National Water-Quality Assessment (NAWQA) Program
Several large national and multistate studies of pesticides in rivers and streams were conducted between the late 1950's and the mid-1970's. In these and most other studies during this period, the focus was on organochlorine insecticides (such as DDT and dieldrin), a few phenoxy acid herbicides (such as 2,4-D), and organophosphorus insecticides (such as diazinon) in use at the time. Use of organochlorine insecticides declined dramatically after the 1960's, and use of organophosphorus and carbamate insecticides increased. In addition, agricultural use of herbicides has increased dramatically during the last 30 years, from an estimated 84 million pounds in 1964 to more than 500 million pounds in 1992.
In response to the changes in pesticide use, the number of different types of pesticides monitored in surface waters from the mid-1970's to the present has increased (Figure 2). The scale of monitoring studies has changed as well. The national and multistate studies conducted during the 1960's and 1970's have been largely replaced by state and local studies, or by regional studies directed at specific river basins. Recent studies have been relatively short-term and their geographic distribution is highly uneven. Iowa, California, Florida, and the Great Lakes region have been the most frequently studied areas. The most extensive regional studies have been conducted in the Mississippi River Basin. Overall, there has been a steady increase in monitoring of pesticides in surface waters over the last several decades.