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Regional variability of nitrate fluxes in the unsaturated zone and groundwater, Wisconsin, USA

February 6, 2018

Process-based modeling of regional NO3− fluxes to groundwater is critical for understanding and managing water quality, but the complexity of NO3− reactive transport processes make implementation a challenge. This study introduces a regional vertical flux method (VFM) for efficient estimation of reactive transport of NO3− in the vadose zone and groundwater. The regional VFM was applied to 443 well samples in central-eastern Wisconsin. Chemical measurements included O2, NO3−, N2 from denitrification, and atmospheric tracers of groundwater age including carbon-14, chlorofluorocarbons, tritium, and tritiogenic helium. VFM results were consistent with observed chemistry, and calibrated parameters were in-line with estimates from previous studies. Results indicated that (1) unsaturated zone travel times were a substantial portion of the transit time to wells and streams (2) since 1945 fractions of applied N leached to groundwater have increased for manure-N, possibly due to increased injection of liquid manure, and decreased for fertilizer-N, and (3) under current practices and conditions, approximately 60% of the shallow aquifer will eventually be affected by downward migration of NO3−, with denitrification protecting the remaining 40%. Recharge variability strongly affected the unsaturated zone lag times and the eventual depth of the NO3− front. Principal components regression demonstrated that VFM parameters and predictions were significantly correlated with hydrogeochemical landscape features. The diverse and sometimes conflicting aspects of N management (e.g. limiting N volatilization versus limiting N losses to groundwater) warrant continued development of large-scale holistic strategies to manage water quality and quantity.

Publication Year 2018
Title Regional variability of nitrate fluxes in the unsaturated zone and groundwater, Wisconsin, USA
DOI 10.1002/2017WR022012
Authors Christopher T. Green, Lixia Liao, Bernard T. Nolan, Paul F. Juckem, Christopher L. Shope, Anthony J. Tesoriero, Bryant C. Jurgens
Publication Type Article
Publication Subtype Journal Article
Series Title Water Resources Research
Index ID 70195031
Record Source USGS Publications Warehouse
USGS Organization National Research Program - Western Branch