2.1 Safe Drinking Water Act (SDWA) Primary & Secondary MCLs

2.1 Safe Drinking Water Act (SDWA) Primary & Secondary MCLs

The Safe Drinking Water Act (SDWA), originally enacted by Congress in 1974 and substantially amended in 1986 and 1996, forms the bedrock of public drinking water regulation across the United States. Under the SDWA, the federal government empowers the United States Environmental Protection Agency (EPA) to set national standards for drinking water quality and oversight. In Arizona, primary enforcement responsibility—known as primacy—is delegated to the Arizona Department of Environmental Quality (ADEQ) pursuant to Arizona Revised Statutes (A.R.S.) Title 49, Chapter 2, and the Arizona Administrative Code (A.A.C.) Title 18, Chapter 4. Certified water operators must master both federal statutory frameworks and Arizona-specific compliance mandates to ensure public health protection and regulatory compliance.

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Statutory Framework & Regulatory Architecture

The SDWA governs all Public Water Systems (PWS), defined as any system providing water for human consumption through pipes or other constructed conveyances that 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. The law categorizes public water systems into three distinct operational classifications:

1. Community Water System (CWS): A public water system that supplies water to the same population year-round (e.g., municipal water utilities, residential subdivisions, mobile home parks).
2. Non-Transient Non-Community Water System (NTNCWS): A public water system that regularly supplies water to at least 25 of the same people for at least six months per year, but not year-round (e.g., schools, rural commercial business parks, industrial manufacturing facilities with private wells).
3. Transient Non-Community Water System (TNCWS): A public water system that provides water in places where people do not remain for long periods of time (e.g., highway rest stops, national park campgrounds, rural gas stations).

NPDWRs vs. NSDWRs: Enforceability and Health Protection

The SDWA divides drinking water standards into two fundamental regulatory tiers:

• National Primary Drinking Water Regulations (NPDWRs): Legally enforceable federal and state standards codified under 40 CFR Part 141 and A.A.C. R18-4-101 et seq. These regulations apply directly to public water systems to limit contaminants that present known or suspected risks to human health.
• National Secondary Drinking Water Regulations (NSDWRs): Non-enforceable federal cosmetic, aesthetic, and technical guidelines codified under 40 CFR Part 143. These guidelines address contaminants that cause objectionable taste, odor, color, foaming, corrosion, or staining of plumbing fixtures. While secondary standards are not federally enforceable, ADEQ monitors them, and certain local utility ordinances or state operating permits may establish mandatory compliance triggers for specific aesthetic thresholds (such as salinity or corrosion control).

MCLGs, MCLs, and Treatment Techniques (TT)

Under the NPDWR framework, the EPA establishes standards through a multi-step toxicological and technological evaluation:

• Maximum Contaminant Level Goal (MCLG): The level of a contaminant in drinking water below which there is no known or expected risk to human health, incorporating an adequate margin of safety. MCLGs are purely health-based, aspirational goals and are non-enforceable. For known human carcinogens and microbial pathogens (such as Giardia lamblia, Cryptosporidium, and Legionella), the EPA sets the MCLG strictly at zero (0).
• Maximum Contaminant Level (MCL): The maximum permissible level of a contaminant in water delivered to any user of a public water system. The MCL is a legally enforceable standard set as close to the MCLG as feasible, taking into account Best Available Technology (BAT), laboratory analytical detection limits, and economic treatment feasibility.
• Treatment Technique (TT): An enforceable procedure or performance standard that public water systems must follow when it is not economically or technologically feasible to ascertain the precise concentration of a contaminant in a routine water sample (e.g., the Surface Water Treatment Rule's filter effluent turbidity standards or the Lead and Copper Rule's corrosion control protocols).

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Primary Inorganic Contaminants: Numerical Standards & Toxicology

Water utility operators in Arizona frequently encounter inorganic contaminants originating from natural mineral dissolution in regional alluvial aquifers, agricultural fertilizers, and historical industrial activities.

