MCLs, MCLGs, and SMCLs — How EPA Sets Drinking Water Limits
Three acronyms come up on every water operator exam and in every regulatory conversation: MCL, MCLG, and SMCL. They look similar, they sound similar, and operators routinely confuse them. They mean three very different things — one is enforceable, one is aspirational, and one is essentially a recommendation. Knowing the distinction is the foundation for reading any drinking water regulation.
This guide walks through what each term means, how EPA actually arrives at the numbers, and the exam patterns that test these distinctions.
TL;DR
- MCL (Maximum Contaminant Level) is the enforceable, health-based legal limit for a contaminant in finished drinking water. Exceeding the MCL triggers a violation, public notification, and corrective action.
- MCLG (Maximum Contaminant Level Goal) is a non-enforceable, purely health-based target — the level below which no known or anticipated adverse health effects occur. For carcinogens, MCLGs are typically zero.
- SMCL (Secondary Maximum Contaminant Level) is a non-enforceable aesthetic guideline for contaminants that affect taste, odor, color, or appearance but not health. Federal recommendations only — some states adopt them as enforceable.
- The relationship: MCLG is the ideal; MCL is the legal limit set as close to MCLG as is technically and economically feasible; SMCL is a separate, lower-stakes category.
- EPA uses cost-benefit analysis and best available technology (BAT) determinations to set the gap between MCLG and MCL.
- Practice with the regulations practice test; related guides include the SDWA overview and the Lead and Copper Rule guide.
The three terms, side by side
| Term | What it is | Enforceable? | Basis |
|---|---|---|---|
| MCLG | Goal — level of no adverse health effect, with margin of safety | No | Health science only |
| MCL | Legal numeric limit in finished water | Yes — federal | Health + technical + economic feasibility |
| SMCL | Aesthetic recommendation | No federally | Taste, odor, color, appearance |
The progression EPA follows when regulating a new contaminant:
- Establish the MCLG — a purely health-based number. What concentration causes zero adverse health effects with a margin of safety?
- Determine best available technology (BAT) — what treatment methods, sampling techniques, and analytical methods are currently feasible?
- Set the MCL as close to the MCLG as is economically and technically feasible using BAT. This is the legally enforceable number.
For some contaminants, MCL = MCLG (e.g., arsenic MCLG was originally zero, then revised to consider feasibility — current MCL is 10 µg/L). For others, MCLG and MCL are the same number (e.g., nitrate MCL and MCLG are both 10 mg/L because nitrate isn't a carcinogen and there's a clear no-effect threshold).
For carcinogens with no known safe threshold, MCLG is zero. The MCL is then set wherever the cost-benefit math lands.
Why MCLG is zero for carcinogens
EPA's policy is that for proven or probable human carcinogens with no observable threshold, no exposure can be considered "safe." So the goal — the MCLG — is set to zero. The MCL is then set above zero because reaching zero in real water systems isn't practical (you can't detect zero, you can't treat to zero, and the cost would be infinite).
Examples of carcinogenic contaminants with MCLG = 0:
- Arsenic: MCLG = 0, MCL = 0.010 mg/L (10 µg/L)
- Benzene: MCLG = 0, MCL = 0.005 mg/L
- Vinyl chloride: MCLG = 0, MCL = 0.002 mg/L
- TCE (trichloroethylene): MCLG = 0, MCL = 0.005 mg/L
- Lead: MCLG = 0, MCL — none (regulated by treatment technique instead; action level 0.015 mg/L)
For non-carcinogenic threshold contaminants (where below a certain level there's truly no effect), MCLG is set above zero based on toxicology data with a safety margin.
Examples of non-carcinogenic contaminants with MCLG = MCL:
- Nitrate: MCLG = MCL = 10 mg/L (as nitrogen)
- Nitrite: MCLG = MCL = 1 mg/L
- Fluoride: MCLG = MCL = 4 mg/L
- Copper: MCLG = 1.3 mg/L (action level = 1.3 mg/L)
MCL — what operators actually have to meet
The MCL is the legally enforceable number. If your finished water exceeds the MCL — either at a single sample or as a running annual average (depending on the rule) — your system has a violation. You have to:
- Notify the state primacy agency, typically within 24-48 hours for acute contaminants and 30 days for non-acute ones
- Issue public notification at the tier the rule requires (Tier 1 within 24 hours for acute health risk; Tier 2 within 30 days for non-acute MCL violations; Tier 3 in the annual CCR for minor administrative violations)
- Implement corrective action — often a Compliance Schedule with the state, plus immediate treatment changes or alternative source use
- Return to compliance and document it
Common MCLs the exam expects operators to know:
| Contaminant | MCL | Category |
|---|---|---|
| Total coliform | Presence triggers RTCR response | Microbiological |
| E. coli | Presence = acute health violation | Microbiological |
| Turbidity (combined filter effluent) | 0.3 NTU 95% of time, never >1 NTU | Treatment technique |
| Free chlorine residual | ≥ 0.2 mg/L at entry, detectable in distribution | Treatment technique |
| TTHM (running annual avg) | 0.080 mg/L | DBP |
| HAA5 (running annual avg) | 0.060 mg/L | DBP |
| Arsenic | 0.010 mg/L | Inorganic |
| Nitrate (as N) | 10 mg/L | Inorganic (acute) |
| Nitrite (as N) | 1 mg/L | Inorganic (acute) |
| Fluoride | 4.0 mg/L | Inorganic |
| Lead (action level) | 0.015 mg/L | Treatment technique |
| Copper (action level) | 1.3 mg/L | Treatment technique |
Most of these come up directly on every level of operator exam. Class C and above is expected to know most or all of them.
