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

- Nitrogen speciation data from 877 wells across the US are presented.
- XSN2 (denitrification-N2) increases as time-of-travel increases.
- NH4+ mobilization primarily from degradation of aquifer organic matter.
- Temporal patterns reflect changing NO3− loads, denitrification and NH4+ mobilization.
- A classification tree predicts denitrification extent with 79% accuracy.

SummaryUncertainties 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.