Oteramika Trial Catchment Groundwater Studies. Studies into Non-point Source Groundwater Effects in Southland. Southland Regional Council. August, 1998 Private Bag, INVERCARGILL, NZ. Prepared by: Jens H Rekker AquaFirma Ltd, Box 5469, DUNEDIN. This report is provided by AquaFirma Ltd solely for the benefit of Southland Regional Council and AquaFirma Ltd shall accept no liability of any kind whatsoever resulting from a third party's use or reliance upon the information in this report for any reason whatsoever. © Southland Regional Council & AquaFirma Ltd 1998. AquaFirma Ltd Southland Regional Council; Oteramika Groundwater Studies. Page 1. Table of Contents. Executive Summary............................................................................................5 1. Introduction. .................................................................................................. 9 1.1 Problem Definition. ........................................................................................................................... 9 1.2 Method. ........................................................................................................................................... 11 1.2.1 Initial Characterisation. .......................................................................................................... 11 1.2.2 Monitoring / Long Term Data Collection. ............................................................................. 12 1.2.2.1 Groundwater Quality. ................................................................................................... 12 1.2.2.2 Groundwater Quantity and Elevation. .......................................................................... 12 1.2.3 Analysis of Field Data. ........................................................................................................... 12 1.2.4 Modelling of Groundwater Flow and Composition. ............................................................... 12 2. Edendale Aquifer Setting. ........................................................................... 13 2.1 Hydrogeology. ................................................................................................................................. 13 2.1.1 Aquifer Composition and Geometry. ..................................................................................... 13 2.1.2 Aquifer Properties. ................................................................................................................. 15 2.1.3 Groundwater Recharge. .......................................................................................................... 16 2.1.4 Groundwater Discharge.......................................................................................................... 17 2.1.5 Groundwater Level Fluctuation. ............................................................................................. 19 2.1.6 Aquifer Conceptual Model. .................................................................................................... 21 2.2 Soils. ............................................................................................................................................... 22 2.3 Surface Water Hydrology................................................................................................................ 24 2.3.1 Hydrological Data. ................................................................................................................. 24 2.3.2 Specific Runoff & Water Balances. ....................................................................................... 25 2.3.3 Water Balances. ..................................................................................................................... 27 2.3.4 Recharging Groundwater Interaction. .................................................................................... 29 3. Groundwater Quality & Contaminant Loading. ........................................ 30 3.1 Introductory Concepts on Groundwater Quality. ............................................................................ 30 3.2 Groundwater Quality Indices. ......................................................................................................... 31 3.2.1 Primary Indices. ..................................................................................................................... 31 3.2.2 Guideline Values. ................................................................................................................... 31 3.3 Results of Long Term Monitoring of Bore and Spring Waters. ...................................................... 32 3.3.1 Nitrate. ................................................................................................................................... 35 3.3.2 Phosphorus. ............................................................................................................................ 36 3.3.3 pH Units. ................................................................................................................................ 37 3.3.4 Electrical Conductivity. .......................................................................................................... 38 3.3.5 Chloride. ................................................................................................................................ 39 3.3.6 Other Cations and Anions. ..................................................................................................... 40 3.3.7 Cation / Anion Balances. ........................................................................................................ 43 3.4 Mass Loading. ................................................................................................................................. 45 Alkalinity ........................................................................................................................................ 45 3.4.1 Mass Load for a Conservative Ion. ........................................................................................ 46 3.5 Groundwater Typing. ...................................................................................................................... 47 4. Groundwater Modelling. ............................................................................. 52 4.1 Modelling Approach. ...................................................................................................................... 52 AquaFirma Ltd Southland Regional Council; Oteramika Groundwater Studies. Page 2. 4.1.1 Groundwater Flow Model. ..................................................................................................... 52 4.1.2 Groundwater Contaminant Transport Model. ........................................................................ 52 4.2 Modelling Objectives. ..................................................................................................................... 53 4.3 Phases of Model Development. ....................................................................................................... 53 4.4 Model Framework Development. .................................................................................................... 54 4.4.1 Model Domain. ...................................................................................................................... 54 4.4.2 Base of Aquifer. ..................................................................................................................... 54 4.4.3 Top of Aquifer. ...................................................................................................................... 54 4.4.4 Lateral Boundaries. ................................................................................................................ 54 4.5 Model Stress Initialisation. .............................................................................................................. 55 4.5.1 Aquifer Recharge. .................................................................................................................. 55 4.5.2 Spring Discharge Zones. ........................................................................................................ 55 4.5.3 River Discharge Zones. .......................................................................................................... 55 4.5.4 Wells. ..................................................................................................................................... 55 4.6 Parameter Assignment. .................................................................................................................... 55 4.7 Order of Simulation......................................................................................................................... 56 4.8 Linkage with Land Use. .................................................................................................................. 56 4.8.1 Magnitude of Nitrate-N Losses. ............................................................................................. 58 4.9 Scenario Modelling. ........................................................................................................................ 59 4.9.1 Scenario 1: All Dairy, High Intensity. .................................................................................... 60 4.9.2 Scenario 2: High Intensity Grazing. ....................................................................................... 60 4.9.3 Scenario 3: Replacement of Forage Cropping.