Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

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 N<sub>2</sub> gas concentrations for 112 observations from 61 UFA springs, we show that excess (i.e. denitrification-derived) N<sub>2</sub> is highly variable in space and inversely correlated with dissolved oxygen (O<sub>2</sub>). Negative relationship between O<sub>2</sub> and δ<sup>15</sup>N<sub>NO<sub>3</sub></sub> across a larger dataset of 113 springs, well-constrained isotopic fractionation coefficients, and strong <sup>15</sup>N : <sup>18</sup>O 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 δ<sup>15</sup>N<sub>NO<sub>3</sub></sub> based on denitrification progression suggest that isotopically-enriched nitrate (NO<sub>3</sub><sup>−</sup>) in many springs of the UFA reflects groundwater denitrification rather than urban- or animal-derived inputs

Identifying sources and processes influencing nitrogen export to a small stream using dual isotopes of nitrate

“…For example, Burns et al [2009] observed linear seasonal patterns of 18 O-NO 3 À : 15 N-NO 3 À but suggested that mixing of two or more distinct sources may have caused to these patterns. In combination with other tracers, Osaka et al [2010] found little evidence of denitrification in soil-waters, whereas they showed evidence of denitrification in the shallow groundwater in their catchment, similar to other shallow groundwater studies [Bottcher et al, 1990;Heffernan et al, 2012;McMahon and Bohlke, 2006;Mengis et al, 1999].…”

Nitrate Pathways, Processes, and Timing in an Agricultural Karst System: Development and Application of a Numerical Model

“…The quick flow contribution increases at the incidence of hydrologic activity and contributes relatively diluted NO 3 − to the spring flux signal. The long residence time of water in phreatic pores could lead to net denitrification (Heffernan et al, ) as pathway results indicate that high NO 3 − concentrations from percolating epikarst water are decreased prior to export by the phreatic pathway.…”

Spatial and temporal variations in nitrogen export from a New Zealand pastoral catchment revealed by stream water nitrate isotopic composition

“…First, assimilation causes significantly less isotopic fractionation than denitrification [Karsh et al, 2014], which would presumably become even weaker under field conditions due to cellular regulation [Kritee et al, 2012] and the simultaneous occurrence of ''reverse'' fractionation from the ongoing mineralization of this organic material [M€ obius, 2013]. Second, if the NO 2 3 pool was primarily controlled by algae and macrophytes, the strong seasonal differences in aquatic plant growth (Environment Canterbury, unpublished data, 2012) would be reflected in its isotopic composition [Heffernan et al, 2012]. However, in the current study, precipitation was a stronger driver of NO 2 3 isotopic composition than season or temperature.…”