How to Read Your WET Testing Results (Without a Toxicologist on Staff)
The Effluent Standard — Post 4 of 12
A practical walkthrough of the lab report: what to check first, what the numbers mean, and how to compare them to your permit.
A WET testing report usually arrives as a PDF somewhere between ten and fifty pages long. It contains tables of survival counts, a list of statistical tests, water chemistry logs, reference toxicant charts, and, somewhere near the front, a short statement of what the lab concluded. For many permit holders, that conclusion is the only part that gets read. If it says "pass," the report goes in the file. If it says anything else, the phone starts ringing.
That approach works until it doesn't. A result that looks like a pass can still carry warning signs, and a result that looks like a failure sometimes turns out to be a problem with the test rather than the effluent. Knowing how to read the full report puts you in a better position to catch both, and to ask your laboratory sharper questions.
This is the fourth post in The Effluent Standard. Earlier in the series we covered what WET testing is, the difference between acute and chronic tests, and the organisms behind the data. Here we put that background to work. You don't need a toxicology degree to review a report; you need to know three questions to ask, in order.
"Every WET report answers three questions in sequence: Was the test valid? What did it find? And how does that compare to my permit? Skip the first one and the other two don't mean much."
Question 1: Was the Test Valid?
Before looking at a single effluent result, check whether the test itself met its quality requirements. EPA's WET methods define test acceptability criteria: performance standards the control organisms must meet for the test to count. Controls are organisms held in clean laboratory dilution water with no effluent at all. If they don't survive and perform well, there is no reliable baseline to measure the effluent against.
The report should state the criteria and show that they were met. The most common ones for the standard species are:
Typical Test Acceptability Criteria (Controls)
Acute tests (fathead minnow, Daphnia): control survival of at least 90 percent
Ceriodaphnia dubia chronic: at least 80 percent control survival, an average of 15 or more young per surviving control female, and at least 60 percent of surviving control females producing three broods
Fathead minnow chronic (larval survival and growth): at least 80 percent control survival and a minimum average dry weight per surviving control larva, as specified in the method
If a control fails, the test is invalid and should be repeated. The effluent results from an invalid test shouldn't be used for compliance, whether they look good or bad.
Two other validity checks are worth finding in the report. The first is the reference toxicant test. The lab periodically exposes its organisms to a known toxicant, often a salt such as sodium chloride, and tracks the response on a control chart over time. A result inside the lab's historical range shows the organisms are healthy and responding as expected. The second is the sample documentation: collection date and time, arrival temperature, and the time between collection and test start. EPA methods set limits on sample holding time, and a sample that sat too long or arrived warm may have changed before the organisms ever saw it.
Question 2: What Did the Test Find?
This is where the vocabulary starts, and where many people lose the thread. The report presents results using a handful of standard terms. They look intimidating, but each one answers a simple question.
LC50 — What it means: The effluent concentration that kills 50 percent of test organisms in the exposure period; Where you'll see it: Acute tests
NOEC — What it means: No Observed Effect Concentration: the highest tested concentration with no statistically significant effect compared to the control; Where you'll see it: Chronic tests
LOEC — What it means: Lowest Observed Effect Concentration: the lowest tested concentration with a significant effect; Where you'll see it: Chronic tests
IC25 — What it means: The concentration estimated to cause a 25 percent reduction in reproduction or growth compared to the control; Where you'll see it: Chronic tests
TUa / TUc — What it means: Toxic Units, acute or chronic. Calculated as 100 divided by the endpoint (for example, 100 ÷ LC50). Higher numbers mean more toxic effluent; Where you'll see it: Summary tables and permit limits
Two practical notes on reading these values. First, results are expressed as a percent effluent, meaning the proportion of effluent in the test solution, so a higher LC50 or IC25 means a less toxic sample. Toxic Units run the opposite way, which is a common source of confusion. Second, you will sometimes see a "greater than" value, such as LC50 > 100%. That means the test never reached 50 percent mortality even in undiluted effluent. It is a good result, but the number is a limit rather than a measured value.
Your report will also name the statistical methods used to calculate each endpoint. You don't need to evaluate them yourself, but you should see them listed, along with a statement of which method was used and why. If the report presents an endpoint with no stated method, ask the lab.
Question 3: How Does It Compare to My Permit?
A WET result only means something relative to the limit or trigger in your permit. That comparison hinges on a value called the Instream Waste Concentration (IWC): the percentage of effluent expected in the receiving water at the critical design flow. A facility that discharges into a large river with plenty of dilution will have a low IWC. A facility discharging into a small stream, or into an effluent-dominated channel, will have a high one.
Permits express the requirement in different ways depending on the state and the permit program. Some set a limit on a specific endpoint, such as an IC25 that must be at or above the IWC. Others set a limit in toxic units. Others call for a response, such as additional testing, when a test shows a statistically significant effect at the IWC. Your permit's WET section spells out which applies to you, and it is worth reading alongside every report.
A Worked Example
The numbers below are illustrative and don't come from a real client. Imagine a municipal facility with an IWC of 25 percent, tested with a seven-day Ceriodaphnia dubia chronic test. The permit requires the chronic result to be no more than 4.0 TUc, which is the same thing as an endpoint of at least 25 percent effluent.
