资源环境科技发展态势分析平台

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 makes 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 O-2, NO3, N-2 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.

Plain Language Summary The degradation of freshwater resources by excess nitrogen from agriculture is one of the most widespread threats to human and environmental health. Even if anthropogenic inputs cease today, nitrate will continue to migrate through the subsurface for decades. Predicting the delayed effects on wells and streams requires advanced methods that can estimate local processes of nitrate movement and reactions across large regions. This study introduces a new "vertical flux method'' (VFM) for efficient estimation of these processes. The VFM was applied to 443 groundwater samples from central-eastern Wisconsin, and successfully reproduced the observed chemical concentrations. Results indicate that (1) the time nitrate spends moving from the soil to groundwater is a substantial portion of the overall time-lag between application and arrival at wells or streams and (2) since 1945 the percent of applied manure-N reaching groundwater has increased, possibly due to increased injection of liquid manure, while the percent of fertilizer-N reaching groundwater has decreased. Under current conditions, the VFM forecasts that 60% of the shallow aquifers' volume will eventually be affected by downward migration of nitrate, with denitrification protecting the remaining 40%.