17.1 Safe Drinking Water Act (SDWA) Framework, MCLs & MCLGs

Legislative Foundation of the Safe Drinking Water Act

The Safe Drinking Water Act (SDWA), originally enacted by Congress in 1974 as Public Law 93-523, serves as the cornerstone of federal drinking water regulation in the United States. Prior to the SDWA, federal drinking water oversight was governed primarily by the Public Health Service Act, which only regulated water provided on interstate carriers (trains, ships, and buses) to prevent the cross-border transmission of communicable waterborne illnesses like cholera and typhoid fever. Municipal water utilities operated under a fragmented patchwork of state and local standards that varied dramatically in technical rigor and analytical oversight.

The 1974 SDWA radically altered this regulatory landscape by directing the newly formed Environmental Protection Agency (EPA) to establish uniform national standards for all public water supplies, protecting public health from both natural microbiological pathogens and industrial chemical contaminants. Over subsequent decades, Congress enacted two comprehensive sets of amendments that transformed the act from a purely reactive monitoring regime into a proactive, multi-barrier public health framework:

1. The 1986 SDWA Amendments: Enacted to dramatically accelerate regulatory rulemaking, these amendments mandated that the EPA set enforceable standards for 83 specific contaminants within three years, establish a dynamic Contaminant Candidate List (CCL) reviewed every 5 years, and promulgate the landmark Surface Water Treatment Rule (SWTR) requiring filtration and disinfection for all surface water supplies and Ground Water Under the Direct Influence of surface water (GWUDI). The 1986 amendments also outlawed the use of lead solder, pipes, and flux in public water systems and residential plumbing, mandated wellhead protection programs to safeguard aquifers, and established enforceable monitoring rules for unregulated contaminants.
2. The 1996 SDWA Amendments: Representing a major paradigm shift toward risk-based prevention and financial sustainability, the 1996 amendments introduced the Drinking Water State Revolving Fund (DWSRF), providing low-interest federal capitalization grants for utility infrastructure upgrades. In addition, the amendments mandated Source Water Assessment Programs (SWAP) to delineate and protect watershed and recharge zones, instituted national guidelines for Water Treatment Operator Certification to ensure minimum operator competency across all 50 states, required utility Capacity Development (evaluating technical, managerial, and financial capabilities), and mandated that all community water systems deliver an annual water quality report—the Consumer Confidence Report (CCR)—directly to all billing consumers. Furthermore, the 1996 amendments required the EPA to balance health risk reductions against compliance costs using comprehensive Health Risk Reduction and Cost Analyses (HRRCA) when proposing new Maximum Contaminant Levels.

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Roles and Regulatory Jurisdiction: Federal EPA vs. State Primacy

The implementation and enforcement of drinking water standards under the SDWA operate as a cooperative federalism framework shared between the Federal EPA and state regulatory authorities:

Federal EPA Responsibilities

• Rulemaking and Standard Setting: Promulgates National Primary Drinking Water Regulations (NPDWRs), which apply uniformly across all U.S. states and territories. NPDWRs establish enforceable maximum concentration limits, mandatory treatment techniques, and standardized analytical testing methodologies.
• Toxicological and Risk Assessment: Conducts epidemiological and toxicological health assessments to establish reference doses, cancer potency factors, and non-enforceable health goals (MCLGs).
• Contaminant Candidate List (CCL) and Six-Year Review: Publishes a Contaminant Candidate List every 5 years identifying unregulated contaminants known or anticipated to occur in public water systems that may require future regulation. Evaluates existing regulations every six years (the Six-Year Review) to determine whether technological advances, analytical improvements, or updated health data justify making existing standards more stringent.
• Method Certification: Approves standardized laboratory analytical methods (EPA Methods, Standard Methods for the Examination of Water and Wastewater, and ASTM) that utilities and certified commercial laboratories must strictly utilize for compliance testing.

