Age of irrigation water in ground water from the Eastern Snake River PlainAquifer, south-central Idaho

Stable isotope data (H-2 and O-18) were used in conjunction with chlorofluo rocarbon (CFC) and tritium/helium-3 (H-3/He-3) data to determine the fracti on and age of irrigation water in ground water mixtures from farmed Darts o f the Eastern Snake River Plain (ESRP) Aquifer in south-central Idaho, Two groups of waters were recognized: ii) regional background mater, unaffected by irrigation and fertilizer application, and (2) mixtures of irrigation w ater from the Snake River with regional background water. New data are pres ented comparing CFC and H-3/He-3 dating of water recharged through deep fra ctured basalt, and dating of young fractions in ground water mixtures. The H-3/He-3 ages of irrigation water in most miutures ranged from about zero t o eight years. The 6FC ages of irrigation water in mixtures ranged from val ues near those based on H-3/He-3 dating to values biased older than the H-3 /He-3 ages by as much as eight to 10 years. Unsaturated zone air had CFC-12 and CFC-113 concentrations that were 60% to 95%, and 50%; to 90%, respecti vely, of modern air concentrations and were consistently contaminated with CFC-11. Irrigation water diverted from the Snake River was contaminated wit h CFC-11 but near solubility equilibrium with CFC-12 and CFC-113, The datin g indicates ground water velocities of 5 to 8 m/d for water along the top o f the ESRP Aquifer near the southwestern boundary of the Idaho National Eng ineering and Environmental Laboratory (INEEL), Many of the regional backgro und waters contain excess terrigenic helium with a He-3/He-4 isotope ratio of 7 X 10(-6) to 11 x 10(6) (R/R-a congruent to 5 to 8) and could not be da ted. Ratios of (CFC data indicate that some rangeland water may contain as much as 5% to 304e young water (ages of less than or equal to two to 11.5 y ears) mixed with old regional background water. The relatively low residenc e times of ground water in irrigated parts of the ESRP Aquifer and the dilu tion ffith low-NO3- irrigation water from the Snake River lon er the potent ial for NO3- contamination in agricultural areas.