National Water-Quality Assessment (NAWQA) Program
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 98-4222
Sacramento, California, 1999
Figure 21. Annual mean detection frequency in relation to agricultural use for (A) herbicides and (B) insecticides at 32 agricultural sites. Each value represents the annual mean detection frequency of a specific compound at one site. Detection frequencies are shown only for compounds with reported use of 100 or more kilograms active ingredient (a.i.) in a particular basin. A common reporting level of 0.01 microgram per liter was used for all compounds. Five agricultural sites were excluded because sampling was not sufficiently frequent throughout the year to calculate an unbiased annual detection frequency. Agricultural-use data are from Gianessi and Anderson (1996).