Isotopic evidence for the cycling of nitrate and nitrous oxide in aquifers beneath an irrigated agricultural district using established and emerging measurement techniques

Abstract

Modern agricultural fertiliser application rates often result in groundwater contamination with nitrogen compounds such as nitrate (NO3-) and nitrous oxide (N2O). Groundwater isotopic measurements can elucidate the hydrogeochemical controls on NO3- and N2O, leading to improved land management practices. Although most groundwater isotopic measurement techniques are well-established, laser spectroscopic methods to acquire groundwater N2O isotopic data are still evolving. However, if proven robust, they may provide potential benefits in terms of cost, ease of use and immediacy of results when compared to the conventional method using isotope-ratio mass-spectrometry (IRMS).

The primary aim of this thesis was to use established multiple stable and radioactive isotope tracers to elucidate the hydrogeochemical controls on the production and reduction of NO3- and N2O in aquifers beneath the Emerald Irrigation Area (EIA), Queensland, Australia. Isotopic evidence indicates that NO3- in EIA aquifers is derived from fertiliser N which is retained in the soil organic N pool prior to mineralisation, nitrification and leaching. Groundwater NO3- occurrence is linked to irrigated agriculture beginning in the 1970s. N2O isotope data acquired via IRMS shows that groundwater N2O is a mixture of nitrification- and denitrification-derived N2O that has undergone reduction. The reduction of NO3- and N2O occurs under two contrasting scenarios: progressively, as groundwater flows from the upgradient oxic basalt aquifer to the downgradient suboxic alluvial aquifer; and beneath leaking irrigation channels, where electron donor-rich suboxic hyporheic zones have manifested. Modifications to current land management practices have been proposed in view of these findings.

The secondary aim of this thesis was to assess the utility of using laser spectroscopy in acquiring accurate N2O isotope data from EIA groundwater samples via comparison with equivalent IRMS data. Several N2O isotope laser spectrometers were tested using laboratory-prepared gaseous mixtures, and a calibration workflow was developed. Subsequent comparison of the laser spectroscopy and IRMS results shows poor agreement, suggesting the proposed calibration procedures cannot account for groundwater gas samples containing significant compositional variability. Several other potential contributing factors are discussed, culminating in recommendations for areas of future research to further refine the calibration workflow.