Nitrate, Nitrite, and Total Nitrogen

Nitrogen compounds represent acute chemical hazards in drinking water:

• Nitrate ($NO_3^-$ as N): MCL = 10 mg/L (or 45 mg/L when expressed as $NO_3^-$ ion).
• Nitrite ($NO_2^-$ as N): MCL = 1.0 mg/L (or 3.3 mg/L when expressed as $NO_2^-$ ion).
• Total Nitrate + Nitrite: MCL = 10 mg/L as N.

Toxicology: Nitrate ingestion is dangerous for infants under six months of age. In an infant's underdeveloped gastrointestinal system, higher gastric pH allows bacteria to reduce ingested nitrate ($NO_3^-$) to nitrite ($NO_2^-$). Nitrite absorbs into the bloodstream and binds with hemoglobin, oxidizing the iron atom from its normal ferrous state ($Fe^{2+}$) to the ferric state ($Fe^{3+}$). The resulting compound, methemoglobin, cannot bind or transport molecular oxygen to tissues, causing systemic hypoxia, cutaneous cyanosis, and potentially fatal suffocation—a clinical condition known as methemoglobinemia or "blue baby syndrome."

Arizona Sources & Treatment: High nitrate concentrations are common in agricultural basins across Maricopa, Pinal, and Yuma counties due to decades of synthetic nitrogen fertilizer use, livestock feedlots, and dense unsewered septic tank clusters. Treatment technologies include ion exchange using strong-base anion (SBA) resins in the chloride form, reverse osmosis (RO), and electrodialysis reversal (EDR). Blending high-nitrate well water with low-nitrate supplies is also widely practiced under ADEQ-approved blending plans.

Arsenic ($As$)

• Primary MCL = 0.010 mg/L (equivalent to 10 µg/L or parts per billion). Lowered from 50 µg/L in 2006.

Arizona Context & Occurrence: Arsenic is among Arizona's most pervasive compliance challenges. It occurs naturally throughout the state's Basin and Range aquifers due to the weathering of volcanic rocks, mineralized granites, and hydrothermal deposits. Elevated arsenic concentrations frequently exceed 10 to 50 µg/L in groundwater across the Verde Valley, Salt River Valley, Gila River Basin, Willcox Basin, and Safford Valley.

Toxicology & Treatment: Chronic ingestion of inorganic arsenic over decades causes thickening and discoloration of the skin (hyperkeratosis), peripheral neuropathy, vascular disease, and increased lifetime risks of bladder, lung, kidney, liver, and skin cancers. Arsenic exists in groundwater primarily as trivalent arsenite ($As(III)$, uncharged $H_3AsO_3$ at neutral pH) or pentavalent arsenate ($As(V)$, negatively charged $H_2AsO_4^-$ or $HAsO_4^{2-}$). Because arsenite is uncharged, it resists adsorption and ion exchange. Therefore, treatment trains must include a pre-oxidation step (using free chlorine, potassium permanganate, or ozone) to convert $As(III)$ to $As(V)$ prior to removal via coagulation/filtration with ferric chloride, granular ferric oxide/hydroxide (GFO/GFH) adsorption media, activated alumina, or reverse osmosis.

Fluoride ($F^-$)

Fluoride occupies a unique regulatory position under both primary and secondary standards:

• Primary MCL = 4.0 mg/L: Legally enforceable standard established to prevent skeletal fluorosis, a severe condition characterized by dense, brittle bones, calcified ligaments, and debilitating joint pain.
• Secondary Standard = 2.0 mg/L: Non-enforceable aesthetic guideline established to protect children during tooth development against dental fluorosis (cosmetic brown mottling, staining, and surface pitting of tooth enamel). Systems exceeding 2.0 mg/L must deliver a mandatory public notice advising parents of alternative water sources for young children.
• Optimal Fluoridation Level = 0.7 mg/L: Recommended target for community water fluoridation to promote dental caries prevention without adverse cosmetic impacts.

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The Lead and Copper Rule (LCR / LCRI)

Unlike contaminants that originate in source water, lead and copper enter drinking water primarily through the corrosion of interior plumbing fixtures, lead solder, brass fittings, and legacy lead service lines (LSLs). Because concentrations vary based on household plumbing materials and stagnation time, compliance is governed by an enforceable Treatment Technique (TT) rather than a plant effluent MCL.

