Moreau-Fournier, M. 2010 Comparison of nitrate analysis results between two laboratories and effects of sample holding time on nitrate concentration analyses Lower Hutt, N.Z.: GNS Science. GNS Science report 2010/14 26 p.
Abstract: The National Groundwater Monitoring Programme (NGMP) is a long-term research and monitoring programme operated by GNS Science in collaboration with 15 regional authorities. The primary aims of the NGMP are to 1) provide a national perspective on groundwater quality in New Zealand, including determination of natural “baseline” groundwater quality, 2) to identify spatial and temporal patterns in groundwater quality and associate them with certain causes, such as human influence, and 3) to develop and convey best-practice methods for groundwater sampling, chemical analysis and interpretation. In April 2009, Auckland Regional Council (ARC) made GNS Science aware of significant differences in the nitrate concentrations in NGMP samples from the Auckland Region that were analysed by two separate laboratories. The ARC has traditionally collected duplicate sets of samples from NGMP sites in the Auckland Region, one set being analysed by Hill Laboratories (Hamilton), and the other set being analysed by the Wairakei Analytical Laboratory (WAL) of GNS Science. The April 2009 paired results of the six NGMP sites located in ARC suggest that nitrate concentrations reported by WAL are consistently lower than the nitrate concentrations determined by Hill’s. Nitrate is one of the primary indicators of anthropogenic degradation of groundwater quality (Daughney and Reeves, 2005), as a result, it is of particular importance that nitrate analyses are robust and that the data can be meaningfully interpreted. This report reviews nitrate concentrations on duplicate samples analysed quarterly at selected NGMP sites by both laboratories between Jan-06 and Oct-09. It also summarises the results of an assessment of the effect of sample holding time on nitrate concentrations conducted at WAL, as a varying sample holding time is likely to be different between the two laboratories. (auth)