1.4 Safe Drinking Water Standards, MCLs & Emerging Contaminants

Statutory Framework: Federal vs. California Safe Drinking Water Act

Public drinking water supplies are co-regulated by the federal government under the Safe Drinking Water Act (SDWA) of 1974 (amended in 1986 and 1996) and the State of California under the California Safe Drinking Water Act (California Health and Safety Code, Division 104, Part 12, Chapter 4). The State Water Resources Control Board Division of Drinking Water (DDW) administers primacy authority on behalf of the US EPA.

[!IMPORTANT] The Strictness Principle: Under state law, California drinking water standards must be at least as stringent as federal US EPA standards. In many cases, California establishes lower numerical limits or regulates emerging contaminants years ahead of federal rulemaking (e.g., hexavalent chromium, perchlorate, 1,2,3-trichloropropane [1,2,3-TCP], and PFAS).

                                  Risk Analysis
                                        │
                                        ▼
                         ┌─────────────────────────────┐
                         │   OEHHA Public Health Goal  │
                         │   (PHG) - Zero-Risk Level   │
                         │   (Non-Enforceable Target)  │
                         └──────────────┬──────────────┘
                                        │
                      Consider Treatment Feasibility, Analytical
                      Detection Limits (DLR), and Economic Cost
                                        │
                                        ▼
                         ┌─────────────────────────────┐
                         │    California Primary MCL   │
                         │  (Legally Enforceable Max) │
                         └─────────────────────────────┘

The Hierarchy of Drinking Water Quality Limits

1. Maximum Contaminant Level Goal (MCLG) / Public Health Goal (PHG):
   • MCLGs are federal targets established by the US EPA; PHGs are California state targets established by the Office of Environmental Health Hazard Assessment (OEHHA).
   • Status: Non-enforceable aspirational health goals.
   • Basis: Calculated exclusively from toxicological and epidemiological risk assessments (representing the concentration in drinking water that poses no significant health risk over a lifetime of exposure). For known carcinogens (e.g., arsenic, benzene, bromate), the PHG is frequently set to zero or near-zero part-per-trillion levels.
2. Primary Maximum Contaminant Level (Primary MCL):
   • Status: Legally enforceable regulatory standard.
   • Basis: Established by DDW as close to the PHG as feasible, taking into account the best available treatment technology (BAT), analytical detection limits for reporting (DLR), and the economic cost of compliance to public water systems.
3. Secondary Maximum Contaminant Level (Secondary MCL / SMCL):
   • Status: Enforceable in California for all community water systems.
   • Basis: Established to protect the aesthetic quality and consumer acceptance of drinking water (preventing objectionable taste, odor, color, staining of laundry/plumbing fixtures, foaming, or excessive mineral scaling).

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Master Summary of Major Regulated Contaminants

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Secondary Maximum Contaminant Levels (SMCLs)

Under Title 22 CCR §64449, California community water systems must monitor for secondary contaminants. For total dissolved solids and specific conductance, California utilizes a three-tier standard (Recommended, Upper, and Short-Term limits):

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Disinfection and Disinfection Byproducts Rules (Stage 1 & Stage 2 D/DBPR)

When chemical disinfectants (primarily free chlorine) react with naturally occurring organic matter (Total Organic Carbon / TOC) in raw surface water, they produce Disinfection Byproducts (DBPs) linked to bladder cancer and reproductive harm.

   Disinfectant Dose                   Raw Water NOM                     Disinfection Byproducts
┌──────────────────────┐           ┌─────────────────────┐              ┌───────────────────────┐
│  Free Chlorine       │     +     │ Natural Organic     │   ──────►    │ TTHM (MCL = 80 µg/L)  │
│  (HOCl / OCl-)       │           │ Matter (NOM / TOC)  │              │ HAA5 (MCL = 60 µg/L)  │
└──────────────────────┘           └─────────────────────┘              └───────────────────────┘

Maximum Residual Disinfectant Levels (MRDLs)

• Chlorine (Free): MRDL = 4.0 mg/L (Running Annual Average)
• Chloramines (Combined): MRDL = 4.0 mg/L (Running Annual Average)
• Chlorine Dioxide (ClO₂): MRDL = 0.8 mg/L (Daily monitoring at plant point of entry)

Regulated Disinfection Byproduct Limits

• Total Trihalomethanes (TTHM): MCL = 0.080 mg/L (80 µg/L). Sum of chloroform, bromoform, bromodichloromethane, and dibromochloromethane.
• Haloacetic Acids (HAA5): MCL = 0.060 mg/L (60 µg/L). Sum of monochloroacetic, dichloroacetic, trichloroacetic, monobromoacetic, and dibromoacetic acids.
• Bromate (BrO₃⁻): MCL = 0.010 mg/L (10 µg/L) (Formed when ozone reacts with raw water bromide).
• Chlorite (ClO₂⁻): MCL = 1.0 mg/L (Byproduct of chlorine dioxide disinfection).

Compliance Calculation: The LRAA Framework

Under the Stage 2 D/DBPR, compliance is calculated using the Locational Running Annual Average (LRAA):

$\text{LRAA} = \frac{\text{Q}_1 + \text{Q}_2 + \text{Q}_3 + \text{Q}_4}{4}$

[!WARNING] Stage 1 RAA vs. Stage 2 LRAA: Under the obsolete Stage 1 Rule, utilities averaged results across all monitoring sites system-wide (Running Annual Average). Under Stage 2, each individual monitoring location must independently maintain an LRAA ≤ 80 µg/L for TTHM and ≤ 60 µg/L for HAA5. An exceedance at even one high-water-age sampling point constitutes a non-compliant Primary MCL violation.

