Whole Effluent Toxicity Testing 101

October 29, 2020

By: Barbara Orelo

Whole Effluent Toxicity (WET) testing is a critical component of monitoring the health of aquatic ecosystems and it is included in many National Pollutant Discharge Elimination System (NPDES) permits. If your NPDES permit includes WET testing, it is important to understand why it exists and how to best implement it in order to save time, money, and other resources.

Why is WET testing important?
In 1972, as part of the U.S. Clean Water Act, the NPDES permit system was created to regulate sources of pollution – the act states that there shall be “no discharge of toxic substances in toxic amounts”. These toxic substances can range from polymers used in wastewater treatment processes to heavy metals that flake off brake pads and are deposited on our roads.

Analytical chemistry allows us to measure these toxic substances, but it is only part of the story, since biological toxicity can be affected by different chemical forms, interactions between multiple contaminants, and environmental parameters (pH, hardness, etc.). Additionally, not all chemicals are measured or even known! WET testing helps us fill this gap.

What does WET testing measure?
WET tests (or bioassays) measure direct biological effects, which we call “test endpoints”. These endpoints determine whether aquatic organisms can survive and also perform basic biological functions. So how does this work?

In short, the test organisms are exposed to the sample and a laboratory control, and the biological responses are compared. When testing, however, there are two main types of bioassays – acute and chronic:

  • Acute tests are shorter in duration and measure mortality.
  • Chronic tests are longer in duration (typically lasting 7 days), and measure mortality as well as a sub-lethal endpoint like growth or reproduction.

Many of the methods used in the U.S. labs are based on guidance from the U.S. Environmental Protection Agency (USEPA). However, guidance for WET testing is also available from other resources such as the American Society for Testing and Materials (ASTM)  and the Organization for Economic Co-operation and Development (OECD). The guidance used to perform toxicity testing will depend on the type of sample and the reason for testing.

Planning for your WET test
Reviewing your NPDES permit is key because it includes a detailed description of your WET testing requirements. This section should include: the type of test required (acute vs chronic), the testing frequency, the approved test species, the permit limitations (Toxic Unit or Pass/Fail), and what to do if there is an exceedance.

Here are some things to consider when planning for your WET test:

  • Generally, the testing frequency depends on the type of discharge, how often, and the volume of discharge. Higher volumes of wastewater discharge may require more frequent testing.
  • The approved test species are based on the body of water in which the discharge will end up. For example: if you discharge into the ocean, you will have to test marine species. If you discharge into a creek, you will have to test freshwater species. Depending on the test species, organisms may need to be ordered from a supplier.
  • Some permits require species sensitivity screening, in which a vertebrate, an invertebrate, and a plant are tested to determine which species is most sensitive to the sample. The most sensitive species should be used for compliance monitoring.
  • Some chronic bioassays require three samples, collected on three different days. Additionally, some bioassays require more sample volume than others.
  • The holding time for bioassays is much shorter than most other tests! Typically, a sample must be tested within 36 hours of collection – that’s because toxicity may change over time depending on the constituents in the sample.
  • Each permit is unique, based on the type of site, discharge, and more, so communicate with your laboratory and plan ahead of time.

Preparing before an exceedance is observed
In addition to routine monitoring, there are proactive steps you can take to help both your business and the environment in case an exceedance is observed. It is important to have a Toxicity Reduction Evaluation (TRE) work plan when you start discharging under your NPDES permit, and depending on which state or region you are in it may be a permit requirement. TRE plans can help you navigate through compliance issues, should they arise, by describing the steps that you should follow if toxicity is detected. A well-designed TRE plan also increases chances for an efficient and effective investigation. Additionally, TRE plans may include a Toxicity Identification Evaluation (TIE), which uses WET test methods to investigate the cause of toxicity.

So, have a plan ready before you need it. And make sure you are working with a quality lab that can assist you in understanding your permit requirements, developing a TRE plan, and investigating the cause of toxicity, in the case of a permit exceedance.

For More Information
Need help with your WET testing requirements? Want to create a TRE work plan to prepare before you face compliance issues? Our tox experts can help. Click here to start a conversation.

Barbara Orelo
Project Manager
Barbara is passionate about environmental biology and brings over 4 years of environmental science experience to the Enthalpy team in her role as Project Manager. She has a B.S. in Biology, with an emphasis in Ecology, Behavior, and Evolution from UCSD.

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