Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

J. B. Heffernan,M. J. Cohen,M. L. Fork,A. R. Albertin,B. G. Katz

doi:10.5194/bg-9-1671-2012

Abstract

Abstract. Aquifer denitrification is among the most poorly constrained fluxes in global and regional nitrogen budgets. The few direct measurements of denitrification in groundwaters provide limited information about its spatial and temporal variability, particularly at the scale of whole aquifers. Uncertainty in estimates of denitrification may also lead to underestimates of its effect on isotopic signatures of inorganic N, and thereby confound the inference of N source from these data. In this study, our objectives are to quantify the magnitude and variability of denitrification in the Upper Floridan Aquifer (UFA) and evaluate its effect on N isotopic signatures at the regional scale. Using dual noble gas tracers (Ne, Ar) to generate physical predictions of N2 gas concentrations for 112 observations from 61 UFA springs, we show that excess (i.e. denitrification-derived) N2 is highly variable in space and inversely correlated with dissolved oxygen (O2). Negative relationships between O2 and δ15NNO3 across a larger dataset of 113 springs, well-constrained isotopic fractionation coefficients, and strong 15N:18O covariation further support inferences of denitrification in this uniquely organic-matter-poor system. Despite relatively low average rates, denitrification accounted for 32 % of estimated aquifer N inputs across all sampled UFA springs. Back-calculations of source δ15NNO3 based on denitrification progression suggest that isotopically-enriched nitrate (NO3–) in many springs of the UFA reflects groundwater denitrification rather than urban- or animal-derived inputs.

Highlights

Anthropogenic increases in reactive nitrogen (N) availability have wide-ranging consequences including eutrophication of aquatic systems, acidification of soils and surface waters, loss of biodiversity, and facilitation of disease transmission (Vitousek, 1994; Galloway et al, 2003; Smith and Schindler, 2009)
Diverse reaction modes, and challenges of direct measurement of N2 all contribute to persistent high uncertainty in local, regional, and global estimates of denitrification (Davidson and Seitzinger, 2006; Groffman et al, 2009)
Throughout parts of northern Florida, the Upper Floridan Aquifer (UFA) is confined by low-permeability, high-clay deposits that preclude infiltration to the UFA except via sinkholes and fractures; these confining layers are largely absent in the central-western portion of the state (Scott et al, 2004)

Summary

Introduction

Anthropogenic increases in reactive nitrogen (N) availability have wide-ranging consequences including eutrophication of aquatic systems, acidification of soils and surface waters, loss of biodiversity, and facilitation of disease transmission (Vitousek, 1994; Galloway et al, 2003; Smith and Schindler, 2009). Denitrification, which reduces NO−3 to N2 gas, mitigates this enrichment by returning N to long-residence-time atmospheric pools, and is an important component of the nitrogen cycle at local, regional, and global scales (David et al, 2006; Seitzinger et al, 2006; Townsend and Davidson, 2006; Schlesinger, 2009; Sigman et al, 2009). Aquifer denitrification is a potentially large component of regional and global nitrogen (N) budgets, with a recent global estimate of 44 Tg N yr−1 (16 % of land-based annual N inputs; Seitzinger et al, 2006). Existing estimates are based on simple models and are extremely poorly constrained (range of estimates from Seitzinger et al, (2006): 0– 138 Tg N yr−1), in large part due to the limited number and Published by Copernicus Publications on behalf of the European Geosciences Union

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