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.