Action Levels and 90th Percentile Calculation

The rule establishes health-based Action Levels (AL) evaluated at consumer taps:

• Lead Action Level = 0.015 mg/L (15 µg/L or ppb).
• Copper Action Level = 1.3 mg/L (1,300 µg/L or ppm).

Sampling Methodology: Compliance samples must be collected as first-draw, one-liter samples taken after the water has stood motionless in the interior plumbing and service lines for a minimum stagnation period of at least six hours. Sampling must target high-risk residential sites classified under Tier 1 criteria (homes with lead service lines, lead interior plumbing, or copper pipes with lead solder installed before local lead bans).

The 90th Percentile Evaluation: Systems arrange all valid sample results in ascending numerical order from lowest to highest. The value of the sample at the 90th percentile position determines compliance:

$\text{Rank Position} = 0.90 \times N$

(where $N$ is the total number of valid compliance tap samples collected)

If the 90th percentile concentration exceeds either 0.015 mg/L for lead or 1.3 mg/L for copper, the system does not receive an automatic civil violation, but triggers mandatory corrective actions:

1. Installation and optimization of Optimal Corrosion Control Treatment (OCCT), such as pH/alkalinity adjustment or orthophosphate corrosion inhibitors.
2. Source water monitoring and treatment.
3. Mandatory public education delivered within 60 days.
4. Accelerated lead service line inventorying and replacement programs.

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National Secondary Drinking Water Regulations (NSDWRs)

Secondary standards protect the aesthetic qualities of finished water, ensuring consumer acceptance and preventing premature degradation of distribution infrastructure and household appliances.

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Public Notification Rule (PNR) Tiers

When a public water system violates a primary drinking water standard, fails to monitor, or experiences an acute emergency, federal and Arizona administrative rules (A.A.C. R18-4-105) require public notification across three structured tiers based on public health severity.

Tier 1: 24-Hour Immediate Notice (Acute Hazards)

• Scope: Violations and operational conditions with significant potential to have serious, adverse effects on human health as a result of short-term, acute exposure.
• Triggers:
  • Violation of the Maximum Contaminant Level for E. coli.
  • Exceedance of the Nitrate (10 mg/L), Nitrite (1 mg/L), or Total Nitrate+Nitrite (10 mg/L) MCL.
  • Exceedance of the Maximum Residual Disinfectant Level (MRDL) for chlorine dioxide (0.8 mg/L) at the treatment plant entry point where one or more distribution samples also exceed the standard.
  • Acute failure of surface water filter turbidity standards where the single exceedance exceeds 1.0 NTU and the system fails to consult with ADEQ within 24 hours, or where ADEQ determines a Tier 1 notice is required.
  • Waterborne disease outbreak or other waterborne emergency (toxic chemical spill, backflow event, natural disaster).
• Mandatory Actions: Issue public notice via broadcast television/radio, reverse emergency calling, conspicuous posting, or hand delivery within 24 hours. Utility must consult with ADEQ within that same 24-hour window.

Tier 2: 30-Day Notice (Non-Acute Violations)

• Scope: Violations that have potential to cause adverse health effects over time, but do not pose an immediate acute risk.
• Triggers: All other non-acute NPDWR MCL exceedances (e.g., Arsenic, Total Trihalomethanes, Haloacetic Acids, Fluoride); all non-acute Treatment Technique violations (e.g., failure to meet enhanced coagulation TOC removal); failure to comply with variance or exemption schedules.
• Mandatory Actions: Provide written notice via direct mail or hand delivery to all billing accounts and affected consumers within 30 days. Notice must be repeated every three months for as long as the violation persists.

Tier 3: Annual Notice / Consumer Confidence Report (CCR)

• Scope: Administrative, testing, and operational infractions that do not directly represent elevated contaminant exposure or health risks.
• Triggers: Monitoring and reporting (M/R) violations (failure to collect or submit compliance samples on schedule); failure to comply with an approved testing procedure or analytical method; operation under a granted variance or exemption.
• Mandatory Actions: Deliver written notice to consumers within 12 months. Community water systems typically satisfy Tier 3 requirements by publishing the violation details in their annual Consumer Confidence Report (CCR), which must be distributed to all customers by July 1 each year.