Nitrate contamination and groundwater chemistry – Ashburton-Hinds plain Report No. R10/143 ISBN 978-1-927146-01-9 (printed) ISBN 978-1-927161-28-9 (electronic) Carl Hanson and Phil Abraham May 2010 Report R10/143 ISBN 978-1-927146-01-9 (printed) ISBN 978-1-927161-28-9 (electronic) 58 Kilmore Street PO Box 345 Christchurch 8140 Phone (03) 365 3828 Fax (03) 365 3194 75 Church Street PO Box 550 Timaru 7940 Phone (03) 687 7800 Fax (03) 687 7808 Website: www.ecan.govt.nz Customer Services Phone 0800 324 636 Nitrate contamination and groundwater chemistry - Ashburton-Hinds plain Executive summary The Ashburton-Hinds plain is the sector of the Canterbury Plains that lies between the Ashburton River/Hakatere and the Hinds River. It is an area dominated by agriculture, with a mixture of cropping and grazing, both irrigated and non-irrigated. This report presents the results from a number investigations conducted in 2004 to create a snapshot of nitrate concentrations in groundwater across the Ashburton-Hinds plain. It then examines data that have been collected since 2004 to update the conclusions drawn from the 2004 data. In 2004, nitrate nitrogen concentrations were measured in groundwater samples from 121 wells on the Ashburton-Hinds plain. The concentrations ranged from less than 0.1 milligram per litre (mg/L) to more than 22 mg/L. The highest concentrations were measured in the Tinwald area, within an area approximately 3 km wide and 11 km long where concentrations were commonly greater than the maximum acceptable value (MAV) of 11.3 mg/L set by the Ministry of Health. Concentrations above the MAV were also measured in samples from some wells outside the Tinwald area, most of them located southeast of State Highway 1. Concentrations indicating anthropogenic nitrate contamination (>3 mg/L) were found at depths of over 60 m below the water table in the upper part of the plain. Higher nitrate nitrogen concentrations were found in the middle and upper parts of the plain, where the soils are lighter, the sediments are highly permeable and the groundwater is well oxygenated. In contrast, reducing conditions were found in the groundwater near the coast, resulting in lower nitrate nitrogen concentrations. This area was formerly covered by swamp and is characterised by heavy soils and low-permeability sediments. The highest nitrate nitrogen concentrations, including those in the Tinwald area, were found near the transition zone between high-permeability sediments beneath the upper plain and the lower-permeability sediments near the coast. Since 2004, concentrations have been monitored annually or quarterly in eight wells on the plain. The data from this monitoring suggest that concentrations have risen in some locations. In particular, strong increasing trends have been measured in one well (K37/0358) in the Tinwald area and another to the southwest of Tinwald (K37/0468). Diffuse nitrate leaching from agricultural land is the main source of nitrate in the groundwater. Cropping and winter fallow activities on soils with low nitrate uptake have probably contributed to the high concentrations in the Tinwald area, while the dilution of nitrate in soil drainage water caused by flood irrigation in the Valetta irrigation scheme area may help to account for the lower concentrations outside the Tinwald area. Environment Canterbury Technical Report i Nitrate contamination and groundwater chemistry - Ashburton-Hinds plain ii Environment Canterbury Technical Report Nitrate contamination and groundwater chemistry - Ashburton-Hinds plain Table of Contents Executive summary .................................................................................................... i 1 Introduction ..................................................................................................... 1 1.1 Background ..................................................................................................................... 1 1.2 Objective of this report .................................................................................................... 1 2 Description of the investigation area ............................................................ 1 2.1 Location .......................................................................................................................... 1 2.2 Topography and rainfall .................................................................................................. 3 2.3 Soils ................................................................................................................................ 3 2.4 Surface water drainage ................................................................................................... 3 2.5 Irrigation and stock water ................................................................................................ 5 2.6 Hydrogeology .................................................................................................................. 5 2.7 Land use ......................................................................................................................... 6 2.8 Potential nitrate sources ................................................................................................. 8 2.9 Pre-2004 groundwater quality data...............................................................................10 3 Nitrate nitrogen concentrations in groundwater in 2004 ........................... 12 3.1 Summary of investigations ............................................................................................12 3.2 Nitrate nitrogen results ..................................................................................................14 3.2.1 General summary.............................................................................................14 3.2.2 Tinwald area .....................................................................................................14 3.2.3 Wider Ashburton-Hinds plain ...........................................................................17 3.2.4 Nitrate nitrogen concentrations and well depth ................................................17 3.2.5 Reducing conditions in the coastal groundwater .............................................18 4 Update on the 2004 results .......................................................................... 21 4.1.1 Overview ..........................................................................................................21 4.1.2 Long-term monitoring .......................................................................................21 4.1.3 2009 investigation ............................................................................................24 4.1.4 Summary ..........................................................................................................25 5 Discussion ..................................................................................................... 26 6 Conclusions .................................................................................................. 28 7 Recommendations ........................................................................................ 28 8 Acknowledgements ...................................................................................... 29 9 References ..................................................................................................... 29 Appendix 1: Analytical results for groundwater samples collected from the Ashburton-Hinds plain, 1988 to 2009 ............................................. 33 Environment Canterbury Technical Report iii Nitrate contamination and groundwater chemistry - Ashburton-Hinds plain Appendix 2: Construction details for wells of the Ashburton-Hinds plain where groundwater samples were collected between 1988 and 2009 ................................................................................................... 43 Appendix 3: Summary of 2004 investigations, sampling procedure and determinands analysed ................................................................... 50 Appendix 4 Procedure G002_02 - Collection of Groundwater Quality Samples............................................................................................. 52 List of Figures Figure 2-1: Ashburton-Hinds plain investigation area, including topography, surface water drainage, springs and irrigation schemes ......................................................................... 2 Figure 2-2: Soils of the Ashburton-Hinds plain investigation area ....................................................... 4 Figure 2-3: Land use on the Ashburton-Hinds plain (Agribase, 2005) ................................................ 7 Figure 2-4: Cropping land on the Ashburton-Hinds plain estimated to have low nitrate uptake due to successive years in winter fallow ........................................................................... 9 Figure 2-5: Map showing maximum nitrate nitrogen concentrations measured in groundwater samples collected on the Ashburton-Hinds plain prior to 2004 .......................................11 Figure 3-1: Map showing the locations of wells on the Ashburton-Hinds plain where groundwater samples were collected during surveys conducted in 2004 ............................................13 Figure 3-2: Nitrate nitrogen concentrations in groundwater, Ashburton-Hinds plain, 2004. ............15 Figure 3-3: Vertical cross section of nitrate nitrogen concentrations in groundwater along a line through the Tinwald area .................................................................................................16