Frequency of pesticide occurrence in streams, groundwater, and fish in agricultural areas. Detection of pesticides indicates that these chemicals are not being fully broken down or contained within the farm system. (Figs. 1, 2)

An ideal measure would address not simply the occurrence of pesticides, but also the frequency with which they are found over established threshold levels. However, such standards have been developed for only a fraction of the pesticides found in water samples, and not for mixtures of different compounds. Active research is ongoing in this area, and it will be monitored for possible reference in future reports

Locations where pesticides are found frequently or at high concentrations, as a means of identifying areas where cropping practices, natural conditions, or other factors result in export of larger quantities of chemicals. (Figs. 3, 4)

Commonly detected individual pesticides, as a means of identifying the most commonly encountered chemicals. (Fig. 5)

Pesticides have varying degrees of toxicity to fish, other animals, and humans, and the presence of a pesticide does not imply that a health-based standard has been exceeded.

In agricultural areas, about 45 percent of the groundwater and more than 80 percent of the streams and fish sampled have detectable amounts of at least one pesticide. Many samples contain mixtures of pesticides. However, fish and stream samples in agricultural areas generally have fewer pesticides than do those in urban or mixed-land-use areas, whereas agricultural ground water have more. About 10 percent of streams in all types of land uses have detectable amounts of 10 or more pesticides.

Two currently used herbicides are found in more than half of the streams sampled in agricultural areas.

No pesticide was found in more than one-third of ground water samples.

Pesticides in Surface & Ground Water (1)	Pesticides in Fish (2)
Technical Note
Source: USGS National Water Quality Assessment Program
These graphs show the percentage of samples (vertical axis) in which different numbers of pesticides were detected (horizontal axis). The lines indicate that more samples contained one pesticide than had two pesticides, and so on. For example, more than 95 percent of the streams in urban and mixed-land-use areas contained at least one pesticide, and only 10 percent had 10 or more pesticides. These graphs show the amounts detectable using current screening methods. For comparison, U.S. Environmental Protection Agency aquatic-life criteria of 0.041 parts per billion (ppb) and 0.100 ppb have been established for chlorpyrifos and malathion, respectively.

Pesticides in Shallow Groundwater in Agricultural Areas (3)	Pesticides in Agricultural Streams (4)
Technical Note
Source: USGS National Water Quality Assessment Program
These maps show the locations of ground water study areas and stream sampling sites in agricultural areas. Areas are ranked according to whether they had high, medium, or low concentrations of pesticides (the index for streams) or more frequent detections relative to other agricultural areas that were sampled (the index for groundwater)