SDWA Fundamentals — Quick Reference
Key Terms & Definitions
| Term | What It Means | Why It Matters |
|---|---|---|
| MCL | Maximum allowed contaminant level (enforceable) | Set as close to MCLG as feasible given cost and technology |
| MCLG | Health-based goal (non-enforceable) | Carcinogen MCLGs are always zero |
| Treatment Technique (TT) | Required process when an MCL isn't feasible | Turbidity limits, lead/copper action levels |
| MRDL | Maximum residual disinfectant level | Chlorine 4.0, chloramines 4.0, ClO2 0.8 mg/L |
| Action Level | Trigger for treatment (not an MCL) | Lead 0.015, copper 1.3 mg/L at 90th percentile |
| BAT | Best available technology for a given contaminant | EPA designates BAT when setting each MCL |
Public Water System Types
| Type | Definition | Regulated? |
|---|---|---|
| CWS (Community) | Same 25+ people year-round | Yes |
| NTNC (Nontransient-Noncommunity) | Same 25+ people, 6+ months/year | Yes |
| TNC (Transient-Noncommunity) | 25+ people, not the same population | Yes |
| Private wells (<15 connections, <25 people) | Not a public water system | No |
EPA's Three-Step Regulatory Test
A contaminant gets regulated only when all three criteria are met:
- May cause adverse health effects
- Known or likely to occur in public water systems
- Regulation presents a meaningful opportunity for health risk reduction
What to Watch on the Exam
- The MCL vs. MCLG distinction is a favorite exam question. MCLGs for carcinogens are always zero, but MCLs are set at feasible levels. Don't confuse them.
- Primacy means the state enforces the SDWA instead of EPA. California has had primacy since the beginning, through DDW under the State Water Resources Control Board. The catch is that state standards must be at least as stringent as federal, and California's often are more stringent.
- Variances vs. exemptions trip people up. A variance lets you exceed an MCL if you can't afford compliance (no unreasonable risk, must install BAT). An exemption gives temporary relief with a compliance schedule capped at 3 years.
- NPDWRs are enforceable. NSDWRs (secondary standards) are not enforceable at the federal level, but California makes its secondary standards enforceable. That's a California-specific detail worth remembering.
California Implementation Note
California administers the SDWA through the Division of Drinking Water (DDW) under the State Water Resources Control Board. State standards are often more stringent than federal, including enforceable secondary standards and California-specific MCLs for perchlorate (0.006 mg/L), hexavalent chromium (0.010 mg/L), and 1,2,3-TCP (0.000005 mg/L).