Control — Survival (%): 100; Young per female: 28.4; Reduction vs. control: —
6.25% — Survival (%): 100; Young per female: 27.9; Reduction vs. control: 1.8%
12.5% — Survival (%): 100; Young per female: 28.1; Reduction vs. control: 1.1%
25% (IWC) — Survival (%): 100; Young per female: 26.5; Reduction vs. control: 6.7%
50% — Survival (%): 90; Young per female: 21.0; Reduction vs. control: 26.1%
100% — Survival (%): 0; Young per female: 0.0; Reduction vs. control: 100%
Illustrative data only.
Working through the three questions:
Valid? The control had 100 percent survival and 28.4 young per female, comfortably above the 80 percent and 15-young criteria. Assume the reference toxicant result and sample handling records also check out.
What did it find? The dose-response pattern is clean: little effect at the lower concentrations, a clear drop in reproduction at 50 percent, and complete mortality at 100 percent. Statistically, the first significant reduction appears at 50 percent, so the LOEC is 50 percent and the NOEC is 25 percent. Interpolating between the 25 and 50 percent results, the point where reproduction falls 25 percent below the control is roughly 49 percent effluent, so the IC25 is about 49 percent. That converts to 100 ÷ 49, or about 2.1 TUc.
How does it compare? The IWC is 25 percent, and the IC25 is well above it. The result of 2.1 TUc is under the 4.0 TUc ceiling. This test passes, with a reasonable margin.
Now change one number. If the IC25 had come back at 18 percent, the same arithmetic gives 100 ÷ 18, or about 5.6 TUc. That exceeds the limit, and the facility would be looking at a failed test and the follow-up steps that come with it. We'll cover that process in detail in the next post.
Reading Between the Lines
A passing number doesn't tell the whole story. A few features of the data are worth a second look even when the headline result is fine.
The shape of the dose-response curve. In a well-behaved test, effects get stronger as concentration rises. When the pattern is erratic, for instance a significant effect at a low concentration but none at the next one up, it can point to a problem with the test, a non-uniform sample, or something unusual in the effluent. The lab should address it in the report narrative.
Water chemistry. Dissolved oxygen, pH, temperature, conductivity, hardness, and alkalinity are measured throughout the test and should stay within the ranges the method allows. Conductivity and hardness deserve extra attention for industrial and mining discharges. Effluents with high dissolved solids can affect sensitive organisms through ion imbalance rather than through any contaminant, and that distinction matters a great deal when you decide how to respond to a result.
Test sensitivity. For chronic tests, EPA guidance includes a measure called the percent minimum significant difference (PMSD), which describes how large a difference the test could reliably detect. EPA publishes upper and lower bounds. A test that falls outside them may have been too insensitive to catch real effects, or so sensitive that it flagged trivial ones. If the report includes a PMSD, check that the lab has addressed any value outside the bounds.
Trends across reports. A single report is a snapshot. Placing the last several results side by side, with the endpoints and the IWC, will show you whether your effluent is stable, drifting toward your limit, or showing intermittent effects. A facility whose IC25 has slipped from 80 percent to 45 percent to 32 percent over a year is in a different position than one holding steady at 45, even though both pass today.
Red Flags in a WET Report
Control performance close to, or below, the acceptability criteria
Sample holding time or arrival temperature outside method limits
Reference toxicant result outside the lab's historical range
Erratic dose-response with no explanation in the narrative
Endpoints reported without the statistical method named
Result that differs sharply from your last several tests with no change in operations
A Five-Minute Report Review
Before You File the Report
Check validity: Did the controls meet the acceptability criteria, and does the report say so?
Check the sample: Collection date, holding time, and arrival temperature all within limits?
Find the endpoint: Which one does your permit use (LC50, IC25, NOEC, TU), and what is the reported value?
Compare to the permit: Is the result on the right side of your IWC or toxic unit limit, and by how much?
Scan the chemistry and curve: Anything unusual in conductivity, pH, or the shape of the response?
Log the trend: Record the result next to your last several tests.
If anything on the list looks off, call your laboratory before the result goes onto a Discharge Monitoring Report. A good lab will walk you through the data, explain what happened, and tell you plainly whether the test stands. That conversation is easier to have before a submission deadline than after a regulator has questions.
Reading results confidently is a skill that builds quickly once you've gone through a few reports with the three questions in mind. If you'd like a second set of eyes on a recent report, or help interpreting how your permit's WET language applies to your results, contact SeaCrest Group. We've worked with permit holders across the Rocky Mountain Region since 1987, and we're glad to help.
The Effluent Standard — Monthly Series by SeaCrest Group
Next month: What Happens When You Fail a WET Test: A Step-by-Step Guide
About SeaCrest Group
Established in 1987, SeaCrest Group is an aquatic toxicology laboratory specializing in Whole Effluent Toxicity (WET) testing, PTIE/TIE/TRE studies, and permit consultation services. We serve municipalities, industrial facilities, and mining operations across the Rocky Mountain Region.
Comments