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Groundwater Flow and Nitrogen Transport Modelling of the Northern Lake Taupo Catchment

TR200739

Report: TR 2007/39

Author: John Hadfield

Abstract

The quality of water in Lake Taupo is under threat, largely as a result of increasing nitrogen from farming in its catchment. Groundwater flow and contaminant transport modelling of the northern catchment using Modflow and MT3D indicate a substantial lag (~250 years) before the full impact of current land use would be realised. Pre-development nitrogen flux from groundwater is estimated at about 65 tonnes annually with about 3,500 tonnes of nitrogen stored in the groundwater system. After 35 years of ‘current’ land use, annual flux from groundwater is estimated to be nearly 175 tonnes at which stage about 10,000 tonnes of nitrogen may be stored in the system. Under simulated equilibrium with current land use, about 300 tonnes of nitrogen flux from the northern groundwater system is predicted annually. Some 18,000 tonnes of nitrogen would be stored in the aquifer. Assumptions include conservative nitrogen transport and porosity reflecting fracture flow in the deeper western model domain. Modelling results are broadly consistent with water age dating and stream water quality information. The extent of groundwater denitrification, currently thought to be minor, is an aspect of ongoing research. The proposed reduction of the manageable load to the lake was found to provide useful mitigation in the northern catchment.

Groundwater Flow and Nitrogen Transport Modelling of the Northern Lake Taupo Catchment [PDF, 826 KB]

Contents
  Acknowledgements i
  Executive summary vii
1 Introduction 1
2 Scope and purpose 1
3 Hydrogeologic characteristics 2
3.1 Introduction 2
3.2 Hydrogeology 2
3.3 Water chemistry 8
3.4 Dating 8
4 Conceptual model 9
5 Model construction 11
5.1 Model grid 11
5.2 Boundary conditions 12
6 Steady state flow modelling 13
6.1 Inputs 13
6.2 Calibration and results 17
6.3 Sensitivity analysis 20
7 Transient contaminant modelling 21
7.1 Introduction 21
7.2 Transport properties 22
7.3 Calibration and results 23
7.4 Sensitivity analysis 32
8 Discussion – implications and predictive uncertainties 35
9 Further research 37
10 Summary 37
  References 39
Appendix I: Well Input Details 43
Appendix II Model Technical Basis 46
Appendix III Field Investigation of Denitrification in Taupo Catchment Groundwater 50