median concentration of 0.6 microgram per liter. INTRODUCTION Nebraska is one of several Midwestern States that rely on large quantities of inorganic fertilizers and herbicides to sustain crop production. Agricultural herbicides usually are applied during the spring to row crops, such as corn, sorghum, and soybeans, for control of broadleaf weeds and grasses. The occurrence of herbicides in surface water depends primarily on their rate of use, rate of degradation (persistence), and on properties determining their rate of transportation in the hydrologic environment (solubility) (Bevans and others, 1993). Within the States that form the Upper Midwest, the herbicides alachlor, atrazine, cyanazine, and metolachlor accounted for about 73 percent of the pesticides used in 1982 (Goolsby and others, 1991). Atrazine has a solubility of approximately 33,000 ng/L (micrograms per liter) and a persistence of approximately 1 year in soil (Gilliom and others, 1985). Atrazine also has been detected during every month of the year in the surface water of the Big Blue River Basin in Nebraska (Bevans, 1991). Although generally referred to as the Big Blue River Basin, the basin consists of the Big Blue River and the Little Blue River watersheds. The Big Blue River is the largest downstream tributary of the Kansas River, contributing 27 percent of the mean flow rate of the Kansas River measured at its confluence with the Missouri River from 1971-86 (Dugan and others, 1991). The occurrence of agricultural pesticides in surface water is an important water-quality issue in eastern Kansas and other areas where crop production is a principal economic and land-use activity, and where publicwater supplies are derived primarily from surfacewater resources (Bevans and others, 1993). Three reports have been written compiling available surface-water-quality data for areas that include the Big Blue River Basin. Jordan and Starrier (1991) analyzed available surface-waterquality data for the lower Kansas River Basin in Kansas and Nebraska. Bevans and others (1993) documented the occurrence and transport of agricultural pesticides in the Tuttle Creek Lake watershed in Kansas and Nebraska. Both reports included available data for surface-water-quality samples, which were analyzed for the presence of atrazine and other herbicides through 1986. Fallon and McChesney (1993) summarized water-quality data from water and sediment samples collected as part of the lower Kansas River Basin National Water-Quality Assessment pilot study (1987-90) in Kansas and Nebraska. This report presents available data from past surface-water-quality studies, within the Big Blue River Basin in Nebraska, in which water samples were analyzed for atrazine and other related herbicides. The number of detections, the range in concentrations, and the median concentration of the herbicides detected are documented. The drinking-water regulations associated with atrazine and other herbicides as stated by the U.S. Environmental Protection Agency (1992) and the water-quality criteria fo...
