Nitrogen and carbon isotopes as tracers of nitrate in groundwater

Nitrate continues to be a problem in New Zealand groundwater supplies. Major sources of nitrate are: mineralisation of soil nitrogen, fertilisers, animal manure (mainly urine) and septic wastes. Loss of ammonia during breakdown of urea causes enrichment in 15N, which in principle can be used to identify the resulting NO3. The authors have a continuous flow mass spectrometer which facilitates analysis of nitrogen and carbon isotopes in samples. Samples are freeze dried, loaded onto the elemental analyser and analysed for 15N and 13C automatically. Present work is looking at the effects of different pretreatments on results, and applying the method to groundwaters from the Bay of Plenty, Waimea Plains and Canterbury. Possible pretreatments may improve the specificity of the measurement; i.e. rather than analysing the total precipitate one may analyse a specific component. Oxidation is producing consistent results. Regular monitoring of groundwater wells is being carried out on the Waimea Plains to trace sources of nitrate. These are being compared with results from areas on the Canterbury Plains, where groundwater nitrate levels are elevated. At the same time CFC concentrations are being used to determine the residence times of the groundwaters sampled, to provide evidence on when the nitrate contamination took place and the time-variation of the contamination