State Primacy (Primary Enforcement Responsibility)

Under Section 1413 of the SDWA, states, territories, and recognized tribal nations may apply to the EPA for Primacy (primary enforcement authority) over their public water systems. To obtain and maintain primacy, a state must demonstrate that it has:

• Adopted state drinking water statutes and administrative regulations that are at least as stringent as federal NPDWRs (states retain legal authority to adopt more stringent standards, but never less stringent ones).
• Established active enforcement procedures, compliance monitoring mechanisms, administrative penalty authorities, and variance/exemption review processes.
• Implemented a rigorous Sanitary Survey program requiring comprehensive on-site engineering and operational reviews of water facilities at least every 3 years for Community Water Systems and every 5 years for Non-Community Water Systems.
• Established a state-administered Operator Certification Program meeting EPA baseline guidelines, ensuring that treatment facilities and distribution networks are supervised by appropriately licensed personnel.
• Maintained an EPA-certified state environmental laboratory infrastructure capable of auditing and certifying municipal and commercial drinking water testing laboratories.

Currently, 49 of the 50 states possess primacy enforcement authority. The single exception is Wyoming, where EPA Region 8 exercises direct federal implementation and enforcement. The District of Columbia is also regulated directly by EPA Region 3.

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Classification of Public Water Systems (PWS)

A fundamental requirement on Class II certification examinations is understanding the statutory definition and classification of a Public Water System (PWS). Under 40 CFR Part 141.2, a system qualifies as a PWS if it satisfies either of the following criteria:

PWS Statutory Threshold:
>= 15 Service Connections  OR  >= 25 Individuals Served Regularly for >= 60 Days/Year

If a water system does not meet either the 15-connection or 25-person threshold for at least 60 days per calendar year, it is classified as a private supply and is entirely exempt from federal SDWA regulations. Once a system meets the definition of a PWS, it is categorized into one of three distinct regulatory classes, which dictate its monitoring frequencies and compliance obligations:

1. Community Water System (CWS)

• Definition: A public water system that supplies piped drinking water to at least 15 service connections used by year-round residents or regularly serves at least 25 year-round residents.
• Facility Examples: Municipal water utilities, county water authorities, residential subdivisions, mobile home parks, and apartment complexes operating dedicated wells.
• Regulatory Mandates: Subject to the full suite of SDWA regulations, including all chemical, radiological, and microbiological NPDWRs; the Lead and Copper Rule; Disinfection Byproduct rules; operator certification requirements; and mandatory annual distribution of Consumer Confidence Reports (CCRs) to all billing customers.

2. Non-Transient Non-Community Water System (NTNCWS)

• Definition: A public water system that is not a community water system and that regularly serves at least 25 of the same persons over at least 6 months per year, but where those consumers do not maintain permanent primary residency.
• Facility Examples: Public and private schools, universities, industrial manufacturing plants, corporate business parks, and daycare facilities that maintain their own independent drinking water supply (such as private groundwater wells).
• Regulatory Mandates: Because the same individuals consume this water consistently over extended durations, chronic toxicological risks apply. NTNCWSs must comply with all chemical MCLs (including VOCs, SOCs, and inorganics), radiological standards, the Revised Total Coliform Rule (RTCR), and the Lead and Copper Rule. However, NTNCWSs are exempt from preparing and distributing annual Consumer Confidence Reports.

3. Transient Non-Community Water System (TNCWS)

• Definition: A public water system that serves at least 25 individuals per day for at least 60 days per year, but where the individuals served do not remain for extended periods and change continuously.
• Facility Examples: Highway rest stops, campgrounds, state and national parks, gas stations, roadside diners, and seasonal ski lodges operating their own wells.
• Regulatory Mandates: Because consumers ingest this water only transiently (rarely more than a few hours or days), these systems pose minimal risk from chronic long-term toxicants like arsenic, trihalomethanes, or radionuclides. Consequently, regulatory oversight is restricted primarily to acute public health hazards: the Revised Total Coliform Rule (microbiological safety), Nitrate (10 mg/L as N), and Nitrite (1.0 mg/L as N). TNCWSs are exempt from lead and copper monitoring, organic chemical testing, and CCR distribution.

