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Energy and Environment Cabinet

Division of Water

Division of Water

Following is a list of common questions about Whole Effluent Toxicity (WET) testing.

What does LC50 mean?

LC stands for lethal concentration. The LC50 is a calculated percentage of effluent at which 50 percent of the organisms die in the test period. It is calculated statistically and used in acute toxicity testing.

What is an IC25?

IC stands for inhibition concentration. The IC25 is also a calculated percentage of effluent. It is the level at which the organisms exhibit 25 percent reduction in a biological measurement such as reproduction or growth. It is calculated statistically and used in chronic toxicity testing.

What is a toxicity unit (TU)?

A toxicity unit is a unit of measure for effluent toxicity. It is not a percentage. The major advantage of using toxicity units to express toxicity test results is that they increase as the toxicity of the effluent increases. A TU of 4.00 is twice as toxic as a TU of 2.00.  The LC50 and IC25 are just the opposite. The lower the toxicity, the higher its percentage. TUs are also used as permit limits. Like metal concentrations and other permit parameters, increasing values reflect higher impacts.

How do the LC50 and IC25 translate into toxicity units?

To convert a percentage into a toxicity unit (TU), the percentage is divided into 100. For example, if an acute LC50 equals 100 percent effluent, the test results would be equivalent to a TUa of 1.00 (100/100=1.00). Or, if a chronic IC25 equals 80 percent effluent, the result would be 100/80=1.25 TUc.

How do I report test results from two grab acute tests on the Discharge Monitoring Report (DMR)?

For acute tests, do not average the results from the two grabs for the same test organism. The permit limit is the maximum; therefore, the maximum (highest) test result should be reported. If you test both species, each maximum is to be reported.

What is a major and minor facility?

A major facility is defined as one that discharges greater than 1 million gallons of effluent per day. A minor facility discharges less than that.

Why are water fleas and fathead minnows used as the test organisms for WET?

The species Ceriodaphnia dubia belongs to a group of freshwater microcrustaceans commonly referred to as water fleas. These invertebrates are a major component of the freshwater zooplankton found in lakes, streams, ponds and rivers throughout North America. The selection of Ceriodaphnia for toxicity testing is appropriate for a number of reasons:

  • They are broadly distributed in fresh water and are present throughout a wide range of habitat.
  • They are an important link in aquatic food chains and are a significant source of food for small fish.
  • They have a short life cycle and are easy to culture in the laboratory.
  • They are sensitive to a broad range of aquatic contaminants.
  • Their small size requires small volumes of test water leading to ease of sampling.

Fathead minnows, Pimephales promelas, belong to the fish family Cyprinidae, or carps and minnows, the dominant freshwater family in terms of number of species. Fathead minnows are native to North America and thrive in ponds, lakes, ditches and slow muddy streams. They are easy to culture in the laboratory, adapting well to the dry commercial fish food and brine shrimp necessary for culturing in the laboratory.

These species have been used for acute and chronic tests for many years. Their life cycles allow for tests that run from two to seven days, thus reducing testing costs and sample volumes considerably.

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