Whole Effluent Toxicity (WET) testing is an important component of the U.S. Environmental Protection Agency’s (USEPA’s) integrated approach for detecting and addressing toxicity in surface waters. The National Pollutant Discharge Elimination System (NPDES) permit program, authorized by the Clean Water Act, controls water pollution by regulating point source discharges into waters of the United States.
Point sources are discrete structures such as pipes or manmade ditches. The wastewater discharges from point sources are commonly called “effluents.” Facilities must obtain permits for direct discharges to surface waters and, in many cases, for storm sewer systems. A permit specifies the conditions that must be met to discharge. Permits often include WET tests as a monitoring requirement and sometimes for compliance determination. “WET” is a term used to describe the adverse effects or toxicity to a population of aquatic organisms caused by exposure to an effluent. This toxicity can be experimentally determined in the laboratory by exposing sensitive organisms (usually surrogate organisms representative of those found in the environment) to effluents using WET tests.
Responses assessed usually include survival, growth, and/or reproduction. This type of test can be used to evaluate the toxicity of effluents, storm water, or ambient surface waters. WET testing is used to assess and regulate the combined effects of all constituents of a complex effluent rather than the conventional methods of controlling the toxicity of single chemicals or constituents.
WET testing exposes laboratory populations of aquatic organisms such as fish, invertebrates, and algae to diluted and undiluted effluent samples under controlled conditions in order to estimate the environmental toxicity of that sample. The information is used to prevent the discharge of toxic amounts of pollutants to surface waters. The standardized procedures of WET tests allow one to determine the actual environmental exposure of aquatic life to an effluent or ambient water without knowledge of the chemical, physical, and biological characteristics of that discharge or ambient water. WET tests can be performed on a variety of commonly used test species under a variety of exposure periods and dilution regimes. Acute tests are conducted for a relatively short period and usually focus on how well an organism survives.
Chronic tests are conducted for longer periods relative to certain organisms’ life cycle or during a very sensitive life stage to evaluate survival, growth, and/or reproduction. Tests may be conducted on an undiluted sample to answer the question, “Is this sample toxic to this test species?” Often, testing includes the undiluted sample along with a series of dilutions of the sample to answer the question, “How toxic is this sample to this test organism?”
Some regulatory approaches focus on controlling toxic amounts of individual chemicals known to be present in the effluent. In contrast, WET testing actually measures the potential toxicity of all chemicals in a solution. Furthermore, WET testing may show that chemicals known to be toxic to aquatic organisms may be rendered non-toxic by particular characteristics of the effluent matrix and/or receiving stream chemistry. When receiving water from a stream, lake, or river is used as the dilution water for an effluent sample in WET testing, the test can account for changes in quality of a discharge as the discharge dilutes and mixes with the receiving water. The synthesis of WET testing results, along with chemical analyses and other information, can provide a more comprehensive and realistic picture of potential effects of discharges into aquatic systems. No other water quality control tool has this capability.