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

Journal of Hydrology
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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 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.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Nitrogen speciation and trends, and prediction of denitrification extent, in shallow US groundwater
Series title Journal of Hydrology
DOI 10.1016/j.jhydrol.2013.11.048
Volume 509
Year Published 2014
Language English
Publisher Elsevier
Contributing office(s) Oregon Water Science Center
Description 11 p.
First page 343
Last page 353
Country United States
Online Only (Y/N) N
Additional Online Files (Y/N) N