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

Abstract

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 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.

Assessing Decadal Trends of a Nitrate-Contaminated Shallow Aquifer in Western Nebraska Using Groundwater Isotopes, Age-Dating, and Monitoring

“…The second point is further suggested by a significant increase in planted corn area, yet significant decrease in estimated fertilizer application in Scotts Bluff County. In other words, it is possible other variables beyond irrigation practice and canal management are currently driving the decreasing trends in groundwater [NO 3 − ], consistent with other studies that emphasize the need to improve both water and nitrogen management in agricultural production [12,13,70,71]. It should be noted, however, that, while this study was unable to detect a significant reduction in recharge rates, limitations and uncertainties associated with groundwater age-dating and recharge calculations may make it difficult to identify a small but meaningful decrease.…”

Recharge and Nitrate Transport Through the Deep Vadose Zone of the Loess Plateau: A Regional‐Scale Model Investigation

“…Previous studies have suggested different types of nitrate budget quantification at a regional scale, which include the transient nature of nitrate storage in deep vadose zones (Roelsma & Hendriks, ). Green et al () introduced a mechanics‐based approach to account for reactive nitrate transport in the unsaturated zone at a regional scale. Subsequently, this method was implemented within a statistical framework based on GIS variables to reduce model complexity (Nolan et al, ).…”

Metamodeling for Groundwater Age Forecasting in the Lake Michigan Basin

“…The candidate predictor variables (see Table ) described in Feinstein et al () are intended to span the physical factors that might influence the age of shallow well discharge. They were initially chosen guided by the common analytical equation for mean travel time in the saturated zone from Vogel (), following the symbology of Green et al (): where is the mean travel time, n s is porosity, R is recharge, H s is the saturated thickness of the aquifer, and z is the vertical position within the aquifer.…”

“…The candidate predictor variables (see Table 1) described in Feinstein et al (2018) are intended to span the physical factors that might influence the age of shallow well discharge. They were initially chosen guided by the common analytical equation for mean travel time in the saturated zone from Vogel (1967), following the symbology of Green et al (2018…”