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Lead and Copper Rule (LCR) & 90th Percentile Action Levels

Lead and copper enter drinking water primarily through the corrosion of interior plumbing materials, lead service lines (LSLs), and brass fixtures rather than source water contamination. Consequently, the Lead and Copper Rule regulates these metals through Action Levels (ALs) rather than traditional plant-effluent MCLs.

Action Levels & Monitoring Protocol

• Lead Action Level: 0.015 mg/L (15 µg/L / 15 ppb)
• Copper Action Level: 1.3 mg/L (1,300 µg/L / 1.3 ppm)
• Sampling Protocol: Requires first-draw 1-liter tap samples collected after water has stood motionless in customer plumbing for a minimum of 6 hours (stagnation period).

$\text{90th Percentile Concentration} = \text{Value at Rank } (0.90 \times N)$ (where N is the total number of valid compliance tap samples sorted in ascending order).

Mandatory Triggers upon Action Level Exceedance

If the 90th percentile concentration exceeds 15 ppb lead or 1.3 ppm copper, the utility has not committed an immediate violation of an MCL, but is legally mandated to execute specific corrective actions:

1. Water Quality Parameter (WQP) Monitoring: Intensive monitoring of pH, alkalinity, calcium, conductivity, and orthophosphate throughout the distribution network.
2. Corrosion Control Treatment (CCT): Installation or optimization of CCT (e.g., raising pH/alkalinity to form calcium carbonate scale or dosing zinc orthophosphate inhibitor).
3. Public Education (PE): Delivering mandatory lead public education notices to all consumers, schools, and healthcare facilities within 60 days.
4. Lead Service Line Replacement (LSLR): Implementing mandatory annual lead service line inventory and replacement programs.

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Revised Total Coliform Rule (RTCR): "Find and Fix" Framework

The Revised Total Coliform Rule (RTCR) treats total coliform bacteria as an operational indicator of distribution integrity, vulnerability, or potential pathogen pathways rather than an acute health threat, replacing the old total coliform MCL with a "Find and Fix" assessment protocol.

                            Routine Distribution Sample
                                        │
                                        ▼
                            Total Coliform Positive?
                                   /         \
                              YES /           \ NO
                                 ▼             ▼
                     Collect 3 Repeat Samples   In Compliance
                     + E. coli Lab Test
                                 │
               ┌─────────────────┴─────────────────┐
               │                                   │
               ▼                                   ▼
        E. coli Positive                  Total Coliform Only
               │                                   │
   ACUTE PRIMARY MCL VIOLATION           Assess Trigger Criteria:
   Tier 1 Public Notice (24 Hr)         - >5.0% Positives (>=40 samples)
   Level 2 Assessment Mandatory         - >=2 Positives (<40 samples)
                                                   │
                                        LEVEL 1 ASSESSMENT TRIGGERED

Coliform Compliance Rules & Assessments

1. Acute E. coli Primary MCL Violation: Triggered if:
   • An E. coli-positive repeat sample follows a total coliform-positive routine sample.
   • A total coliform-positive repeat sample follows an E. coli-positive routine sample.
   • A utility fails to collect all required repeat samples following an E. coli-positive routine sample.
   • Mandatory Action: Tier 1 Public Notification (Boil Water Advisory within 24 hours) and a mandatory Level 2 Assessment.
2. Level 1 Assessment: Triggered by operational vulnerabilities (e.g., >5.0% total coliform positive samples in a month for systems collecting ≥40 samples/month, or ≥2 positive samples for small systems). Conducted by the utility operator to inspect storage tanks, booster stations, main breaks, and sampling taps.
3. Level 2 Assessment: Triggered by an E. coli MCL violation or a second Level 1 assessment trigger within a rolling 12-month window. A comprehensive sanitary investigation conducted by DDW staff or a DDW-approved certified third party.

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California Emerging Contaminants: PFAS & Hexavalent Chromium

1. Per- and Polyfluoroalkyl Substances (PFAS)

PFAS are synthetic organofluorine chemical compounds ("forever chemicals") containing strong carbon-fluorine bonds, historically used in firefighting foams (AFFF), non-stick cookware, waterproof textiles, and industrial plating. California regulates two primary advisory tiers:

• Notification Level (NL): Health-based advisory level that triggers mandatory written notification to the utility's governing body (city council / board of directors) within 30 days.
  • PFOA (Perfluorooctanoic acid) NL: 5.1 ng/L (ppt)
  • PFOS (Perfluorooctanesulfonic acid) NL: 6.5 ng/L (ppt)
• Response Level (RL): Advisory level at which DDW strongly recommends taking the contaminated drinking water well off-line or installing advanced treatment.
  • PFOA RL: 10 ng/L (ppt)
  • PFOS RL: 40 ng/L (ppt)
• Note: In 2024, the US EPA promulgated National Primary Drinking Water Regulations establishing enforceable federal Primary MCLs of 4.0 ppt for PFOA and PFOS, which California utilities must achieve within the federal multi-year compliance schedule.

2. Hexavalent Chromium (Chromium-6 / Cr-VI)

While total chromium has a California Primary MCL of 50 µg/L (stricter than the federal 100 µg/L limit), California established a dedicated state Primary MCL for Hexavalent Chromium of 10 µg/L (10 ppb), reflecting OEHHA's carcinogenic risk assessments. Water systems must comply through granular activated carbon, ion exchange (strong base or weak base), or reduction-coagulation-filtration (RCF) treatment.