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Source: GNS Science

In 2019, Otago Regional Council scientists increased the scale and density of their urban Dunedin groundwater monitoring network from four sites to 23, covering the wider Harbourside-South Dunedin area. The groundwater levels and temperature data has been recorded for a year and is stored in their groundwater database.

Every 15 minutes, automated recorders collect data on how water beneath the city responds to rainfall, seasonal variation, hillslope runoff, pumping and the effect of tides.

 “The first year of monitoring has yielded quite a few nice surprises,” Simon Cox, Principal Scientist at GNS Science and the report’s lead author, says. 

“Before we started, we heard widespread stories, such as people having to wait for low-tide before digging to get a dry hole in their garden.

“We thought the monitoring would show groundwater is strongly tidal – but in reality, there are very few places where groundwater changes more than 20 mm between low- and high-tide.

“Chemical analyses also indicate saline-rich seawater barely infiltrates more than 800m inland from the sea or harbour.”

This suggests water is not moving rapidly through the ground, and that is good news. Further monitoring and analysis needs to continue to improve our understanding of the relationship between present day tides and groundwater levels and the effects of future sea level rise.

Dr Simon Cox

The monitoring also indicates that some of the flat low-lying land beneath parts of the city is much less permeable than initially expected.  

“Gentle pumping of wells is able to disturb the chemistry of groundwater long after water-levels have recovered, and many places have ‘topsy turvey’ temperatures that are warmest in June and coldest in December,” Dr Cox says.

“This suggests water is not moving rapidly through the ground, and that is good news. Further monitoring and analysis needs to continue to improve our understanding of the relationship between present day tides and groundwater levels and the effects of future sea level rise.

South Dunedin Future, a collaborative project between Otago Regional Council and Dunedin City Council are pleased to see these results from the groundwater data being collected.

ORC General Manager Operations, Dr Gavin Palmer says, “Technical staff from both councils will use the analyses presented in this report to help inform the next phase of scientific work, while consulting with the community on potential options to mitigate against natural hazards and climate change impacts through the South Dunedin Future project. Ongoing monitoring and further analyses of the likely effects of climate change on groundwater are still needed to confirm these initial observations based on one year of monitoring only.” 

DCC Infrastructure Services General Manager Simon Drew says, “The DCC has budgeted $35 million to spend on flood reduction in the South Dunedin area over the next decade. This new report will form an important part of the technical basis for developing options for exactly how that money will be spent.

“We look forward to further collaboration with the Otago Regional Council, GNS Science, the community and others as we work towards safely adapting to the effects of climate change.”

Dunedin groundwater is monitored by Otago Regional Council and GNS Science, with chemical analyses completed by University of Otago. The city is seen as a test case for developing methods to monitor, map, model and help mitigate the effects of rising groundwater and sea-level rise. This research forms part of the NZSeaRise Endeavour Programme (www.searise.nz funded by MBIE).

The baseline data presented in the report are accessible through contacting ORC, who maintain the Dunedin groundwater database along with rainfall and river levels.

Map showing the median elevation of subsurface groundwater in Dunedin during 2019-2020, with colours representing different heights of the water table in New Zealand vertical datum NZVD2016. (Mean sea level is presently -0.25 m in this datum). The position of the shoreline in the 1850s is shown by the blue line. From the new report published by GNS Science (Cox et al. 2020, GNS Science report SR2020/11).

ENDS

Notes for editors:

Cox SC, Ettema MHJ, Mager SM, Glassey PJ, Hornblow S, Yeo S. 2020. Dunedin groundwater monitoring and spatial observations. Lower Hutt (NZ): GNS Science. 86 p. (GNS Science report; 2020/11). doi:10.21420/AVAJ-EE81.

Available for free download  ground_water_monitoring_observations.pdf (6.07 MB)

Background:

  • Dunedin City is one of many coastal urban areas in New Zealand facing a massive challenge with climate change and sea-level rise. The city has a large number of assets and critical infrastructure sitting on a low-lying coastal plain that is underlain by a largely unseen and relatively poorly understood hazard. Shallow groundwater limits the unsaturated ground available to store rain and runoff, promotes flooding and creates opportunities for infiltration into stormwater and wastewater networks. Groundwater levels are expected to rise as sea level rises, causing greater frequency of flooding and/or direct inundation once it nears the ground surface.
  • Science to monitor groundwater (levels and chemistry), characterise causes of its natural fluctuations and understand future behaviour is an important step in gathering the information needed to support decisions regarding how long to continue investment to protect and maintain assets that serve the community.  Monitoring of groundwater in Dunedin took a major step forward in 2019 when a consortium of interested parties contributed to development of an improved network. The partnership involved efforts by staff from Otago Regional Council, GNS Science, Dunedin City Council, the Earthquake Commission, New Zealand Centre for Earthquake Resilience (QuakeCoRE) through the University of Canterbury, Oceana Gold and the University of Otago.  Subsequent collection and management of groundwater data was led by Otago Regional Council in conjunction with GNS Science, with water chemistry studies undertaken at University of Otago. The new report
  • Work for the new report drew upon contributions from the NZ SeaRise Programme
    (an Endeavour programme funded by the Ministry of Business, Innovation and Employment, led by Victoria University of Wellington) and an Urban Groundwater project (GNS Science’s Strategic Science Investment Fund).  Report authors are Simon Cox & Phil Glassey (GNS Science), Marc Ettema & Sharon Hornblow (ORC), Sarah Mager and Sarah Yeo (University of Otago School of Geography).
  • ‘South Dunedin Future’ is a joint DCC/ORC programme of work, aimed at effectively responding to the climate-driven challenges facing the area. More information is available at www.dunedin.govt.nz/southdunedin

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