Groundwater in the Okataina caldera: Model of future nitrogen loads to Lake Tarawera CBER Report 94 Prepared for Environment Bay of Plenty and Lake Tarawera Ratepayers’ Association By Nicolas Gillon1, Paul White2, David Hamilton1 and Warwick Silvester1 1. Centre for Biodiversity and Ecology Research, School of Science and Engineering, The University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand 2. Institute of Geological and Nuclear Sciences, Private Bag 2000, Taupo, 3352, New Zealand September 2009 1 Table of contents List of figures……………………………………………………………………………. 4 List of tables………………………………………………………………………………6 1. Introduction ................................................................................................... 6 2. Geomorphology and Land Use .................................................................... 7 2.1 Okataina Volcanic Centre ................................................................................................ 7 2.2 Lake Tarawera surface catchment ................................................................................... 9 3. Geology ......................................................................................................... 12 3.1 Geological history and lithological description of the Okataina Caldera complex ....... 12 3.1.1 Whakamaru Ignimbrite ......................................................................................................... 12 3.1.2 Matahina Ignimbrite ............................................................................................................. 12 3.1.3 Pokopoko-Onuku Pyroclastics .............................................................................................. 13 3.1.4 Kaingaroa Ignimbrite ........................................................................................................... 13 3.1.5 Mamaku Ignimbrite .............................................................................................................. 13 3.1.6 Rotoiti Pyroclastics ............................................................................................................... 14 3.1.7 Huka Group Sediments ......................................................................................................... 14 3.1.8 Recent Okataina Rhyolites .................................................................................................... 14 3.2 Tarawera geological model ........................................................................................... 18 4. Hydrogeology............................................................................................... 21 4.1 Aquifer layers ................................................................................................................ 21 4.2 Geothermal features ....................................................................................................... 22 5. Surface Hydrology ...................................................................................... 24 6. Water Budget .............................................................................................. 26 6.1 Okataina Caldera water budget ...................................................................................... 26 6.1.1 Lake Rerewhakaaitu catchment ........................................................................................... 27 6.1.2 Lake Okaro catchment ......................................................................................................... 29 6.1.3 Lake Rotomahana catchment ............................................................................................... 30 6.1.4 Lake Tikitapu catchment ...................................................................................................... 33 6.1.5 Lake Rotokakahi catchment ................................................................................................. 34 6.1.6 Lake Okareka catchment ..................................................................................................... 36 2 6.1.7 Lake Okataina catchment .................................................................................................... 37 6.1.8 Lake Tarawera catchment .................................................................................................... 38 6.2 Groundwater transfers between catchments ................................................................. 40 6.3 Lake Tarawera water budget ........................................................................................ 42 6.3.1 Water budget components..................................................................................................... 42 6.3.2 Water budget ......................................................................................................................... 43 7. Lake Tarawera Groundwater Flow Model .............................................. 44 7.1 Lake Tarawera sub-catchments ..................................................................................... 44 7.2 Conceptual model .......................................................................................................... 47 7.2.1 Initial properties of the proposed model .............................................................................. 47 7.2.2 Boundary conditions and inflows ......................................................................................... 47 7.3 Introduction to the FEFLOW software .......................................................................... 49 7.4 Model build.................................................................................................................... 49 7.5 Model running ............................................................................................................... 50 7.6 Steady state solution ...................................................................................................... 52 7.6.1 General results ..................................................................................................................... 52 7.6.2 Estimated groundwater flow properties .............................................................................. 53 7.6.3 Water budget analysis........................................................................................................... 53 8. Land Use Scenarios ..................................................................................... 54 Scenario 1 : Pre-historic land use ....................................................................................... 56 Scenario 2 : Current land use .............................................................................................. 58 Scenario 3: Cattle farming extension .................................................................................. 61 Scenario 4: Dairy extension ................................................................................................ 62 9. Septic Tanks ............................................................................................................. 64 Scenario 5 : Tourist development (with pasture retirement) ............................................... 64 Scenario 6 : On-site effluent treatment ................................................................................ 66 Scenario 7: Wastewater treatment plant ............................................................................. 67 10. Information gaps and recommendations .................................................. 68 11. Summary ...................................................................................................... 70 12. References .................................................................................................... 72 3 List of Figures Figure 1 - Topographic map (MapToaster Topo NZ 1:250,000) of the Okataina Volcanic Centre; east of Lake Rotorua. ...................................................................................... 9 Figure 2 - Lake Tarawera catchment topographic map. .......................................................... 11 Figure 3 - Land use of the Lake Tarawera catchment in 2003. ............................................... 11 Figure 4 - Geological map of the Okataina Volcanic Centre (Nairn 2002). ........................... 16 Figure 5 – Lake Tarawera catchment NW-SE cross section (New Zealand Geological Survey (1989). ........................................................................................................................ 19 Figure 6 - Three-dimensional model of Okataina Caldera complex (Pamer pers. comm.)..... 20 Figure 7 - North-south vertical geological cross section of the Lake Tarawera catchment (Pamer pers. comm.). ................................................................................................. 20 Figure 8 - Geothermal areas (in orange) in the OVC (Nairn, 2002). ...................................... 23 Figure 9 - Lake Tarawera surface flow monitoring sites ........................................................ 24 Figure 10 - Correlation between lake water level and lake volume for Lake Rerewhakaaitu. 27 Figure 11 - Correlation between lake water level and lake volume for Lake Okaro. ............. 29 Figure 12 - Correlation between lake water level and lake volume for Lake Rotomahana. ... 31 Figure 13 - Geological cross section between Lake Rotomahana and Lake Tarawera indicating the geological link between the two lakes (from the Earth Vision geological model of the OVC, Pamer, pers. comm.). ...............................................