The Big Blue River Basin, which includes the Little Blue River Basin, covers approximately 9,690 square miles in southeast Nebraska and northeast Kansas. Approximately seventy-five percent of the basin is in Nebraska. The primary streams within the basin are the Big Blue and Little Blue Rivers, the West Fork of the Big Blue River, the Black Vermillion River, Lincoln Creek, Turkey Creek, and Big Sandy Creek (Figure 1). The Little Blue River accounts for approximately 36 percent of the mean annual flow of the Big Blue River below the point of confluence, which occurs approximately midway between the Nebraska-Kansas border and the confluence of the Big Blue and the Kansas Rivers near Manhattan, Kansas.
 

Land use within the basin is primarily agricultural, with over 70 percent of the land area devoted to row crops--mainly corn, sorghum and soybeans. In Nebraska the predominant land use is cropland (77%). Irrigated cropland dominates the central and western portions of the basin, north of the Little Blue River. South of the Little Blue River in Nebraska, dryland cropland, pasture and range are the most common land uses. In the eastern part of the basin in Nebraska, dryland cropland and pastureland are prevalent. In the Kansas portion of the basin, cropland is also the dominant land use (60 %), and pasture and range is common.
 

Herbicides are used extensively to control agricultural weeds. This practice has led to the appearance of herbicides, particularly atrazine, in surface waters of the basin. Atrazine, which is used primarily on corn and sorghum, is by far the most widely used herbicide in the basin. This compound's moderate solubility and adsorption, and slow rate of degradation make it highly prone to transport by surface runoff.
 

A 1987-1990 study conducted by the U.S. Geological Survey (USGS) documented that atrazine concentrations in surface waters of southeast Nebraska and northeast Kansas occasionally exceed the maximum contaminant level of 3.0 g/L established by the U.S. Environmental Protection Agency under the Safe Drinking Water Act. This is of particular concern for public drinking water supplies because atrazine is not effectively removed by conventional water treatment. Tuttle Creek Lake, located on the Big Blue River approximately 10 miles upstream from its confluence with the Kansas River, supplies water for Topeka, Lawrence, and Kansas City, Kansas. High concentrations of atrazine in the Big Blue River may cause atrazine concentrations in the Kansas River to exceed 3.0 g/L.
 
 

If all other factors are equal, steeper slopes will generate more atrazine runoff than gentle relief. In general, the north and western portions of the Blue River Basins are relatively flat, with slopes that are typically less than 3 percent. In the remainder of the basin, extensive dissection results in slopes that generally exceed 10 percent.
 

Figure 3 shows slopes within the Big Blue River Basin, as derived from 1:250,000-scale digital elevation model (DEM) data developed by the USGS. Because of the relatively coarse resolution of the dat, actual slopes are likely to be underestimated in this figure, as the peaks and valleys in the landscape are not fully captured at this scale. However, the general pattern of relative slope values shown here is consistent with descriptions found in the literature.
 

Soil properties influence the transport of atrazine into surface waters through their tendency to allow water to either infiltrate or runoff. Soils with greater runoff potential are likely to generate more runoff and soil erosion that can transport atrazine to surface waters either in solution, or bound to eroded soil particles.
 

The soil hydrologic group, a classification developed by the Natural Resources Conservation Service, groups together soils that have the same runoff potential under similar storm and cover conditions. This classification can be used to represent the soil properties that influence runoff potential. Figure 4 shows the distribution of these soil hydrologic groups within the Big Blue River Basin, as derived from the State Soil Geographic (STATSGO) database. The large majority of soils in the basin range in infiltration rate from moderate, to slow to very slow.
 

In general, higher rainfall amounts, intensity, and frequency are all correlated with increased atrazine runoff. Within the context of these factors, the timing of precipitation is critical. Previous research has indicated that the first runoff event following application usually carries the greatest pesticide concentration and accounts for the greatest losses. Losses are generally greatest when the first runoff event occurs immediately after application. According to computer simulation model results, the time frame within which atrazine is most vulnerable to runoff is about 10 to 15 days following application.
 

In the Big Blue Basin, precipitation amounts increase from northwest to southeast across the basin. Average annual precipitation, ranges from 25 inches per year in the northwestern part of the basin, to 34 inches per year in the southeast (Figure 5).
 

Runoff amount is estimated by the USGS using stream flow records. As with precipitation, the general pattern is an increase from northwest to southeast across the basin. Values range from a low of 2 inches per year in the northwest to 6 inches per year in the southeast.
 

Last updated September 18, 1998 klo