Water Resources of Idaho
Water Resources of Idaho |
By Michael G. Rupert ¹
Factors related to contamination of ground water by dissolved nitrite plus nitrate as nitrogen (NO2+NO3-N) in parts of the upper Snake River Basin were evaluated at regional and local scales. Regional-scale relations between NO2+NO3-N concentrations and depth to first-encountered ground water, land use, precipitation, and soils were evaluated using a geographic information system. Local-scale relations between NO2+NO3-N concentrations and other nutrients, major ions, nitrogen isotopes, stable isotopes, and tritium in five areas with different hydrogeologic settings, land use, and sources of irrigtaion water were evaluated to determine the factors causing differences in NO2+NO3-N. Data were collected and analyzed as part of the U.S. Geological Survey's National Water-Quality Assessment Program, which began in 1991.
Regionally, where depth to first-encountered ground water was between 0 and 300 feet, NO2+NO3-N concentrations in ground water were significantly higher than where depth to water was between 301 and 900 feet. Ground water in urban areas contained the highest NO2+NO3-N concentrations; ground water in irrigated agriculture areas contained the next-highest concentrations. Ground water in rangeland, dryland agriculture, and forest areas contained similar low NO2+NO3-N concentrations. There was no correlation between precipitation and NO2+NO3-N concentrations in ground water. Ground water in areas with excessively drained soils contained the highest NO2+NO3-N concentrations; ground watre in areas with poorly drained soils contained the lowest NO2+NO3-N concentrations. Ground water in counties where residual nitrogen input was greater than 3,000,000 kilograms contained significantly higher nitrogen concentrations than in counties where residual total nitrogen input was less than 3,000,000 kilograms.
The maps showing the probability of ground-water contamination by NO2+NO3-N were developed; one used data on depth to first-encountered ground water, land use, and soils, and the other incorportated additional data on nitrogen input. Probability categories were based on correlations between NO2+NO3-N concentrations and those four hydrogeologic and land-use factors. Correlations between NO2+NO3-N concentrations in ground water and probability categories demonstrated significant differences at a greater-than-99-percent confidence level. Because the probability maps are calibrated to actual ground-water quality data, they are a significant improvement over ground-water vulnerability maps generated using the DRASTIC technique.
Local-scale evaluation determined that high NO2+NO3-N concentrations in ground water were correlated with high major ion concentrations, heavy hydrogen and oxygen isotopes, and ground-water ages that postdate atmospheric nuclear testing (1952). The heavy hydrogen and oxygen isotope values indicated that high NO2+NO3-N was associated with water that had undergone evaporation; high major ion concentrations indicated that concentrated ions and NO2+NO3-N may be leaching from the soil by excess irrigation water. NO2+NO3-N concentrations in ground water in parts of the A&B study area were high because of continual recycling of ground water as it is pumped from the ground, applied to fields, and then leached back to the water table. NO2+NO3-N and major ion concentrations decrease as ground water travels from the A&B area to the Eden area because of dilution by Snake River irrigation water. NO2+NO3-N concentrations and stable isotope ratios increase from background concentrations as ground water travels through the Jerome-Gooding area, similar to conditions observed in the A&B area, but at a lesser rate, possibly due to less infiltration of contaminated recharge water.
Long-term NO2+NO3-N data are available for three springs that discharge water from the eastern Snake River Plain aquifer and for several wells in the A&B study area. Long-term overall NO2+NO3-N concentrations from all three springs do not appear to have changed significantly since 1980. However, NO2+NO3-N concentrations in samples collected from Briggs and Box Canyon Springs during the fall months have increased since the mid-1980's. It is unknown whether concentrations will change as the result of a large increase in the number of dairy cattle from 1990 through 1995 in areas upgradient from these springs. NO2+NO3-N concentrations in ground water in several parts of the A&B study area increased dramatically during 1980-95. Concentrations in parts of the A&B study area could exceed the drinking water maximum contaminant level of 10 mg/L in the next 10 to 15 years if the current rate of increase continues.
¹ U.S. Geological Survey, 230 Collins Rd. Boise, Idaho 83702-4520
U.S. Department of the Interior | U.S. Geological Survey
URL: id.water.usgs.gov/nawqa/reports/RUPERT.4174.html
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