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Groundwater chemistry of the Piako catchment, Hauraki Plains

TR 1993/07
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Report: TR 1993/07

Author: JC Hadfield

About this report

This study is part of a larger investigation of the Piako catchment groundwater resources.

The Piako catchment comprises an area of approximately 1435km2 of predominantly dairy farmland in the Hauraki Plains. Surface water resources in the catchment area is limited. Groundwater supplies some 80 per cent of farms and about half of the total water supply to the area.

The major content of this report is a presentation of the results from a groundwater chemistry survey undertaken in December 1992. Time series analyses are not included but are recommended for future state of environment monitoring.

Contents
1 Introduction  1
2 Hydrogeologic setting 3
2.1 Structure and geomorphology 3
2.2 Stratigraphy and formation distribution 3
2.2.1 Tauranga Group 3
2.2.2 Ignimbrites 6
2.2.3 Kiwitahi Volcanics 7
2.3 Groundwater flow regime 7
3 Sampling and data analysis 9
3.1 Sampling 9
3.2 Data analysis 9
4 Groundwater chemistry characteristics 13
4.1 Introduction 13
4.2 Groundwater chemical characteristics of geologic groupings 15
4.2.1 Hinuera Formation 15
4.2.2 Non-Hinuera Formation Tauranga Group Sediments 18
4.2.3 Ignimbrites 19
4.2.4 Kiwitahi Volcanics 20
4.3 Spatial distribution of chemical characteristics 20
4.3.1 Lateral distribution 20
4.3.2 Vertical distribution 22
5 Discussion and review of factors influencing groundwater chemistry 29
5.1 Introduction 29
5.2 Chemical influence of rainwater 29
5.3 Chemical influence of formation 30
5.3.1 Reduction and oxidation processes 31
5.4 Organic and microbial influences 32
5.5 Saltwater influence 33
5.6 Geothermal influence 34
5.7 Chemical influence on streams 36
5.8 Human influence  36
6 Environmental isotopes  37
6.1 Introduction  37
6.2 Tritium  37
6.3 Oxygen and deuterium  38
7  Groundwater quality and implications for use 39
7.1  Introduction 39
7.2 Nitrate contamination 39
7.3 Excessive iron problem 40
7.4 Domestic use 41
7.5 Agricultural use 43
7.6 Industrial use 44
7.7 Land disposal practices 45
8 Summary and conclusions 47
  References 49
  Appendix I: Groundwater chemical analyses 53
  Appendix II: Ionic ratios 59
  Appendix III: Comparative box plots 61
  Appendix IV: Chemistry of Hauraki Plains thermal waters 65

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