Temporal changes in δ18O and δ15N of nitrate nitrogen and H2O in shallow groundwater: Transit time and nitrate-source implications for an irrigated tract in southern Idaho

Intensive irrigated agriculture in semiarid southern Idaho contributes to nitrate loads in shallow groundwater. To determine the temporal character and source of leached nitrate in the Twin Falls Irrigation Tract, and investigate the soil N cycling process involved, we measured stable isotope ratios of nitrate (δ15N-NO3, δ18O-NO3) and water (δ2H-H2O, δ18O-H2O) in 1) tunnel drain and irrigation waters during 2003-07, and 2) leachate from incubated urea- and manure-amended soil endmembers. The tunnel water δ2H-H2O vs. δ18O-H2O plots showed clear overlap with waters of Milner reservoir, which stores the tract’s irrigation water prior to distribution. The δ18O-H2O time series and a correlation analysis with climate/hydrologic parameters indicated a 20 to 23 month, transit time between the surface and shallow groundwater. The tunnel waters had a mean δ15N-NO3 of +6.1 ± 0.7‰ (±Std. Dev.) and mean δ18O-NO3 of −6.1 ± 0.7‰. Decreasing tunnel-water δ15N-NO3, δ18O-NO3, and nitrate concentrations in tunnel drains located closer to the Snake R indicate increasing contributions of regional groundwater to shallow groundwater and dilution of the latter’s NO3–N content as proximity to the river increases. Relative to all-other tunnels, water from the two tunnels closest to Snake River were depleted in δ15N-NO3 (+5.4‰ vs. + 6.3‰) and included a greater contribution of regional groundwater (71% vs. 47%). Nitrate δ15N-NO3 and δ18O-NO3 ratios for tunnel waters plotted between those of urea (fixed-N) amended soil (4.6 ± 0.5‰ and −4.9 ± 1.4‰), manure-amended soil (13.4 ± 1.3‰ and −4.4 ± 1.2‰), and regional groundwater endmembers. A dual-isotopic element, three-source, simple linear mixing model indicated that, on average, 1.5X more N is sourced from fertilizer and fixed N than animal waste. The dominant N-cycling process in the system at the scale observed here is the nitrification of NH4-N derived from applied fertilizer and manure, whereas denitrification has a minor influence.