Regional Variability of Nitrate Fluxes in the Unsaturated Zone and Groundwater, Wisconsin, USA

Abstract

AbstractProcess‐based modeling of regional fluxes to groundwater is critical for understanding and managing water quality, but the complexity of reactive transport processes makes implementation a challenge. This study introduces a regional vertical flux method (VFM) for efficient estimation of reactive transport of in the vadose zone and groundwater. The regional VFM was applied to 443 well samples in central‐eastern Wisconsin. Chemical measurements included O2, , 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 , with denitrification protecting the remaining 40%. Recharge variability strongly affected the unsaturated zone lag times and the eventual depth of the 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.