Investigation of groundwater in the Upper Motueka River Catchment Science Report Michael K. Stewart, Timothy Y.-S. Hong, Stewart C. Cameron, 2003/32 Chris Daughney, Tamara Tait and Joseph T. Thomas September 2005 Investigation of groundwater in the Upper Motueka River Catchment Michael K. Stewart Timothy Y.-S. Hong Stewart C. Cameron Chris Daughney Tamara Tait Institute of Geological & Nuclear Sciences Joseph T. Thomas Tasman District Council, Richmond Institute of Geological & Nuclear Sciences science report 2003/32 Institute of Geological & Nuclear Sciences Limited Lower Hutt, New Zealand September 2005 Gracefield Research Centre, 69 Gracefield Road, P O Box 30368, Lower Hutt, New Zealand, Telephone: +64-4-570 1444, Facsimile: +64-4-570 4600 A Crown Research Institute BIBLIOGRAPHIC REFERENCE Stewart, Michael K., Hong, Timothy Y.-S., Cameron, Stewart C., Daughney, Chris, Tait, Tamara and Thomas, Joseph T. 2005. Investigation of groundwater in the Upper Motueka River Catchment. Institute of Geological & Nuclear Sciences science report 2003/32. 53p. Michael K. Stewart, Institute of Geological & Nuclear Sciences Limited, Wellington Timothy Y.-S.. Hong, Institute of Geological & Nuclear Sciences Limited, Taupo Stewart C. Cameron, Institute of Geological & Nuclear Sciences Limited, Taupo Chris Daughney, Institute of Geological & Nuclear Sciences Limited, Wellington Tamara Tait, Institute of Geological & Nuclear Sciences Limited, Taupo Joseph T. Thomas, Tasman District Council, Richmond © Institute of Geological & Nuclear Sciences Limited, 2005 ISSN 1171-9184 ISBN 0-478-09828-6 CONTENTS EXECUTIVE SUMMARY................................................................................................................................... ii KEYWORDS........................................................................................................................................................ iii 1. INTRODUCTION ...................................................................................................................................1 1.1 Background .................................................................................................................................1 1.2 Study area....................................................................................................................................1 1.3 Hydrological monitoring network ...............................................................................................2 2. HYDROGEOLOGY................................................................................................................................4 2.1 Groundwater recharge ...............................................................................................................12 2.1.1 Groundwater discharge from the Moutere Gravel......................................................12 2.1.2 Rainfall infiltration .....................................................................................................12 2.1.3 River flow loss............................................................................................................13 2.2 Groundwater storage .................................................................................................................16 2.2.1 Stream Depletion ........................................................................................................17 3. GROUNDWATER-RIVER-RAINFALL MODELLING ..................................................................21 3.1 Study objective..........................................................................................................................21 3.2 Methodology .............................................................................................................................21 3.2.1 Dynamic artificial neural network ..............................................................................21 3.2.2 Sensitivity Analysis Procedures .................................................................................22 3.3 Model development...................................................................................................................22 3.3.1 Data Selection.............................................................................................................22 3.3.2 Variable selection .......................................................................................................23 3.3.3 Model Performance Criteria .......................................................................................24 3.4 Modelling results.......................................................................................................................24 4. GEOCHEMISTRY................................................................................................................................31 4.1 Introduction ...............................................................................................................................31 4.2 Chemical Compositions.............................................................................................................31 4.2.1 Methods ......................................................................................................................31 4.2.2 Results ........................................................................................................................31 4.2.2.1 Motueka-type waters.....................................................................................35 4.2.2.2 Motupiko-type waters ...................................................................................37 4.3 Oxygen-18 Concentrations........................................................................................................38 4.4 Groundwater Dating ..................................................................................................................39 4.4.1 Monthly δ18O values .................................................................................................39 4.4.2 CFC and Tritium Dating.............................................................................................43 5. DISCUSSION AND CONCLUSIONS .................................................................................................44 6. REFERENCES ......................................................................................................................................47 ©Institute of Geological & Investigation of groundwater in the Nuclear Sciences Limited i Upper Motueka River Catchment EXECUTIVE SUMMARY Groundwater investigation in the Upper Motueka catchment is a key end user-driven component of the Integrated Catchment Management (ICM) project, a six year programme which commenced in July 2000. The goal of the ICM project is to conduct multi-disciplinary, multi-stakeholder research to provide information and knowledge that will improve the management of land, freshwater, and near-coastal environments in catchments with multiple, interacting, and potentially conflicting land uses. The principal aims of the investigations are to understand the hydrogeology of the Upper Motueka valley (including parts of the Motupiko and Tadmor valleys), the aquifer hydraulic properties, the occurrence of groundwater (recharge, storage and discharge), and the connectivity to the rivers. Understanding these will give a conceptual model of the river/groundwater system to constrain computational models. The study area lies within the Moutere Depression at lower altitude and has moderate rainfall (900-1300 mm). Higher land to the east with rainfall up to 2000 mm feeds the upper reaches of the Motueka River. Comparison of the Motueka River flow at the gorge and upstream of the Wangapeka River confluence shows that much of the flow, and particularly much of the low flow, are generated within the comparatively small gorge headwater area, comprising ultramafics and Maitai Group sediments. Below the gorge, the catchment contains Moutere Gravel. The Motupiko and Tadmor catchments are predominantly Moutere Gravel. The hydrogeology of the study area is described in Chapter 2. Within the Upper Motueka, Motupiko and Tadmor river valleys, groundwater is abstracted from shallow, thin (<15m), unconfined, alluvial gravel aquifers that receive recharge from river flow loss and local rainfall. Four potentially groundwater-bearing river terrace formations have been identified, which from youngest to oldest are: recent river gravel deposits, Speargrass Formation, Tophouse Formation, and Manuka Formation. The groundwater-bearing formations are underlain by the relatively low permeability Moutere Gravel Formation. At present, groundwater is abstracted from the lower three terrace formations only. The piezometric contours in the valleys are close to being at right angles with the rivers, showing that groundwater flow is primarily along the valleys, but piezometric contours, river flow measurements, and river stage RL heights have identified reaches where river flow is lost to groundwater and reaches where groundwater contributes to river flow. River flow loss occurs from Higgins and Quinneys Bush to Hyatts, river gain from Hyatts to Tapawera Bridge, river loss from Tapawera Bridge to Glenrae, and river gain below Glenrae where the valley narrows. Pump test and groundwater level recorder data have been used to estimate the hydraulic connection between the river and the formations from which groundwater is abstracted. Rainfall infiltration of 0.33 m/yr (based on annual rainfall