Effects of surface-water irrigation on sources, fluxes, and residence times of water, nitrate, and uranium in an alluvial aquifer

Effects of surface-water irrigation on an alluvial aquifer were evaluated using chemical and isotopic data including δ²H, δ¹⁸O, ³H, δ³He, Ar, Ne, N₂, δ¹⁵N, and ²³⁴U/²³⁸U activity ratios in a transect of nested wells in the North Platte River valley in western Nebraska, USA. The data were used to evaluate sources and fluxes of H₂O, NO3-">NO3- and U, all of which were strongly affected by irrigated agriculture. Combined results indicate that the alluvial aquifer was dominated by irrigation water that had distinctive chemical and isotopic features that were inherited from the North Platte River or acquired from agricultural soils or recharge processes. Values of δ²H, δ¹⁸O, Ar and Ne indicate that most of the ground water in the alluvial aquifer was irrigation water that was derived from the North Platte River and distributed during the growing season. The irrigation water was identified by an evaporated isotopic signature that was acquired by the river in major upstream reservoirs in Wyoming, and by relatively warm gas-equilibration temperatures related to warm-season recharge. Apparent ³H–³He ages indicate that the ground water in the alluvium was stratified and mostly <30 a old, with apparent recharge rates varying widely from about 0.2 to >3.0 m/a. Age gradients and NO3-">NO3- concentrations indicate that recharge occurred by a combination of focused leakage from irrigation canals (rapid local recharge, low NO3-">NO3-) and distributed infiltration beneath the irrigated agricultural fields (lower recharge, high NO3-">NO3-). Large amounts of U with relatively low ²³⁴U/²³⁸U activity ratios were present in the alluvial aquifer as a result of irrigation with U-bearing river water, and minor amounts of U with higher ²³⁴U/²³⁸U activity ratios were added locally from basal and underlying volcanic-rich sediments. Distributions of NO3-">NO3-, &#x3B4;15N[NO3-]">δ15N[NO3-], dissolved gases, and ground-water ages indicate that NO3-">NO3- concentrations increased and &#x3B4;15N[NO3-]">δ15N[NO3-] values decreased in distributed recharge in the last few decades, possibly in relation to a documented increase in the agricultural use of artificial fertilizers. Canal leakage caused substantial dilution of NO3-">NO3-within the alluvial aquifer, whereas denitrification occurred mainly near the bottom of the alluvium. The average residence time of the irrigation water within the aquifer was relatively short (about 9 a) and reactions such as respiration, denitrification and U exchange in the saturated zone had relatively little effect on the overall composition of the alluvial ground water in comparison to what they might have had in the absence of irrigation recharge.