wAbstract
@Effects of surface-water irrigation on an alluvial aquifer were
evaluated using chemical and isotopic data including Β2H,
Β18O, 3H, Β3He, Ar, Ne, N2, Β15N, 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-
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 Β2H,
Β18O, 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 rate varying widely
from about 0.2 to 3.0 m/a. Age gradients and NO3-
concentrations indicate that recharge occurred by a combination
of focused leakage from irrigation canals (rapid local recharge,
low NO3-) and distributed infiltration
beneath the irrigated agricultural fields (lower recharge, high
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-, Β15N [NO3-], dissolved gases, and ground-water
ages indicate that NO3- concentrations
increased and Β15N [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- 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.x
1. Introduction
2. Hydrogeology and land use
3. Methods
@3.1. Sample sites
@3.2. sample collection and analyses
@3.3. Calculation of recharge temperature, excess air, and excess
nitrogen in ground water
4. Results and discussion
@4.1. Chemical and hydraulic evidence for local recharge by
irrigation canal leakage
@4.2. Hydrogen and oxygen isotopes and sources of water
@4.3. Dissolved gases and ground-water recharge temperatures
@4.4. Ground-water age gradients and recharge rates
@4.5. Distribution of uranium and uranium activity ratios
@4.6. Nitrogen isotopes and sources of nitrate
@4.7. Rates of loading, NO3- recharge,
and denitrification
5. Summary and conclusion
Acknowledgements
Appendix A. Supplemental material
References