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Water Quality in the Upper Snake River Basin, Idaho and Wyoming, 1992-95

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MAJOR ISSUES AND FINDINGS--
IS NITRATE A CONCERN IN STREAMS AND GROUND WATER IN THE SNAKE RIVER BASIN?

On the basis of data collected during the NAWQA study, nitrate concentrations in streams in the upper Snake River Basin were generally low, and none exceeded the USEPA drinking-water standard of 10 milligrams per liter. However, in some streams of the basin, nitrate was a contributing factor to the overabundance of aquatic plants. In contrast to stream samples, some ground-water samples contained nitrate concentrations in excess of 10 milligrams per liter. Nitrate contamination in streams and ground water may become a more serious problem in the future as population growth and industrial development in the basin continue.

Nitrate (NO3) is, in general, the predominant form of nitrogen in streams and ground water. Nitrate is highly soluble in water, stable over a wide range of environmental conditions, and readily transported in streams and in ground water. Nitrate is common in the environment; rainwater contains small concentrations. Although nitrate is an essential plant nutrient, in large concentrations it can be a pollutant in water. Excessive nitrate in streams, in combination with other nutrients, can lead to overabundance of aquatic plants and degraded water-quality conditions. Large concentrations of nitrate in drinking water also have been identified as the cause of blue-baby syndrome in infants, which is characterized by a reduced capacity of the blood to carry oxygen. Nitrate also has been associated with a high incidence of non-Hodgkin's lymphoma and miscarriages [5 ,15]. Because of the health concerns associated with nitrate, the USEPA established the current drinking-water standard of 10 milligrams per liter for nitrate as nitrogen.

The major sources of nitrate in the upper Snake River Basin are fertilizers, cattle manure, and legume crops. These three sources contribute about 93 percent of the nitrate input to the basin [9]. Precipitation contributes an additional 6 percent of nitrate input to the basin. Domestic septic systems contribute less than 1 percent; however, in densely populated areas, domestic septic systems can contribute substantial amounts of nitrate to ground water [9].

In most areas of the Nation, nitrate concentrations in streams are not as large as in ground water and rarely exceed the drinking-water standard [16]. This is also true in the upper Snake River Basin, where concentrations of nitrate were less than 2.0 milligrams per liter in 95 percent of 527 stream samples collected during the NAWQA study [4]. However, concentrations of nitrate in many streams were sufficient, in combination with sufficient phosphorus and a suitable substrate, to result in the overabundance of aquatic plants, particularly in the downstream reaches of the Snake River.

At 19 stream sites sampled during 1992 through 1995 (12 as part of the NAWQA and 7 as part of other U.S. Geological Survey water-quality studies), concentrations of nitrate were largest downstream from agricultural areas. Samples collected at main-stem and tributary sites between Milner Dam and King Hill contained the largest concentrations of nitrate [4]. Nitrate concentrations in samples collected from the Snake River at King Hill during the 16-year period from 1980 through 1995 show no upward or downward trend. Concentrations of nitrate at King Hill were smallest when streamflows were larger than normal and largest when streamflows were smaller than normal. Discharge of ground water to the Snake River from numerous springs between Milner Dam and King Hill is a constant source of nitrate to the river during most years, accounting for about 70 to 80 percent of the nitrate leaving the upper Snake River Basin at King Hill [4, 17]. Nitrate in spring water is derived primarily from fertilizers, cattle manure, and legume crops [9].

Photo: Feedlot (25,130 bytes)

Cattle manure, which contributes about 30 percent of the nitrogen annually introduced to the basin, is used as a fertilizer on crops. Manure may become an even larger source of nitrogen in the future as the dairy and beef cattle industries in the basin expand (photograph by M.G. Rupert, U.S. Geological Survey).

Photo:Nitrogen fertilizer application (60,595 bytes)

Nitrogen fertilizer is applied to crops to enhance production. It is generally added to irrigation water prior to field application. Excess irrigation water can carry nitrate to underlying ground water or back to streams (photographs by M.G. Rupert, U.S. Geological Survey).

Graph:Concentrations of nitrate by well location (12,991 bytes)

Concentrations of nitrate in ground water are increasing in areas of the upper Snake River Basin. Water from four wells in the shallow (mean well depth of 40 feet) alluvial aquifer in the Minidoka Irrigation District north of Burley, Idaho, shows an increase in nitrate concentrations since 1985.

