Assessing and Tracking Nitrate Contamination from a Point Source and the Effects on the Groundwater Systems in Mid Canterbury, New Zealand

Water is a valuable and crucial resource, the protection of which poses environmental, social and economic challenges. Fundamental to the sustainable use of water is effective management. In the Canterbury region of New Zealand, nitrate contamination has become a resource management issue due to changes in land use and intensification, which have placed pressure on the region’s groundwater and surface water systems. The purpose of this study was to assess and track nitrate concentrations on the Central Canterbury Plains with specific emphasis on a local point source of nitrate, the Ashburton Meat Processors plant. To make this assessment review of historical data was followed by the collection of 131 groundwater and 25 surface water samples to analyse the geochemical properties of the water and the stable isotopic composition of nitrate in the water. It was hypothesised that nitrate concentrations at a regional scale have increased since regular records began and that the stable isotopic composition of different nitrate sources are not discernable. Nitrate concentrations across the Canterbury region were found to have increased, prompting concerns about water quality. Concentrations are elevated above natural background levels across much of the Canterbury Plains and extreme concentrations are associated with local point sources of nitrate. Nitrate concentrations down gradient of the Ashburton Meat Processing plant are shown to have declined approximately 5% per year for the past ten years, which is in contrast to the rest of the region, where average concentrations have nearly doubled in 20 years. The reduction of contamination from the point source is most likely the result of the implementation of better wastewater management practices in the early 21st century. The δ18O and δ15N values of nitrate were found to be relatively homogenous over the Canterbury Plains. Therefore, it is suggested by this study that the dual-isotope approach alone, is not a viable tool for nitrate source identification in the region. The uniform nitrate stable isotopic composition in Canterbury could be attributed to a single, principle source of nitrate, such as clover, that overprints other isotopic compositions of nitrate source, or may also be the result of soil processes and the farming techniques used in the region. This research presents important findings for the future of identifying and managing nitrate sources in the Canterbury region. Better management practices are required for the diffuse source(s) of nitrate contributing to the widespread contamination. Critical thinking and the willingness of stakeholders to engage in the identifying, documenting and solving problems is necessary to ensure the effective management and sustainability of this precious resource