Pesticide Toxicity Index for Freshwater Aquatic Organisms

By Mark D. Munn and Robert J. Gilliom



Water Resources Investigation Report 01-4077


Sacramento, California 2001

PDF Version (3.7 MB PDF) Download Acrobat reader)





Purpose and Scope


Development of the Pesticide Toxicity Index

Applications of the Pesticide Toxicity Index

Example Application

Limitations of the Pesticide Toxicity Index

Summary and Conclusions

References Cited


  1. Concentrations of the 18 most common pesticides found in Little Buck Creek, Indiana
  2. Pesticide Toxicity Index for bluegills in Little Buck Creek, Indiana, and the contributions of selected pesticides
  3. Pesticide Toxicity Index for bluegills in Little Buck Creek, Indiana, compared with total pesticide concentration


  1. Pesticides analyzed in surface waters for the National Water-Quality Assessment (NAWQA) Program
  2. Summary of toxicity values by species
  3. Summary of taxa included in bioassay data set and number of bioassays and compounds
  4. Summary of EC50 median toxicity concentrations for cladocerans
  5. Summary of LC50 median toxicity concentrations for benthic invertebrates
  6. Summary of LC50 median toxicity concentrations for freshwater fish
  7. Relative toxicity of the pesticides within each of three taxonomic groups: cladocerans, benthic invertebrates, and fish


The U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program is designed to assess current water-quality conditions, changes in water quality over time, and the effects of natural and human factors on water quality for the Nation's streams and ground-water resources. For streams, one of the most difficult parts of the assessment is to link chemical conditions to effects on aquatic biota, particularly for pesticides, which tend to occur in streams as complex mixtures with strong seasonal patterns.

A Pesticide Toxicity Index (PTI) was developed that combines pesticide exposure of aquatic biota (measured concentrations of pesticides in stream water) with toxicity estimates (standard endpoints from laboratory bioassays) to produce a single index value for a sample or site. The development of the PTI was limited to pesticide compounds routinely measured in NAWQA studies and to toxicity data readily available from existing databases. Qualifying toxicity data were found for one or more types of test organisms for 75 of the 83 pesticide compounds measured in NAWQA samples, but with a wide range of bioassays per compound (1 to 65). There were a total of 2,824 bioassays for the 75 compounds, including 287 48-hour EC50 values (concentration at which 50 percent of test organisms exhibit a nonlethal response) for freshwater cladocerans, 585 96-hour LC50 values (concentration lethal to 50 percent of test organisms) for freshwater benthic invertebrates, and 1,952 96-hour LC50 values for freshwater fish. The PTI for a particular sample is the sum of toxicity quotients (measured concentration divided by the median toxicity concentration from bioassays) for each detected pesticide. The PTI can be calculated for specific groups of pesticides and for specific taxonomic groups.

While the PTI does not determine whether water in a sample is toxic, its values can be used to rank or compare the toxicity of samples or sites on a relative basis for use in further analysis or additional assessments. The PTI approach may be useful as a basis for comparing the potential significance of pesticides in different streams on a common basis, for evaluating relations between pesticide exposure and observed biological conditions, and for prioritizing where further studies are most needed.

Water Resources of California

U.S. Department of the Interior, U.S. Geological Survey
Persistent URL:
Page Contact Information: Contact USGS
Last modified: Thursday, September 01 2005, 03:32:05 PM
FirstGov button  Take Pride in America button