Review of groundwater monitoring data (2004-2010) from the Waihora well field, Lake Taupo catchment

TR 2011/27
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Report: TR 2011/27

Author: Roland Stenger (Lincoln Ventures Ltd)

Abstract

Lincoln Ventures was contracted by Waikato Regional Council to provide a comprehensive review of groundwater level and chemistry data collected at two water table well (WTW) transects in the Waihora well field for the period 2004 – 2010.

Land use at the site was relatively low intensity sheep and beef grazing as found widespread in the Lake Taupo catchment. The WTWs comprised of relatively long screen lengths that intersected the groundwater table, hence yielded composite samples of groundwater drawn from a water column of variable thickness.

In contrast, multilevel wells (MLWs) that were additionally installed in 2007 enabled sampling of discrete depth ranges of 50 cm
thickness, which allowed for detection of vertical groundwater chemistry stratification. Data from these MLWs have been used to facilitate the interpretation of the WTW data. A discussion of the implications of the different well types on monitoring data has been included in this report..

Review of groundwater monitoring data (2004-2010) from the Waihora well field, Lake Taupo catchment [PDF, 819 KB]

Contents
  Executive summary 1
1 Introduction 4
1.1 Background 4
1.2 Scope of work 4
2 Site information 6
2.1 Location and land use 6
2.2 Well field set-up and stratigraphy 10
2.2.1 Lithological stratigraphy 10
2.2.2 Water table wells (WTWs) 10
2.2.3 Multi-level wells (MLWs) 11
2.3 Monitoring programme 12
2.3.1 Groundwater levels 12
2.3.2 Groundwater chemistry 12
3 Groundwater monitoring results 13
3.1 Water table wells (WTWs) 15
3.1.1 Groundwater levels (relative to sea level) 15
3.1.2 Groundwater levels (relative to ground level) 17
3.1.3 Nitrate nitrogen (NO3-N) 19
3.1.4 Ammoniacal nitrogen (NH4-N) 20
3.1.5 pH 23
3.1.6 Electrical conductivity (EC) 25
3.1.7 Dissolved total carbon (DTC) 27
3.1.8 Dissolved inorganic carbon (DIC) 29
3.1.9 Dissolved organic carbon (DOC) 31
3.2 Multi-level wells (MLWs) 33
3.2.1 Vertical groundwater redox-gradient (exemplified by WR20) 33
3.2.2 Data from MLWs located in the upper and lower transacts 36
4 Discussion 42
4.1 Groundwater levels 43
4.2 Groundwater chemistry 44
4.2.1 Spatio-temporal variation of selected parameters 45
4.2.2 Effect of groundwater denitrification on nitrate nitrogne concentrations 47
4.2.3 Redox stratificationof shallow groundwater 51
4.2.4 Groundwater nitrate nitrogen vs estimated root zone losses 54
4.3 Effect of well screen length and position 56
5 References 60
6 Acknowledgements 61
7 Appendix I 62
8 Appendix II 63
9 Appendix III 66