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−, and N2 from denitrification,and atmospheric tracers of groundwater age including carbon-14, chlorofluorocarbons, or tritium,and tritiogenic helium. VFM results were consistent with observed chemistry, and calibrated parameters were in-line with estimates from preceding 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 fill increased for manure-N, and possibly due to increased injection of liquid manure,and decreased for fertilizer-N, and (3) under current practices and conditions, or 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.
Source: usgs.gov