Water Quality in the South Platte River Basin, Colorado, Nebraska, and Wyoming, 1992-95

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MAJOR ISSUES AND FINDINGS: Have Agricultural Chemicals Affected Water Quality?

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Irrigated fields south of Greeley, Colo. Corn, hay, dry beans, and other small grains are the principal irrigated crops in the basin.

Agriculture accounts for about 37 percent of the land use in the South Platte River Basin, and agricultural chemicals (fertilizers and pesticides) are used to enhance production. It is estimated that more than 2 million pounds of active pesticide ingredients are applied in the basin each year. Eleven chemicals account for about 90 percent of the pesticide usage: eight herbicides, which are applied predominantly on corn during spring planting; and three insecticides, which are applied as needed throughout the growing season (Dennehy and others, 1995). Fertilizer use is predominantly nitrogen and phosphorus compounds applied in the form of chemical fertilizer or manure. It is estimated that about 40,000 tons of phosphorus and 200,000 tons of nitrogen are applied in the basin each year (Litke, 1996). Residual chemicals migrate into the hydrologic system by ground- and surface-water irrigation return flows or storm runoff.

Pesticides were detected in surface-water (Kimbrough and Litke, in press), ground-water (Bruce and McMahon, 1998), bed-sediment, and fish-tissue(Tate and Heiny, 1996) samples collected as part of NAWQA studies during 1992-95. Generally, the most commonly detected pesticides were those that were most commonly used. DDT, which was banned in 1972, was detected in bed sediment and fish tissue; and dieldrin, which also was banned in the 1970's, was detected in all four media. The common occurrence of some pesticides, and the persistence of others, can be an ecological and human health concern. Although individual pesticide concentrations were small, generally less than MCLs, the effects of long-term exposure to pesticide mixtures are unknown.

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Feedlot near Fort Lupton, Colo. Feedlots generate substantial quantities of manure, which are applied to nearby fields as supplemental fertilizer.

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Nitrate concentrations exceeded the U.S. Environmental Protection Agency drinking-water standard of 10 milligrams per liter in about one-half of the wells sampled. Locations of exceedances are scattered and depend on fertilizer application rates and soil leaching potential.

Nitrate was the predominant form of nitrogen in ground and surface water within the basin. Nitrate concentrations in alluvial ground water were high in some areas where agricultural fertilizers and manure were applied to the land. Previous work has shown that the location of nitrate "hot spots" is related to soil texture and to the locations of combined organic/inorganic fertilizer applications (Wylie and others, 1993). At nitrate hot spots, nitrate concentrations exceeded the drinking-water standard (10 mg/L), and have resulted in the restricted use of ground water as a rural drinking-water supply. Nitrate-contaminated ground water generally cannot be avoided by drilling deeper wells because nitrate concentrations typically are similar throughout the depth of the alluvial aquifer, most likely due to mixing of water in the aquifer caused by irrigation pumpage. Mixing was documented by age-dating of the ground water, which showed small age differences between the top and bottom of the aquifer (McMahon and Böhlke, 1996).

Ground water contributes substantially to the flow of the South Platte River in the agricultural part of the basin, but nitrate concentrations in surface water were much smaller than in ground water and did not exceed the USEPA drinking-water standard. Studies have shown that microbial activity in streambed sediments removes a substantial portion of the nitrate in the incoming ground water (McMahon and Böhlke, 1996), and that as much as 90 percent of the nitrate can be removed from surface water over short distances (Sjodin, and others, 1997). This natural removal process does much to improve nitrate concentrations in the South Platte River, but concentrations are still large enough that, when excessive phosphorus concentrations also are present, increased growth of nuisance algae and other aquatic plants can occur in streams and reservoirs.

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Nitrate concentrations in surface water are substantially lower than concentrations in wells adjacent to the South Platte River. Microbial action removes some nitrate from ground water as it moves from the aquifer to the river.

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Drinking water for sale near Greeley, Colo., in an area where the ground-water supply is contaminated by high nitrate concentrations.

U.S. Geological Survey Circular 1167

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