Understanding Why You Need WET Tests

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Understanding Why You Need WET Tests

Many industries throughout the country rely on the use of water for their business and manufacturing processes. (Think about it – you do your laundry in a washing machine at home. After the wash cycle, the “used” water (filled with high efficiency soap, maybe a little bit of grease or tomato sauce that you spilled on your shirt at lunch, oh and the potty training accident that happened this morning), is now flushed out to the sewer system. The few gallons from your load of laundry, not such a big deal. But now, multiply that for your dry cleaner. How much laundry do they do? That’s A LOT of wastewater for industries that need to utilize water as part of their manufacturing or service process.)


To balance the need for and use of water, and the creation of wastewater while protecting the environment, the EPA introduced the National Pollutant Discharge Elimination System (NPDES). All point source dischargers (this generally refers to industries that discharge treated wastewater back to the environment through methods such as a pipe, ditch, channel, tunnel, conduit, etc.) must obtain and comply with a NPDES permit to ensure that they have effectively treated their wastewater prior to sending it back to the environment. Point source dischargers may include:

  • Industrial facilities (manufacturing, mining, shipping, oil and gas extraction, and service industries)
  • Municipal governments and government facilities
  • Certain agricultural facilities

Permitted facilities are required to submit frequent reports to the EPA or their state/local regulatory agency to show that their wastewater is being effectively treated and discharged, meeting the compliance requirements of their individual permits. WET Testing provides a means to measure the aggregate acute and chronic toxicity in the effluent (or the final discharge) to be measured.

What is WET Testing

Enter Whole Effluent Toxicity (WET) Testing, which refers to the aggregate toxic effect to aquatic organisms from all pollutants contained in a facility’s wastewater (effluent). WET tests measure the effects of the effluent on specific test organisms’ ability to survive, grow, and reproduce.


Permits may specify independent limits for chemicals and nutrients, but this doesn’t allow for a comprehensive look at the total effects of the effluent. WET testing provides a measurement for the potential toxicity of all aspects of the effluent. This also allows the test to account for dilution that may be experienced when the effluent stream is mixing with a larger water body such as a lake, river, or stream. As a result, WET test results combined with additional analysis can provide a comprehensive overview of the potential effects from a point source discharger.


The frequency of WET tests varies based on the site and is specified in their individual discharge permits.

Chronic vs. Acute WET Tests

With a test length of 48-96 hours, the acute test is used to determine the concentration of effluent that causes an organism to die due to short term exposure under controlled conditions. If 50% of the organisms die in the test period, this is considered a lethal concentration.


The chronic test is used to estimate the effluent concentration that interferes with the normal growth or reproductive potential of the test organisms. The test length is determine by species: In most cases, Fish = 7 days, C.dubia = up to 8 days.  A 25% reduction in a biological measurement would be an indicator of an inhibition concentration.

How are WET Tests Conducted?

The WET methods listed below are specified at 40 CFR 136.3, Table I A. WET test methods consist of exposing living aquatic organisms (plants, vertebrates and invertebrates) to various concentrations of a sample of wastewater, usually from a facility’s effluent stream.


WET tests are used by the National Pollutant Discharge Elimination System (NPDES) permitting authority to determine whether a facility’s permit will need to include WET requirements.


All manuals include guidelines on laboratory safety, quality assurance, facilities and equipment, dilution water, effluent sampling methods and holding times and temperatures, data analysis, report preparation, and organism culturing and handling.


WET Tests must be completed by a certified laboratory – Certified by the local, state, or by the US EPA, certified labs have passed on-site audits to ensure testing capabilities and accuracy.


Based on laboratory availability, WET testing may need to be scheduled months in advance. As the date of the WET Test nears, site personnel will want to ensure that sampler tubing has been replaced and composite sample collection jugs have been properly cleaned (not with soap) and effluent rinsed.


To begin the WET test, the certified laboratory will send appropriate sample containers and a total of three coolers to the site that will need to be returned to the lab on ice and via overnight shipping. This is to ensure the samples are kept at appropriate temperatures.


Cooler 1: Grab sample of the receiving water stream & Composite effluent sample

Coolers 2 and 3: Effluent samples only


Once the samples are received in the laboratory, pre-specified organisms are exposed to various effluent concentrations from the samples. Receiving water is used to dilute the effluent, simulating the effect of mixing in the real aquatic environment.


Species used for WET tests were chosen for their sensitivity to toxic substances, their necessity for the overall health of the food chain, and they are representative population present in the in the environment of the discharge area. They refer to the chosen species as indicator organisms. Commonly used species include C.dubia and fathead minnows.


Ceriodaphnia dubia (C.dubia), also referred to as water fleas, are used in toxicity testing for a number of reasons including but not limited to:

  • Present in many habitats
  • Significant source of food for small fish
  • Short life cycle
  • Easy to culture in the laboratory
  • Sensitive to a broad range of contaminants

Fathead minnows are most closely related to carps and minnows, which are the dominant freshwater species. They are used in toxicity testing because:

  • Native to North America
  • Thrives in ponds, lakes, ditches, and streams
  • Good laboratory fish
  • Easy to culture in the laboratory

These two species life cycles allow for tests that run from 2 to 7 days; reducing cost and necessary sample volumes.

Causes of Toxicity

Effectively treating wastewater generally includes complex biological components and reactions. For example, chlorine may be added to control unwanted biological growth, but it is important to consider that chemicals may retain their toxic qualities even once they’re discharged. While the chlorine may have controlled growth in the wastewater, excessive amounts of chlorine in the effluent will harm natural species once discharged as well.

Common factors that can influence toxicity include:

Chemical Factors
  • Inorganic chemicals (e.g., ammonia, chlorine, and heavy metals)
  • Organic chemicals (e.g., dioxins, polychlorinated biphenyls, and surfactants)
  • Pesticides (e.g., chlorpyrifos, diazinon, and heptachlor)
Physical Factors
  • Dissolved and suspended solids
  • Temperature
  • pH
Biological Factors
  • Biological Factors
  • Bacteria, fungi, and parasitic invertebrates.

Receiving Your Results

In most cases, results are available approximately two weeks after the WET test samples are received. The results of these tests are considered pass or fail. If you fail a WET test Wisconsin will require 2 retests be completed within 90 days. In the event that the first retest is failed, WI in most cases will require a written report detailing what is being done to resolve the toxicity and what steps have been taken. This report would be due to the WDNR within 60 days of the retest failure.

By |2018-11-21T13:59:44-06:00November 1st, 2018|News, Resources, Uncategorized|0 Comments

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