Chart:Nitrate concentration - Snake River (3,004 bytes)

Samples collected during the NAWQA study show that median concentrations of nitrate in the Snake River and at the mouths of major Snake River tributaries upstream from Milner Dam were all less than 1.0 milligrams per liter. Downstream from the dam, however, median nitrate concentrations increased substantially in response to large inflows of nitrate-enriched ground water, effluent from industrial and wastewater-treatment facilities, and irrigation return flows.

In 1992, the USEPA reported that nitrate was a principal ground-water pollutant in 49 States [16]. In the upper Snake River Basin, elevated nitrate concentrations in ground water were detected in numerous counties, primarily on the Snake River Plain [13]. Nitrate concentrations exceeded the 10 milligrams per liter drinking-water standard in water from wells at the Idaho National Engineering and Environmental Laboratory, in the Fort Hall area north of Pocatello, and in agricultural areas north of the Snake River between Burley and Hagerman [5]. Concentrations in 25 percent of 726 wells (mostly domestic and public supply) sampled basinwide from 1991 through 1995 as part of the NAWQA and other USGS water-quality studies exceeded 3.0 milligrams per liter. Concentrations in only 3 percent of the samples exceeded the drinking-water standard [5]. However, in some areas of the basin, nitrate in ground water is becoming a serious concern. In agricultural areas north of the Snake River between Burley and Hagerman, water from 10 percent of 105 wells sampled during the NAWQA study, ranging in depth from 10 feet to more than 500 feet, contained concentrations of nitrate in excess of 10 milligrams per liter [5]. Water from 24 percent of 29 domestic wells sampled in the shallow (mean well depth of 40 feet) alluvial aquifer in the Minidoka Irrigation District north of Burley contained nitrate concentrations in excess of 10 milligrams per liter. In the A&B Irrigation District north of Burley, nitrate concentrations in water from 10 percent of 31 wells sampled (mean well depth of 230 feet) exceeded 10 milligrams per liter. In some agricultural areas, concentrations of nitrate have increased substantially since the early 1980's [5]. If the current rate of increase continues, nitrate concentrations in a large part of the A&B Irrigation District's ground-water supply may exceed the 10 milligrams per liter drinking-water standard early in the 21st century [5].

Chart:Nitrate Concentration in feet below land surface (2,613 bytes)

Ground-water samples collected during 1991-95 indicate that nitrate concentrations were largest in wells less than 200 feet deep. None of the samples collected from wells deeper than 500 feet contained nitrate concen-trations in excess of the drinking-water standard of 10 milligrams per liter.

On the basis of data collected during the NAWQA study, probability maps were developed to identify areas in the upper Snake River Basin where the potential for ground-water contamination by nitrate is high. Results indicate that the most vulnerable areas are those where urban or irrigated agricultural land uses are predominant, where the depth to water is shallow, and where soils are well drained. Other studies across the Nation also noted that land-use and soil-drainage characteristics markedly influence the concentration of nitrate in ground water [18]. In the upper Snake River Basin, ground water beneath agricultural lands adjacent to the Snake River, especially near Burley and Twin Falls, and between Idaho Falls and Pocatello is particularly vulnerable to nitrate contamination [5].

Map:Probability map for nitrate concentration study (8,452 bytes)

Probability maps are valuable for determining areas potentially vulnerable to ground-water contamination. This map for nitrate, produced during the NAWQA study and based on depth to ground water, soil characteristics, and land use, shows that ground water near the Snake River between Idaho Falls and the outlet of the basin at King Hill is the most vulnerable to nitrate contamination.


U.S. Geological Survey Circular 1160

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Suggested citation:
Clark, G.M., Maret, T.R., Rupert, M.G., Maupin, M.A., Low, W.H., Ott, D.S., 1998, Water Quality in the Upper Snake River Basin, Idaho and Wyoming, 1992-95: U.S. Geological Survey Circular 1160, on line at <URL: https://water.usgs.gov/pubs/circ1160>, updated June 18, 1998 .

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Last modified: Tue Sep 8 13:51:30 1998