WHOLE EFFLUENT TOXICITY TESTING

Whole effluent toxicity (or WET) tests are a method for detecting the presence 
of potentially harmful chemicals in wastewater.  Various plants and animals are 
exposed to a sample of water and their survival, growth, and reproduction rates 
are compared to those in a toxicity-free water.  Such tests are much more 
powerful than chemical analysis for determining whether waterbodies are "free of 
toxic substances in toxic amounts" (one of the goals of the Clean Water Act).
WET tests have been conducted on wastewater samples in Indiana for about 35 
years.  Started as a fact-finding collaboration between State and Federal 
regulatory agencies, WET testing began in the late 1970s using acute Daphnia 
magna tests.  When toxicity occurred, the discharger was informed of the result 
but no follow-up was required.  Several dischargers also commissioned acute WET 
tests at the request of regulatory agencies but the results had no legal 
consequences.

With the new emphasis on toxicity-based effluent limits in the mid 1980s, the 
process began to change.  The first NPDES permit written in Indiana with a 
"biomonitoring" requirement was issued in 1987.  The next year, testing using 
the three freshwater species recommended in the U.S. EPA "chronic manual" was 
required in several new permits.  The testing requirement in NPDES permits has 
remained essentially unchanged during the past 15 years, although algae tests 
are no longer routinely used.

During the first ten years of "fact-finding" WET testing, 43% of the 106 
effluents sampled were acutely toxic.  Industrial wastewaters were more 
frequently toxic (51% of all tests) than municipal WWTP effluents (15% of all 
tests).  When toxicity occurred, municipal effluents were far less toxic 
(average LC50 of 92% effluent) than industrial wastewaters (average LC50 of 39% 
effluent).

Approximately 150 dischargers have now conducted some form of whole effluent 
toxicity testing statewide as part of an NPDES requirement.  During this past 
fifteen years of testing by wastewater dischargers, the percentage of acutely 
toxic effluents has declined sharply compared to the first ten-year testing 
period.  Only 5% of municipal effluent tests now exhibit acute toxicity.  Also 
during this time, the number of tests of industrial wastewaters has sharply 
declined (many industrial dischargers no longer have direct discharges to public 
waters but discharge to WWTPs instead).

Chronic toxicity is still commonly observed but only about 20% of municipal 
wastewater tests exhibit chronic toxicity today.  Ceriodaphnia are generally 
more sensitive to effluents than fathead minnows.  When both species are tested 
simultaneously, Ceriodaphnia have been more sensitive than minnows 90% of the 
time.

Ten wastewater dischargers have conducted WET tests on more than 10 different 
occasions.  Three of these (Connersville, Princeton, and Muncie) have had no 
chronic toxicity in any test.  All of these treatment plants have significant 
industrial discharge components (an indication of the effectiveness of their 
industrial pretreatment programs).

When toxicity occurs at concentrations with potential effects on a receiving 
stream, the discharger is required to conduct a toxicity reduction evaluation 
(TRE).  The goal of this evaluation is to identify the source of toxicity and 
reduce or eliminate it.  Approximately 20 TREs have been conducted in Indiana.  
The most commonly identified toxicants have been chlorine, ammonia, polymers, 
surfactants, nickel, chloride, and sulfate.  Although found to be an important 
toxicant elsewhere, the pesticide diazinon has not yet been determined to be a 
primary cause of effluent toxicity in Indiana. 

Other states have begun compiling similar WET test summaries.  The proportion of 
toxic effluents in Maryland is very similar to Indiana's (20% with acute 
toxicity, 60% with chronic toxicity, industrial effluents more toxic than 
municipal effluents, all effluents less toxic now than ten years ago).  North 
Carolina appears to have a more serious toxicity problem than Indiana (75% of 
the effluents are chronically toxic and municipal effluents are toxic about as 
frequently as industrial effluents).

REFERENCES
Water Environment Research 70:101
Environmental Toxicology and Chemistry 9:1019