Tritium, stable isotopes and nitrogen in flow from selected springs that discharge to the Snake River, Twin Falls-Hagerman area, Idaho, 1990-93

Abstract

In 1990-93, tritium concentrations in water from 19 springs along the north side of the Snake River near Twin Falls and Hagerman ranged from 9.2{+-}0.6 to 78.4{+-}5.1 picocuries per liter (pCi/L). The springs were placed into three categories on the basis of their locations and tritium concentrations: Category I springs are the farthest upstream and contained from 52.8{+-}3.2 to 78.4{+-}5.1 pCi/L of tritium; Category It springs are downstream from those in Category I and contained from 9.2{+-}0.6 to 18.5{+-}1.2 pCi/L; and Category III springs are the farthest downstream and contained from 28.3{+-}1.9 to 47.7{+-}3.2 pCi/L. Tritium concentrations in precipitation and in the Snake River were relatively large in the 1950`s and 1960`s owing to atmospheric testing of nuclear weapons. Conversely, tritium concentrations in ground water with a residence time of several tens to a few hundred years, as occurs in the Snake River Plain aquifer hydraulically upgradient from the Category II springs, are comparatively small because of the 12.4-year half-life of tritium. The conclusion that recharge from excess applied-irrigation water from the Snake River has affected tritium in the Snake River Plain aquifer is supported by differences in the deuterium (2H) and oxygen-18 (180) ratios of water. These ratios indicate that water discharged by the springs is recharged by waters of different origins. Irrigation recharge is more enriched in 2H and 180 than the regional ground water. Water from Category I springs is more enriched in 2H and 180 than is water from Category II or III springs because a large proportion of irrigation recharge mixes with the regional ground water in Category I springs. Nitrite plus nitrate as nitrogen concentrations also are greater in water from Category I springs than in water from Category II springs.