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Title: Occurrence and controls on N2O accumulation in the lower Namoi alluvial aquifer, Australia
Authors: Harris, SJ
Cendón, DI
Hankin, SI
Kelly, BFJ
Keywords: Nitrous oxide
Ground water
Issue Date: 8-Apr-2018
Publisher: Copernicus Publications
Citation: Harris S. J., Cendón D. I., Hankin S. I., & Kelly B. F. (2018). Occurrence and controls on N2O accumulation in the lower Namoi alluvial aquifer, Australia. Paper presented to the European Geosciences Union General Assembly 2018, Vienna, Austria, 8–13 April 2018. In Geophysical Research Abstracts, Vol 20, EGU2018-10576. Retrieved from:
Abstract: The lower Namoi alluvial aquifer (LNAA) in northwest Australia supports a multibillion-dollar agricultural sector focused around cotton growing established in the 1960s. Investigations into N2O emissions from the LNAA and possible perturbations from agriculture and natural processes are lacking. To determine groundwater N2O concentrations and production processes in the LNAA, we sampled groundwater from 23 bores (8.4 – 33.6 m depth) in the lower Namoi catchment. To the best of our knowledge, this is the first study to quantify N2O in groundwater at a catchment scale in Australia. Dissolved N2O-N concentrations ranged from 1.2 – 11.9 μg L-1, and NO3-N concentrations ranged from <0.02 – 5.1 mg L-1. N2O-N and NO3-N concentrations were weakly, yet positively, correlated (r2 = 0.2, p = 0.01). The highest concentrations measured in groundwater were beneath intensely cropped farms (N2O-N ranging from 1.9 – 11.9 μg L-1; and NO3-N ranging from 1.3 – 5.1 mg L-1). An exception to this occurred along a groundwater transect within cropped farmland, where both N2O-N and NO3-N concentrations were lower (1.2 – 2.0 μg L-1 and 0.02 – 0.3 mg L-1, respectively). Spatially, this groundwater transect is located where the Great Artesian Basin (GAB), the largest artesian basin in the world, discharges into the LNAA (Iverach et al. 2017). Here, GAB input causes the groundwater to have low dissolved oxygen (0.2 – 0.4 mg L-1) and warmer temperatures (23 – 26 ºC), which promotes the reduction of NOǯ 3 to gaseous N2O and N2via denitrification. Mean emission factors for indirect N2O emissions (EF5g; N2O-N / NO3-N) from groundwater bores located on farm (EF5g = 0.2%) were lower than IPCC default EF5g (EF5g = 0.25%), while estimates from riparian zone groundwater (EF5g = 3.0%) were higher. Importantly, EF5g values from groundwater affected by GAB discharge (EF5g = 3.4%) were also significantly greater than the IPCC default EF5g, despite being located beneath intensely cropped farmland and having low N2O-N contents. The proximity of GAB discharge to major basement faults (FrogTech 2006) suggests these geological features may act as principal conduits for GAB input into the LNAA. By extension, this highlights a fundamental geological control on N2O emissions and nitrogen cycling – a concept that has been largely ignored in the literature. © Author(s) 2018. CC Attribution 4.0 license.
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