SMCL — the aesthetic side
Secondary MCLs cover contaminants that affect aesthetics — taste, odor, color, or appearance — but don't directly affect health at typical levels. SMCLs are non-enforceable federally; EPA recommends them but states are not required to enforce them. Some states do adopt SMCLs as enforceable, especially for iron and manganese in distribution-system service quality regulations.
Common SMCLs operators encounter:
| Contaminant | SMCL | Aesthetic effect |
|---|---|---|
| Aluminum | 0.05–0.2 mg/L | Discoloration of finished water |
| Chloride | 250 mg/L | Salty taste; corrosion concerns |
| Color | 15 color units | Visible color |
| Copper | 1.0 mg/L | Metallic taste, blue-green staining |
| Corrosivity | Non-corrosive | Pipe corrosion |
| Fluoride | 2.0 mg/L | Dental fluorosis (cosmetic) |
| Foaming agents | 0.5 mg/L | Aesthetic foam |
| Iron | 0.3 mg/L | Rusty color, metallic taste, staining |
| Manganese | 0.05 mg/L | Black-to-brown color, staining |
| Odor | 3 TON (threshold odor number) | Odor |
| pH | 6.5–8.5 | Corrosion and taste |
| Silver | 0.10 mg/L | Skin discoloration |
| Sulfate | 250 mg/L | Salty taste, laxative effect |
| TDS | 500 mg/L | Mineral taste |
| Zinc | 5 mg/L | Metallic taste |
Note that some contaminants appear in both primary and secondary lists with different limits — fluoride is the canonical example. The primary MCL (4.0 mg/L) is health-based for skeletal fluorosis; the secondary SMCL (2.0 mg/L) is aesthetic for dental fluorosis (tooth discoloration in children).
How EPA actually sets MCLs
The process EPA follows when regulating a new contaminant under SDWA:
- Identification — through the Contaminant Candidate List (CCL) process or the Unregulated Contaminant Monitoring Rule (UCMR), EPA identifies contaminants that may pose a health risk.
- Risk assessment — toxicology, epidemiology, and exposure assessment determine what concentrations cause adverse health effects.
- MCLG determination — set the goal: zero for carcinogens, no-effect level + safety margin for non-carcinogens.
- Best available technology (BAT) determination — what treatment methods can actually remove this contaminant, at what cost?
- Cost-benefit analysis — EPA estimates national compliance costs and health benefits at various MCL levels.
- Proposed rule — EPA publishes a proposed MCL in the Federal Register and opens public comment.
- Final rule — after comments and revision, EPA promulgates the final MCL.
- Compliance schedule — typically 3+ years for systems to come into compliance.
The classic recent example: PFOA and PFOS (PFAS contaminants). EPA proposed MCLs of 4 parts per trillion in 2023 and finalized them in 2024 — among the lowest MCLs ever set, reflecting both the toxicity at very low levels and the development of detection capabilities to that level.
Action levels (the lead/copper exception)
Lead and copper don't have MCLs — they're regulated by treatment technique with action levels instead. An action level is a concentration that triggers required corrective action (corrosion control optimization, lead service line inventory, public education) but is not itself a violation.
- Lead action level: 0.015 mg/L (15 µg/L) at the 90th percentile of household tap samples
- Copper action level: 1.3 mg/L at the 90th percentile of household tap samples
Exceeding the action level doesn't mean the water at every tap is over 15 µg/L of lead — it means that at least 10% of sampled taps are over. The treatment technique response is required regardless.
This is a perennial exam trap: students confuse action level with MCL. Action levels are for treatment-technique-regulated contaminants; MCLs are for everything else.
Common exam pitfalls
Confusing MCL with MCLG. MCL is the legally enforceable number; MCLG is the health-based goal (often lower).
Confusing MCL with SMCL. Primary (MCL) is health-based and enforceable. Secondary (SMCL) is aesthetic and non-enforceable federally.
Confusing MCL with action level. Action levels apply only to lead and copper (and a few specific treatment-technique rules). Most contaminants have MCLs.
Thinking MCLG = 0 means MCL = 0. MCLGs for carcinogens are zero, but MCLs are set above zero based on feasibility. Lead MCLG = 0, but lead "action level" is 15 µg/L. Arsenic MCLG = 0, but MCL = 10 µg/L.
Forgetting that "running annual average" matters. TTHM and HAA5 MCLs are based on the locational running annual average (LRAA), not single samples. A single TTHM result of 0.085 mg/L isn't a violation if the LRAA stays below 0.080.
Treating SMCL violations as enforceable. Federally, they're not. Your state may have adopted some as enforceable — check before assuming.
Quick reference
- MCL: enforceable legal limit, health-based, in finished water
- MCLG: non-enforceable health goal; for carcinogens, equals zero
- SMCL: non-enforceable aesthetic guideline (federal recommendation)
- Action level: trigger for required treatment-technique response (lead, copper)
- TTHM MCL: 0.080 mg/L as LRAA
- HAA5 MCL: 0.060 mg/L as LRAA
- Arsenic MCL: 0.010 mg/L
- Lead action level: 0.015 mg/L (90th percentile)
- Nitrate MCL: 10 mg/L (as N) — acute, triggers immediate response if exceeded
Practice and next steps
- Free regulations practice test — 50 questions on MCLs, MCLGs, treatment techniques, and rule compliance.
- Free sampling practice test — sample collection for compliance monitoring.
- Safe Drinking Water Act overview — the parent law that authorizes all of these.
- Consumer Confidence Reports — where MCL violations appear in your annual report.
- Lead and Copper Rule guide — the action-level approach in detail.
- Disinfection byproducts explained — TTHM and HAA5 MCLs in operator terms.
MCL, MCLG, and SMCL are three letters apart but worlds apart in meaning. Operators who can hold the distinction straight pass regulation questions cold and read every rule's structure correctly.