Water Quality Assessment of the South Platte River Basin, Colorado , Nebraska, and Wyoming

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age) represent the major irrigated crops in the basin (table 28). The leading counties in the basin, in terms of irrigated -crop production, are Weld, Colorado; Lin- coln, Nebraska; Morgan, Colorado; Perkins, Nebraska; and Logan, Nebraska. Table 2 lists the estimated net monthly irrigation requirements for selected crops in the Greeley, Colorado area. Alfalfa hay has the largest irrigation requirement, followed by sugar beets and corn. Most of the irrigation water is applied in July and August. Winter wheat is the major non - irrigated crop grown in the basin (80 percent of all non - irrigated acre- age). In terms of acres harvested, non - irrigated winter wheat represents the most important crop grown in the basin. The leading counties in the basin, in terms of non - irrigated -crop production, are Washington, Weld, Lincoln, Adams, and Logan, all of which are in Colo- rado. In addition to field -crop production, cattle feed- lot operations are an important form of agriculture on the alluvial deposits. Recent work by the U.S. Depart- ment of Agriculture, Agricultural Research Service (ARS) (Schuff, 1992), has indicated that ground water in aquifers in the vicinity of large feedlot operations along the South Platte River typically has elevated con- centrations of dissolved nitrate. One possible source of the nitrate in ground water suggested in the ARS study is the leaching of nitrogen from manure after the manure has been applied to fields. Table 29, in the "Supplemental Data" section in the back of the report, lists approximate planting dates, days to harvest, and rates and timing of application of common fertilizers and pesticides (herbicides and insecticides) for the crops referred to above. Most of the crops are planted between the middle of April and the end of May, and most fertilizer and pesticide appli- cations occur around planting time. The rates and tim- ing of fertilizer and pesticide application varies depending on numerous factors, including soil nutrient content and the kind of weed and insect problems encountered. The most commonly applied pesticides in the South Platte River Basin are listed in table 3. Herbicide application data by county are available and are listed in table 30 in the "Supplemental Data" section at the back of this report. Insecticide application data by county are not available. The common names, trade names, compound classes, and typical applications of the pesticides are listed in table 31 in the "Supplemen- tal Data" section at the back of this report. The most commonly applied herbicides were 2,4 -D, EPTC, atra- zine, alachlor, and butylate. Counties with the greatest herbicide applications were Weld, Colorado; Lincoln, Nebraska; Morgan, Colorado; Perkins, Nebraska; and Logan, Colorado. The herbicide 2,4 -D is the most applied herbicide in the basin. Its primary uses are for broadleaf control in winter wheat and rangeland. Because most of the acreage associated with wheat and rangeland is located on the terrace deposits and bench lands, the potential effect of 2,4 -D on surface -water quality may be smaller than that of pesticides applied predominantly on irrigated crops on the alluvial depos- its. For example, EPTC, which is the second most applied herbicide in the basin, is used on irrigated crops like corn, dry beans, and sugar beets. Because most of the irrigated acreage is located on the alluvial deposits, EPTC can potentially have a greater effect on surface - water quality than 2,4 -D. The most applied insecti- cides, in terms of pounds of active ingredient applied, were terbufos, chlorpyrifos, propargite, disulfoton, and carbofuran. Although the pesticides listed in table 3 represent some of the more important compounds used in the basin, they are only a few of the possible pesti- cides that could be applied. Nitrogen was the most commonly applied fertil- izer in the South Platte River Basin, followed by phos- phorus (as P2O5) and potassium (as K2O). Estimated fertilizer use by county for 1991 is listed in table 32 in the "Supplemental Data" section at the back of this report. The leading counties for fertilizer applications were Weld, Colorado; Lincoln, Nebraska; Perkins, Nebraska; Washington, Colorado; and Keith, Nebraska. Water - Quality Issues in the Basin Water - quality issues in the South Platte River Basin have been summarized by the Colorado Depart- ment of Health (1990). Standards have not been met in surface water for dissolved oxygen and un- ionized ammonia. Nutrient concentrations were determined to be elevated (but within standard limits) downstream from wastewater- treatment plants. Of 40 stream seg- ments identified by the Colorado Department of Health as impaired, 14 were impaired by un- ionized ammonia, 3 by nutrients, and 2 by suspended sediment. Of 68 lakes and reservoirs in the basin, 33 have been assessed to date, and of these, 21 were categorized by the Colorado Department of Health as eutrophic. Ground water in shallow alluvial aquifers has been determined in some areas to contain concentrations of nitrate in excess of the drinking -water standard. Effected areas include the area around Greeley, Colorado (Schuff, 1992), and in southeast Deuel County, Nebraska (Rod Horn, South Platte Natural Resources District, personal commun., 1993). There are little data on pesticide concentrations in water of the South Platte River Basin, which is in itself a concern. 16 Water - Quality Assessment of the South Platte River Basin, Colorado, Nebraska, and Wyoming -- Analysis of Available Nutrient, Suspended - Sediment, and Pesticide Data, Water Years 1980-62