The Current State of Groundwater Quality in the Waimakariri CWMS Zone
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The current state of groundwater quality in the Waimakariri CWMS zone Report No. R16/48 ISBN 978-0-947511-76-0 (print) 978-0-947511-77-7 (web) 978-0-947511-78-4 (cd) The current state of groundwater quality in the Waimakariri CWMS zone Report No. R16/48 ISBN 978-0-947511-76-0 (print) 978-0-947511-77-7 (web) 978-0-947511-78-4 (cd) Report prepared by Lisa Scott Raymond Wong Sungsoo Koh October 2016 Name Date Prepared by : Lisa Scott, Raymond Wong & 19 September Sungsoo Koh 2017 Reviewed by : Carl Hanson 24 May 2018 Groundwater Science Manager External review by: Lee Burbery 29 August 2017 ESR Approved by: Tim Davie 21 June 2018 Chief Scientist Report No. R16/48 ISBN 978-0-947511-76-0 (print) 978-0-947511-77-7 (web) 978-0-947511-78-4 (cd) 200 Tuam 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 The current state of groundwater quality in the Waimakariri CWMS zone Summary Background Environment Canterbury and Waimakariri District Council (WDC) are working with the Waimakariri Water Zone Committee and the local community to improve water quality and quantity outcomes for the Waimakariri zone. This is one of a series of reports being written to help inform the Zone Committee and the local community about the current state and trends in water quantity and quality within their zone. The problem We needed to summarise groundwater quality data for the Waimakariri zone and its water management subzones and analyse any recent trends in concentrations to understand the current state and stresses on the system. Nitrogen, in the form of nitrate, is the nutrient we are most concerned about in groundwater. What we did We compiled what was known about how groundwater behaves in the Waimakariri zone: where it comes from; where it flows to and how it is connected with the surface water systems. We also looked at the distribution of rock and soil types, land uses and wastewater discharges in the zone. These can all influence the patterns we see in groundwater quality. We extracted groundwater quality data from Environment Canterbury’s water quality database and plotted them on maps. We looked at time series data for nitrate concentrations in long-term monitoring wells to see if and how these were changing in the zone. What we found Groundwater quality is generally very good and in most places meets drinking-water standards. Poorer quality groundwater occurs naturally in some areas, often linked to organic-rich coastal soils and sediments in old swamp areas where iron, manganese and sometimes arsenic can affect water quality. Users of shallow private wells are most at risk from pathogens (disease-causing microorganisms), especially near effluent disposal or animal grazing areas. Nitrate concentrations in groundwater could pose a health risk to a small number of users of shallow domestic wells. Community supply protection zones have been established to protect the quality of the groundwater resource accessed for public supply of drinking-water, especially from microbiological contamination. The Waimakariri zone has a long history of farming land uses. Some farming activities have released nitrogen to the environment which has leached into groundwater as nitrate. Nitrate concentrations were already high in some monitoring wells when we first began regular sampling on the Ashley-Waimakariri plains in the 1980s. Since then nitrate concentrations have fluctuated, but there has been no overall long-term trend. Intensification of land use in the Eyre River subzone is causing an increase in nitrate in some wells and springs in the Kaiapoi River catchment. Some of the nitrate load from the current land use is likely still moving through the groundwater to the surface waterways. What does it mean? Groundwater affected by natural contaminants (iron and manganese) cannot be managed by controls on land use, but can only be avoided or treated at the point of supply. Pathogens are generally controlled by good design and treatment in wastewater systems, livestock and irrigation management, and careful disposal of animal effluent. Owners of shallow private wells should regularly test their drinking-water for E. coli (a pathogen indicator) or install disinfection systems. Diffuse sources of nitrogen leaching from land use are the main threat to groundwater quality in the Waimakariri zone. Any increases of nitrate in groundwater are likely to affect the ecology of spring-fed streams, especially the Cust Main Drain and the Ohoka Stream and Kaiapoi River. Groundwater nitrate is elevated in parts of the Eyre River, Loburn, Cust and Kowai subzones. In the Cust and Kowai subzones, nitrate concentrations are close to the drinking-water limit in places. Groundwater has a naturally greater capacity to assimilate nitrate in the Ashley and Coastal Wetland subzones. Management of pathogens and nutrient losses will contribute greatly to helping the community achieve some of their desired outcomes. Limiting the leaching of nitrogen from farming is particularly important for the health of spring-fed streams, so that they maintain or improve opportunities for mahinga kai gathering and diverse aquatic life. Managing diffuse nitrogen discharges from farms and controlling the discharge of pathogens, particularly within community drinking water supply protection zones, will also help communities in this zone maintain their access to safe drinking water from groundwater. Environment Canterbury Technical Report i The current state of groundwater quality in the Waimakariri CWMS zone ii Environment Canterbury Technical Report The current state of groundwater quality in the Waimakariri CWMS zone Table of contents Summary ........................................................................................................... i 1 Project background and purpose ............................................................. 1 2 Project area ................................................................................................ 1 2.1 General hydrology and topography .......................................................................... 2 2.2 Geology and soils ..................................................................................................... 3 3 Groundwater ............................................................................................... 7 3.1 Groundwater occurrence .......................................................................................... 7 3.2 Groundwater age ...................................................................................................... 7 3.3 Groundwater-surface water interaction..................................................................... 8 3.4 Groundwater recharge .............................................................................................. 8 3.5 Groundwater flow direction ....................................................................................... 9 3.6 Groundwater users ................................................................................................... 9 3.6.1 Drinking water wells ................................................................................... 10 3.6.2 Groundwater-fed surface waterways ......................................................... 12 3.7 Groundwater quality ................................................................................................ 13 4 Groundwater quality data sources ......................................................... 13 4.1 Groundwater quality monitoring .............................................................................. 13 4.2 Groundwater quality investigations ......................................................................... 15 5 Natural groundwater quality ................................................................... 15 5.1 Conductivity ............................................................................................................ 15 5.2 Groundwater redox state ........................................................................................ 16 5.3 Dissolved oxygen .................................................................................................... 18 5.4 Iron, manganese and arsenic ................................................................................. 19 6 Groundwater contamination ................................................................... 21 6.1 Types of contaminants ............................................................................................ 21 6.2 Point source discharges ......................................................................................... 22 6.3 Diffuse pollution ...................................................................................................... 24 6.4 Pathogens in groundwater ...................................................................................... 26 6.5 Nitrogen .................................................................................................................. 27 6.5.1 Sources and transport of nitrogen ............................................................. 27 6.5.2 Why we care about nitrate-N concentrations in groundwater? .................. 27 6.5.3 Nitrate leaching risk ................................................................................... 27 6.5.4 Nitrate-N concentrations in groundwater ..................................................