Table 17.1.1: Public Water System (PWS) Classifications & Regulatory Obligations

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Regulatory Standards Terminology: MCLG, MCL, TT, and AL

Operators must precisely differentiate between four distinct regulatory mechanisms used by the EPA to enforce drinking water safety:

Maximum Contaminant Level Goal (MCLG)

• Definition: A non-enforceable public health target established at the concentration level at which no known or anticipated adverse effects on human health will occur, allowing for an adequate margin of safety.
• Scientific Determination: MCLGs are purely health-based numbers derived by EPA toxicologists from toxicological studies and reference doses. For non-carcinogenic substances with an identifiable toxicological threshold, the MCLG is calculated based on an acceptable daily intake. For known or probable human carcinogens (such as benzene, vinyl chloride, arsenic, and bromate) and microbial pathogens (including Giardia lamblia, Cryptosporidium, enteric viruses, and Legionella), the EPA enforces a strict policy of setting the MCLG at exactly zero, reflecting the toxicological premise that no safe threshold of exposure exists.
• Enforceability: MCLGs carry no direct legal enforceability; utilities cannot be cited, fined, or issued violations solely for exceeding an MCLG.

Maximum Contaminant Level (MCL)

• Definition: The maximum permissible concentration of a contaminant delivered to any user of a public water system. The MCL is the legally enforceable primary drinking water standard.
• Development and Feasibility: Under Section 1412 of the SDWA, the EPA must establish the enforceable MCL as close to the MCLG as is technically and economically feasible. In setting an MCL, the agency evaluates Best Available Technology (BAT), treatment efficiencies, analytical laboratory detection capabilities (Practical Quantitation Limits, or PQLs), and the national economic costs versus health benefits. If a chemical has an MCLG of zero, the enforceable MCL is set at a low, measurable detection limit achievable by certified municipal treatment systems (e.g., benzene has an MCLG of zero but an enforceable MCL of 0.005 mg/L).

Treatment Technique (TT)

• Definition: An enforceable operational procedure, engineering control, or performance specification mandated by the EPA in lieu of an MCL when it is economically or technologically unfeasible to ascertain the concentration of a contaminant in field treated water.
• Application: Measuring microscopic protozoan pathogens (Giardia cysts and Cryptosporidium oocysts) or viruses in finished water requires complex, labor-intensive, and expensive laboratory concentration techniques that cannot provide real-time compliance feedback. Consequently, the EPA established Treatment Techniques under the Surface Water Treatment Rules mandating specified multi-barrier processes: continuous coagulation, chemical rapid mixing, flocculation, sedimentation, granular media filtration, and chemical disinfection to achieve verified log-removal and log-inactivation. Similarly, corrosion control treatment under the Lead and Copper Rule and enhanced coagulation for Total Organic Carbon (TOC) removal are Treatment Techniques.

Action Level (AL)

• Definition: A concentration threshold for a specific contaminant measured at the consumer's cold water kitchen tap that triggers mandatory operational, chemical, and public notification requirements.
• Application: The Lead and Copper Rule establishes Action Levels of 0.015 mg/L (15 µg/L) for Lead and 1.3 mg/L for Copper. Compliance is evaluated based on the 90th percentile level of targeted first-draw customer tap samples. An exceedance of an Action Level is not an immediate violation of an MCL, but it triggers mandatory statutory responses: comprehensive Water Quality Parameter (WQP) monitoring, lead public education within 60 days, optimal corrosion control treatment (CCT) installation, and, if exceedances persist, systematic Lead Service Line Replacement (LSLR).

Table 17.1.2: Regulatory Standards Terminology & Enforceability Framework

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National Secondary Drinking Water Regulations (NSDWRs / SMCLs)

Under Section 1412 of the SDWA, the EPA established National Secondary Drinking Water Regulations (NSDWRs), which define Secondary Maximum Contaminant Levels (SMCLs) for 15 parameters. Unlike NPDWRs, SMCLs are non-enforceable federal guidelines that address aesthetic qualities (taste, odor, color), cosmetic effects (skin discoloration, dental fluorosis), or technical interference (corrosion, pipe scaling, equipment staining). However, state primacy agencies possess the statutory authority to adopt SMCLs as legally enforceable state standards within their own jurisdictions.

Water treatment operators must know the specific SMCL thresholds and their operational manifestations, as customer complaints regarding secondary contaminants represent the primary interaction between the public and water treatment utilities:

Table 17.1.3: National Secondary Maximum Contaminant Levels (SMCLs) & Aesthetic Effects