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

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(commercial, industrial, livestock, mining, and other) account for only 6.2 percent of total offstream water use. Commercial and industrial uses primarily occur in the Front Range urban corridor in Colorado. Mining water use includes sand and gravel operations, hard - rock mining, and oil and gas production. However, most the water used in the basin is returned to the hydrologic system; only 44 percent of the water with- drawn is consumptively used. Irrigation -return flows amount to 1,200 Mgal /d, primarily as ground -water return flows. Urban Practices Urban and built -up areas comprise only 3 percent of the total land use in the South Platte River Basin, but because of the variety and intensity of urban activities, there can be a disproportionate effect on water quality. Urban practices can lead to both point source and non - point source effects on water quality. For example, a study on urban storm runoff in the Denver Metropoli- tan area (Ellis and others, 1984), reported that point sources were substantial contributors to nutrient loads in the South Platte River, while storm runoff was a major contributor of suspended solids, total organic carbon, lead, and zinc. Point sources to surface water are permitted by the U.S. Environmental Protection Agency ( USEPA) National Pollution Discharge Elimination System (NPDES) program. The ten largest wastewater- treat- ment plants in the study unit, which discharge at least 4,200 acre -ft (6 ft3 /s), are listed in table 1. Effluent dis- charges from these plants can make up a substantial part of the streamflow downstream from their dis- charge points. For example, the largest discharger in the basin, Metro Wastewater Reclamation District (MWRD), on an annual basis can account for about 69 percent of the flow and as much as 100 percent on a given day in the South Platte River downstream from the discharge point. In addition to wastewater- treat- ment plants there are other point dischargers; in Colo- rado in 1990 there were 253 facility discharge permits of which 142 were wastewater - treatment plants. Other urban -area dischargers include drinking water- treat- ment plants (25), breweries (3), meat - packing plants (2), and sugar -beet plants (3). Industrial dischargers generally recycle their water in -plant and pre -treat their effluent prior to discharging to a stream or sewer sys- tem. Point sources to ground water include landfills, gasoline- storage tanks, and Comprehensive Environ- mental Response, Compensation, and Liability Act ( CERCLA) and Resource Conservation and Recovery Act (RCRA) sites. There are nine USEPA Superfund CERCLA sites in the South Platte River Basin. They include Lowery Landfill in Arapahoe County, Marshall Landfill in Boulder County, Martin Marietta site and Rocky Flats site in Jefferson County, and the Sand Creek site and Rocky Mountain Arsenal in Denver and Adams Counties. Non -point urban sources include urban storm runoff and urban lawn and turf irrigation return flows. Storm runoff currently is being investigated in urban areas to prepare for discharge permit applications. Cul- tivation of turf in urban lawns, parks, and golf courses is a widespread activity. It has been estimated that 42 percent of the total water demand by Denver Water Department customers is for summer seasonal use (U.S. Army Corps of Engineers, 1986, p.40); if this ratio is applied to the total water demand in the Denver metropolitan area for 1992, then it can be estimated that about 125,000 acre -ft are used in this area primarily to grow lawns. Since about 2 ft of water are needed to grow turf in a year, about 60,000 acres of turf may be grown. Turf - growing professionals recommend (Follett and others, 199 1) that about 601bs /acre of nitrogen fertilizer be applied annually to lawns at four times throughout the growing season: late April —mid May, mid —late June, late August —early September, and October —early November, but actual application rates may vary widely. Pesticides also are applied to this acreage; however, there are little available data with which to characterize urban pesticide use. Agricultural Practices Agriculture accounts for about 37 percent of the land use in the South Platte River Basin. Essentially all of this acreage is located in the Plains east of the Rocky Mountains (fig. 4). Agriculture in the plains is located primarily in two areas: on the alluvial deposits of the South Platte River and its major tributaries and on the terrace deposits and bench lands above and adjacent to the alluvial deposits. The total amount of land in crops in counties in the basin is about 4.1 million acres (see table 28 in the "Supplemental Data" section at the back of this report). Of this acreage, about 1.4 million acres are irrigated, 1.6 million acres are non - irrigated, and the remaining acreage is partially irrigated. Irrigated farming predominates on the alluvial deposits, whereas non - irrigated (dryland) farming predominates on the terrace deposits and bench lands. Corn (57 percent of all irrigated acreage), hay (26 percent of irrigated acreage), dry beans (7 percent of irrigated acreage), winter wheat (6 percent of irri- gated acreage), and barley (3 percent of irrigated acre- DESCRIPTION OF THE SOUTH PLATTE RIVER BASIN 15