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

Stephen R. Hinkle; Anthony J. Tesoriero

doi:10.1016/j.jhydrol.2013.11.048

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

Stephen R. Hinkle·Anthony J. Tesoriero

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Abstract

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 NO 3 − and XSN 2 (N 2 from denitrification), reflecting active/ongoing denitrification and/or a mixture of undenitrified and denitrified groundwater. NO 3 − and NH 4 + essentially do not co-occur, indicating that the dominant source of NH 4 + at these sites likely is not dissimilatory reduction of NO 3 − to NH 4 + . Positive correlations of NH 4 + with apparent age, CH 4 , dissolved organic carbon, and indicators of reduced conditions are consistent with NH 4 + mobilization from degradation of aquifer organic matter and contraindicate an anthropogenic source of NH 4 + for most sites. Glacial aquifers and eastern sand and gravel aquifers generally have lower proportions of NO 3 − and greater proportions of XSN 2 than do fractured rock and karst aquifers and western sand and gravel aquifers. NO 3 − dominates in the youngest groundwater, but XSN 2 increases as residence time increases. Temporal patterns of nitrogen speciation and concentration reflect (1) changing NO 3 − loads over time, (2) groundwater residence-time controls on NH 4 + 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.

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Stephen R. Hinkle, Anthony J. Tesoriero. 2014. Nitrogen speciation and trends, and prediction of denitrification extent, in shallow US groundwater. https://doi.org/10.1016/j.jhydrol.2013.11.048

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