Traditionally when an effluent was found to be toxic to aquatic organisms, a sample of
the effluent was analyzed for 126 "priority pollutants". If any of these
priority pollutants were found to be present, their concentrations were subsequently
compared to toxicity data for that toxicant.
This approach, however, often fails to pin-point the cause of toxicity in an effluent
for two main reasons.
- The so-called "priority pollutants" represent only a tiny fraction of all the
potential chemicals that can cause toxicity to aquatic organisms. The cost to
analyze an effluent for every possible toxic chemical compound would be prohibitive.
- This approach tells us nothing about the bio-availability of chemical substances found
in the effluent. Thus, even if one or more of the "priority pollutants"
are found in the effluent sample, that does not necessarily mean that they are responsible
for the observed toxicity. This is because other factors such as total suspended
solids, pH, hardness and alkalinity can change the bioavailabilty and therefor the
toxicity of a given toxicant.
This approach first uses the responses of test organisms to show changes in toxicity as
the sample is subjected to various chemical and/or physical manipulations. In this
way, knowledge of the physical/chemical characteristics (such as solubility, volatility,
filterability, pH dependence, etc.) of the toxic substance is gained, and the number of
possible toxic agents is greatly reduced before actual chemical analysis begins.
A toxicity based TIE typically has three phases: characterization,
tentative identification, and confirmation. In the
first phase, the toxicant is characterized ( i.e. general
physical/chemical characteristics of the toxic substance are determined). In the
second phase, toxicant-specific methods are used to isolate, analyze and tentatively
identify the toxicant. In the third phase, confirmation tests are carried
out to confirm that the identified toxicant is the substance responsible
for the effluent toxicity.
To be successful, toxicity-based TIEs require that toxicity be consistently
present in the effluent so that repeated testing can be carried out to characterize,
identify, and confirm the toxicants. Speed in conducting the tests is usually
important because effluents may decay during storage. The TIE tests should be conducted by
a multi-disciplinary team of experienced toxicologists and chemists who are aware of the
many factors that can affect test results.
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