Geographic Distribution of Pesticides

Figure 5. Average air concentration and detection frequency per site for p,p'-DDT compared to use on cropland in 1971.

It should be no surprise that the highest concentrations of pesticides detected in air and rain occur in those areas where they are used most frequently and in the highest amounts. Occurrence and geographic distribution of pesticides in air and rain, and the relation between the measured air concentrations, detection frequencies, and national use were examined by comparing results from national and multistate studies to national pesticide use patterns. National use and air concentration data from the early 1970's show general correlations among pesticides in the atmosphere and regional use and cropping patterns. For example, the highest air concentrations and most frequent detections of DDT generally occurred in the highest use areas (see Figure 5). DDT was primarily used on cotton in the Southeast and Mississippi delta states. It was also used in the Appalachian and Southern Plains states on soybeans and grains, and to a lesser extent on wheat, tobacco, and corn. DDTs regional use corresponds with many of the high average air concentrations at sites located in the Mississippi delta, Southeast, and Appalachian regions. A more recent 1990-1991 study of triazine and acetanilide herbicides in rain shows a much more direct relation between use and observed concentration (see Figure 6).

Figure 6. Concentrations of atrazine in precipitation in the midwestern and northeastern United States, mid-April through mid-July, 1990 and 1991, compared to atrazine use in 1988 (Goolsby and others, 1994).

The use of triazine herbicides has increased from 24 million pounds in 1966 to nearly 104 million pounds in 1988, accounting for about 23 percent of national herbicide use. Studies that searched for these compounds in the atmosphere did not begin until the late 1970's, when atrazine was detected in Maryland precipitation. Since then, atrazine and other triazine herbicides have been detected in rain in many midwestern and northeastern states, sometimes at high levels.

The acetanilide herbicides, which include alachlor and metolachlor, comprised about 26 percent of total herbicide use in 1988, up from only 5 percent in 1966, and now have use comparable to the triazines. These herbicides are frequently used in conjunction with triazine herbicides. Although they are not as long-lived in the environment as the triazines, they have been detected at equivalent and even higher concentrations in rain.