Spatial and temporal variations and trends in groundwater quality in the Manawatu-Wanganui region

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Daughney, C.J.; Meilhac, C.; Zarour, H. 2009 Spatial and temporal variations and trends in groundwater quality in the Manawatu-Wanganui region. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/02 137 p. + 1 CD

Abstract: Horizons Regional Council (Horizons) commissioned GNS Science to undertake a comprehensive review of all groundwater quality data in order to: identify and interpret spatial and temporal variations and trends in groundwater quality across the Manawatu-Wanganui Region; provide an assessment of the suitability of the groundwater quality in the Region for various purposes, including human consumption, irrigation and livestock consumption; provide information to underpin the planned review of the Region’s state of the Environment (SoE) groundwater quality monitoring programme; and provide information to assist with the on-going development of strategies for management of the Region’s water resources. The Horizons groundwater quality dataset allows for a high level of interpretation with respect to the status and spatial variation of groundwater quality across the Region. Major conclusions from this study are as follows: untreated groundwater is generally of suitable quality for livestock consumption; naturally occurring high levels of iron, manganese, ammonia, hardness and pH may act as a barrier to use of untreated groundwater for human consumption and irrigation; in relation to the influence of human activities on groundwater quality - there is no evidence of seawater intrusion into coastal aquifers and only about 10% of the sites considered in this investigation display nitrate (NO3) concentrations in excess of the expected natural background, highlighting a localised part of the Horowhenua and adjacent areas south of the Manawatu River where groundwater is not suitable for human consumption; spatial variations in groundwater quality across the Region appear to reflect differences in the origin and type of materials comprising the various aquifers, allowing for mapping of areas in which specific water quality issues are most likely to occur; temporal trends in the Region’s groundwater quality are very difficult to assess due to the limitations in the length and continuity of data for most monitoring sites, but for the few sites with sufficient data, groundwater quality is not changing substantially over time. (auth)