR 141.6, Effective dates (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 40 CFR 141.80, General requirements and action level (as of Nov 3, 2026)Checked November 6, 2026
Other Drinking water operator study guides: Maximum contaminant levels, Total coliform (RTCR), Filtration and disinfection, Disinfectant residuals and byproducts, Lead and copper, Public notification, Secondary standards and lead free plumbing