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].
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].
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].
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.
