What to Do After a WET Test Failure: Understanding TRE Requirements in California
- Jul 15
- 4 min read
Justin (Lab Technician 1) Enumerating an Urchin Fertilization Bioassay Test
A Whole Effluent Toxicity (WET) test failure can be one of the most stressful compliance issues for facilities operating under an NPDES permit in California. Unlike routine monitoring exceedances, a toxicity failure indicates that a discharge may be harmful to aquatic life.
However, a failed test does not automatically mean enforcement action. Instead, it typically initiates a structured regulatory response designed to identify the cause and restore compliance—most commonly through a Toxicity Reduction Evaluation (TRE).
Understanding what happens next—and how to respond—is critical for minimizing regulatory risk and returning to compliance quickly.
What Constitutes a WET Test Failure?
A WET test failure in California generally occurs when test results exceed permit-defined toxicity limits. This may include:
Exceeding chronic toxicity endpoints (e.g., IC25, NOEC, or TUc limits)
Significant mortality or sublethal effects in test organisms
Statistically significant differences from control samples
Permit language varies, but most NPDES permits in California clearly define what constitutes a reportable toxicity exceedance.
Immediate Steps After a Toxicity Failure
When a whole effluent toxicity failure occurs, the first priority is to confirm the result and meet permit requirements.
1. Review Data Quality Confirm that:
All QA/QC criteria were met
Control performance was acceptable
No sampling or handling errors occurred
2. Conduct Required Retesting
Most permits require accelerated or confirmatory testing following a failure.
3. Notify the Appropriate Regulatory Agency
Facilities are typically required to notify their Regional Water Quality Control Board within a specified timeframe.
4. Evaluate Recent Operational Changes
Consider whether any recent changes may have contributed to toxicity, such as:
New chemicals or suppliers
Process upsets
Stormwater or influent variability
Understanding Toxicity Reduction Evaluations (TREs)
A Toxicity Reduction Evaluation (TRE) is a structured process required when toxicity persists or meets specific triggers under an NPDES permit.
The goal of a TRE is to:
Identify the source(s) of toxicity
Implement corrective actions
Demonstrate a return to compliance
TREs are not one-size-fits-all—they are tailored to the facility, discharge type, and suspected causes of toxicity.
The Role of Toxicity Identification Evaluations (TIEs)
Many TREs include a Toxicity Identification Evaluation (TIE), which is a series of controlled laboratory procedures used to determine the class of toxicant causing the effect.
A TIE may help identify whether toxicity is due to:
Metals (e.g., copper, zinc)
Ammonia
Chlorine or oxidants
Organic compounds
Surfactants
TIEs are often conducted in phases and provide critical information for targeting corrective actions.
Common Causes of WET Test Failures
Facilities conducting wastewater toxicity testing in California often encounter similar sources of toxicity:
Metals
Copper and zinc are among the most common contributors, especially in industrial and urban discharges.
Ammonia
Elevated ammonia can cause chronic toxicity, particularly in freshwater tests.
Chlorine and Disinfection Byproducts
Residual chlorine or chloramines can be highly toxic, even at low concentrations.
Organic Compounds
Solvents, hydrocarbons, and pesticides may contribute to toxicity, sometimes in combination.
Surfactants and Detergents
These can affect membrane integrity in aquatic organisms and are often overlooked.
Strategies for Resolving Toxicity
Addressing a WET test failure requires a combination of investigation and operational control.
Source Identification
Use TIE results, chemical data, and process knowledge to identify likely contributors.
Process Optimization
Adjust treatment processes to improve removal of suspected toxicants.
Chemical Management
Evaluate chemical usage, storage, and substitution options.
Pretreatment Controls
For facilities receiving industrial influent, strengthening pretreatment programs can reduce toxicity.
Ongoing Monitoring
Track improvements through follow-up bioassay testing and trend analysis.
Regulatory Outcomes and Returning to Compliance
If toxicity is successfully reduced:
Monitoring frequency may return to normal
TRE requirements may be closed out
Compliance status is restored
If toxicity persists:
Additional TRE steps may be required
Permit conditions may be modified
Enforcement actions may be considered
Early and proactive response is the most effective way to avoid escalation.
Key Takeaways
A WET test failure in California is a serious but manageable compliance issue
Prompt response, retesting, and evaluation are critical first steps
TREs and TIEs provide structured pathways to identify and resolve toxicity
Understanding common causes can significantly shorten resolution time
Maintaining consistent operations helps prevent recurring failures
References
United States Environmental Protection Agency (EPA).
Toxicity Reduction Evaluation (TRE) Guidance for Municipal Wastewater Treatment Plants (EPA/833B-99/002).
United States Environmental Protection Agency (EPA).
Toxicity Identification Evaluation (TIE) Methods for Characterizing Effluent Toxicity.
United States Environmental Protection Agency (EPA).
Short-Term Methods for Estimating the Chronic Toxicity of Effluents (EPA-821-R-02-013 & 014).
California State Water Resources Control Board.
NPDES Permit Program and Water Quality Regulations.
California State Water Resources Control Board.
Policy for Implementation of Toxics Standards (SIP).
California Environmental Laboratory Accreditation Program (ELAP).
Laboratory Accreditation Requirements.
Our Commitment
Our mission is to support California’s public agencies, utilities, and businesses by providing exceptional bioassay testing services that meet the highest scientific and regulatory standards.
Whether you’re preparing for a permit update, addressing toxicity concerns, or simply maintaining compliance, our team is here to help you stay ahead of the changing regulatory landscape.
For further details or support regarding California NPDES specifics, please contact us.
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