Transport and potential attenuation of nitrogen in shallow groundwaters in the lower Rangitikei catchment, New Zealand

Abstract

Intensive agricultural activities are generally associated with nitrogen leaching from agricultural soils, and this nitrogen has the potential to percolate and contaminate groundwater and surface waters. We assessed surface water and groundwater interactions, and nitrogen leaching and its potential attenuation in shallow groundwater in the lower Rangitikei River catchment (832 km²), New Zealand. We combined regional- and local-scale field surveys and experiments, nutrient budget modelling, and hydraulic and geochemical methods, to gain an insight into leaching, transformation and transport of nitrogen via groundwaters to the river in the study area. Concurrent river flow gaugings (in January 2015) and a piezometric map, developed from measured depths to groundwater in 110 bores (in October 2014), suggest groundwater discharges to the Rangitikei River in the upper parts of the study area, while there is groundwater recharge near the coast. The groundwater redox characterisation, based on sampling and analysis of 15 mostly shallow bores (< 30 m below ground level (bgl)), suggests groundwater across the lower Rangitikei catchment in general is under anoxic/reduced conditions. The groundwater typically has low dissolved oxygen (DO) concentrations (< 1 mg/L), suggesting the subsurface environment is conducive to potential attenuation by ‘denitrification’ of NO₃-N in groundwater. We further measured NO₃-N attenuation in shallow groundwater piezometers (3–6 m bgl) using single-well push-pull tests. We found generally low levels (< 0.5 mg/L) of NO₃-N in shallow groundwater piezometers (> 5 m bgl), despite being installed under intensive land uses, such as dairying and cropping. Our in-field push-pull tests showed NO₃-N reduction at four shallow groundwater piezometers, with the rates of reduction varying from 0.04 mg N L⁻¹ h⁻¹ to 1.57 mg N L⁻¹ h⁻¹. This highlights the importance of a sound understanding of not only the sources, but also transport and transformation, or fate, of nutrients leached from farms, to mitigate the likely impacts of land use on water quality and ecosystem health in agricultural catchments.