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Integrated Deep Soil N and Groundwater Isotope Investigation of N Sources Captured by Municipal Wells

Spalding, Roy F., Hirsh, Aaron J., Exner, Mary E., Stange, Marty, Aravena, Ramon
Ground water monitoring & remediation 2019 v.39 no.2 pp. 22-31
ammonium fertilizers, atrazine, deethylatrazine, denitrification, drinking water, feedlots, fine-textured soils, groundwater, growing season, irrigated farming, isotopes, leachates, leaching, nitrates, sprinkler irrigation, water quality, weather, wells, Nebraska
Identification of major nitrate sources that adversely impact groundwater quality in municipal well capture zones in areas of emerging nitrate contamination is essential to minimize leaching and prevent exceedance of the nitrate drinking water standard. Vertical profiles of nitrate leachate in deep soils provide an estimate of the amount of nitrate in transit beneath irrigated, row‐cropped fields; depths of peak leachate; and the approximate rate of downward movement. Profiles of pore‐water soil‐nitrate concentrations in thick 60‐feet (~18 m), fine‐textured soils near Hastings, Nebraska clearly indicate that considerably more nitrate leached beneath furrow‐irrigated than center‐pivot irrigated fields. Peak leaching appeared to correlate with recorded periods of poor weather conditions during some growing seasons and may best be controlled by “spoon feeding” fertilizer to the crop through the sprinkler irrigation system at times of nutrient need. The presence of trace levels of atrazine and deethylatrazine to 60 feet (18 m) in core samples indicates that larger, more complex anthropogenic molecules also leach through the fine‐textured soils. The light δ¹⁵NNO₃ values in the surficial groundwater beneath fertilized and irrigated cropland indicate that ammonium fertilizer is a major N source and suggest that the natural soil‐N contribution is negligible. δ¹⁵NNO₃ values were most enriched in irrigation wells located within municipal well capture zones downgradient of a large feedlot. Dual isotope method (DIM) δ¹⁵NNO₃ and δ¹⁸ONO₃ values suggest that the Hastings’ municipal wells farther downgradient are contaminated with a mixture of nitrate from manure and commercial ammonium‐based fertilizer. DIM values indicate an absence of denitrification, which has implications for long‐term management of the water resources.