Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

[摘要] Aquifer denitrification is among the most poorly constrained fluxes inglobal and regional nitrogen budgets. The few direct measurements ofdenitrification in groundwaters provide limited information about itsspatial and temporal variability, particularly at the scale of wholeaquifers. Uncertainty in estimates of denitrification may also lead tounderestimates of its effect on isotopic signatures of inorganic N, andthereby confound the inference of N source from these data. In this study,our objectives are to quantify the magnitude and variability ofdenitrification in the Upper Floridan Aquifer (UFA) and evaluate its effecton N isotopic signatures at the regional scale. Using dual noble gas tracers(Ne, Ar) to generate physical predictions of N₂ gas concentrations for112 observations from 61 UFA springs, we show that excess (i.e. denitrification-derived) N₂ is highly variable in space and inverselycorrelated with dissolved oxygen (O₂). Negative relationships betweenO₂ and δ¹⁵N_NO3 across a larger dataset of 113springs, well-constrained isotopic fractionation coefficients, and strong¹⁵N:¹⁸O covariation further support inferences of denitrificationin this uniquely organic-matter-poor system. Despite relatively low averagerates, denitrification accounted for 32 % of estimated aquifer N inputsacross all sampled UFA springs. Back-calculations of source δ¹⁵N_NO3 based on denitrification progression suggest thatisotopically-enriched nitrate (NO₃^–) in many springs of the UFAreflects groundwater denitrification rather than urban- or animal-derivedinputs.

[学科分类] 地球化学与岩石