Nitrogen speciation and trends, and prediction of denitrification extent, in shallow US groundwater

Uncertainties surrounding nitrogen cycling complicate assessments of the environmental effects of nitrogen use and our understanding of the global carbon–nitrogen cycle. In this paper, we synthesize data from 877 ambient-monitoring wells across the US to frame broad patterns of nitrogen speciation and trends. At these sites, groundwater frequently contains substantial co-occurring NO₃⁻ and XSN₂ (N₂ from denitrification), reflecting active/ongoing denitrification and/or a mixture of undenitrified and denitrified groundwater. NO₃⁻ and NH₄⁺ essentially do not co-occur, indicating that the dominant source of NH₄⁺ at these sites likely is not dissimilatory reduction of NO₃⁻ to NH₄⁺. Positive correlations of NH₄⁺ with apparent age, CH₄, dissolved organic carbon, and indicators of reduced conditions are consistent with NH₄⁺ mobilization from degradation of aquifer organic matter and contraindicate an anthropogenic source of NH₄⁺ for most sites. Glacial aquifers and eastern sand and gravel aquifers generally have lower proportions of NO₃⁻ and greater proportions of XSN₂ than do fractured rock and karst aquifers and western sand and gravel aquifers. NO₃⁻ dominates in the youngest groundwater, but XSN₂ increases as residence time increases. Temporal patterns of nitrogen speciation and concentration reflect (1) changing NO₃⁻ loads over time, (2) groundwater residence-time controls on NH₄⁺ mobilization from solid phases, and (3) groundwater residence-time controls on denitrification. A simple classification tree using readily available variables (a national coverage of soil water depth, generalized geology) or variables reasonably estimated in many aquifers (residence time) identifies categorical denitrification extent (<10%, 10–50%, and >50%) with 79% accuracy in an independent testing set, demonstrating a predictive application based on the interconnected effects of redox, geology, and residence time.