11.5 Secondary MCLs, Notification Levels & Response Levels
Key Takeaways
- Secondary Contaminants is a named sub-topic in the SWRCB treatment Expected Range of Knowledge, distinct from Primary Contaminants.
- California secondary MCLs under 22 CCR 64449 include color at 15 units, odor threshold at 3, turbidity at 5 units, iron at 0.3 mg/L, and manganese at 0.05 mg/L.
- Total dissolved solids has consumer acceptance ranges of 500 mg/L recommended, 1,000 mg/L upper, and 1,500 mg/L short term rather than a single value.
- Notification levels are non-regulatory health-based advisory levels for chemicals that have no MCL, and exceeding one requires notifying the system's governing body.
- As of October 29, 2025 the California notification level for both PFOA and PFOS is 4.0 ng/L, with response levels of 10 ng/L for PFOA and 40 ng/L for PFOS.
Two Separate Standards, Two Separate Purposes
The Expected Range of Knowledge lists Primary Contaminants and Secondary Contaminants as separate sub-topics, and the distinction is real:
| Primary MCL | Secondary MCL | |
|---|---|---|
| Protects | Health | Taste, odor, and appearance (aesthetics) |
| Federal status | Enforceable | Advisory only |
| California status | Enforceable | Enforceable |
| Violation response | Public notification by tier, corrective action | Notify the State Board, notify customers, develop a plan |
| Also called | - | Consumer acceptance contaminant levels |
[!IMPORTANT] California enforces secondary MCLs; the federal government does not. This is one of the most distinctive features of California drinking water regulation and a reliable exam item. A California system exceeding the 0.05 mg/L manganese secondary MCL has a real compliance obligation, whereas the same system in a state with only federal requirements would face only customer complaints.
The California Secondary MCLs (22 CCR 64449)
| Constituent | Secondary MCL |
|---|---|
| Color | 15 units |
| Odor - threshold | 3 (threshold odor number) |
| Turbidity | 5 units |
| Iron | 0.3 mg/L |
| Manganese | 0.05 mg/L |
| Aluminum | 0.2 mg/L |
| Copper | 1.0 mg/L |
| Zinc | 5.0 mg/L |
| Silver | 0.1 mg/L |
| Chloride | 500 mg/L (recommended range) |
| Sulfate | 500 mg/L (recommended range) |
| Foaming agents (MBAS) | 0.5 mg/L |
| Thiobencarb | 1 µg/L |
| Total dissolved solids | Range: 500 recommended / 1,000 upper / 1,500 short term (mg/L) |
| Specific conductance | Range: 900 / 1,600 / 2,200 µS/cm |
The Ranges
Chloride, sulfate, total dissolved solids, and specific conductance are expressed as consumer acceptance contaminant level ranges rather than single numbers:
- Recommended - the level at which most consumers are satisfied
- Upper - acceptable where a lower level is not reasonably achievable
- Short term - acceptable only for an existing system on a temporary basis while corrective action is planned
A system above the recommended level is not automatically in violation, but it must demonstrate that the recommended level is not reasonably achievable and it may need a waiver under 22 CCR 64449.2.
Why the Numbers Are Where They Are
| Constituent | Consumer experience at the secondary MCL |
|---|---|
| Iron 0.3 mg/L | Reddish-brown staining of laundry and fixtures, metallic taste |
| Manganese 0.05 mg/L | Black to purple-black staining; deposits in mains that release as black water |
| Copper 1.0 mg/L | Blue-green staining, bitter metallic taste |
| Color 15 units | Visible yellow-brown tint from natural organic matter or metals |
| Odor TON 3 | Detectable earthy, musty, or chlorinous odor to a sensitive panel at 60 °C |
| Zinc 5.0 mg/L | Astringent taste, milky appearance |
| TDS 500 mg/L | Salty or mineral taste; scaling in appliances |
Aesthetic exceedances drive customer perception of safety. A system that meets every primary MCL but delivers black water will be perceived as unsafe, and it will lose the public confidence it needs when a real health event occurs.
Notification Levels and Response Levels
California created a third tier for chemicals that have been detected in drinking water but for which no MCL has been adopted yet. This framework is unique among the states and is a major reason California regulatory questions differ from generic ABC-style questions.
| Term | Definition |
|---|---|
| Notification level (NL) | A non-regulatory, health-based advisory level established for contaminants in drinking water for which MCLs have not been established |
| Response level (RL) | Set higher than the NL; the recommended concentration at which water systems consider taking a source out of service or providing treatment |
What a System Must Do
- Exceeding an NL: notify the system's governing body (city council, board of directors) and inform the State Water Board; DDW also recommends customer notification.
- Exceeding an RL: under Health and Safety Code section 116378, the system must either remove the source from service, treat the water delivered, or provide public notification.
Current PFAS Notification and Response Levels
| PFAS | Notification level | Response level | Effective |
|---|---|---|---|
| PFOA | 4.0 ng/L | 10 ng/L | October 29, 2025 |
| PFOS | 4.0 ng/L | 40 ng/L | October 29, 2025 |
| PFHxS | 3.0 ng/L | 10 ng/L | October 29, 2025 |
| PFHxA | 1.0 µg/L | 10 µg/L | October 29, 2025 |
| PFBS | 500 ng/L | 5,000 ng/L | March 5, 2021 |
The October 2025 revision tightened the PFOA notification level from 5.1 ng/L and the PFOS notification level from 6.5 ng/L, both to 4.0 ng/L, while leaving the response levels unchanged.
[!TIP] Keep three PFAS number sets straight. (1) California notification levels - advisory, above triggers governing-body notification. (2) California response levels - above triggers source removal, treatment, or public notification under HSC 116378. (3) Federal MCLs for PFOA and PFOS at 4.0 ng/L, with the associated federal rule currently in flux. A California operator answers first from the state framework, because it has applied continuously since 2019 and is being tightened while the federal rule is contested.
Other notable California notification levels include 1,2,3-trichloropropane (which subsequently received an MCL of 0.005 µg/L), NDMA at 10 ng/L, 1,4-dioxane at 1 µg/L, and hexavalent chromium, which now carries a California MCL of 10 µg/L. California has repeatedly used the NL framework as a staging ground before adopting an MCL, so a chemical on the NL list today is a candidate for regulation tomorrow.
A California water system's finished water contains 0.09 mg/L manganese. What is the compliance situation?
A groundwater source is found to contain PFOS at 22 ng/L. Under the current California framework, what is required?
Total dissolved solids in a system's finished water is 780 mg/L. How is this evaluated under 22 CCR 64449?