title: Cyclone Gabrielle 2023 event-response groundwater sampling in Hawke's Bay flooded areas
Creators: name: Moreau, Magali
name: Cameron, Stewart G.
name: Harper, Simon
name: Close, Murray
name: Taves, Mike
name: Sarris, Theo
name: Morton, Graham
name: Parsons, D
name: Grant, Steve
name: Brakenrig, Thomas
name: Coup, Lauren
name: Lawrence, Mark J.F.
name: Mitchell, F
name: Lencek, A
name: Kersel, M
name: Hewitt, K
name: Friar, B
name: Clark, Gemma
name: Coble, Matthew
Publisher: name: GNS Science
Year: 2024

description: In mid-February 2023, Cyclone Gabrielle caused severe flooding and damage across parts of the North Island. In the month following the event, Hawke’s Bay Regional Council (HBRC) initiated a project to identify potential contamination of groundwater in three areas of concern that were subject to significant floodwater inundation: the central north-western part of the Heretaunga Plains; Esk Valley and Ōtāne. The project was the first coordinated event-response to assess the impact of widespread flooding on groundwater quality undertaken in New Zealand. The Ministry of Business, Innovation and Employment (MBIE) contracted GNS Science (GNS) to lead the project and to work alongside HBRC and the Institute of Environmental Science and Research (ESR). Planning and execution were also supported by in-kind contributions from Napier City Council (NCC) and Hastings District Council (HDC) in consultation with Taumata Arowai.

A methodology was developed to account for operational constraints, including budget, and the high likelihood of non-detection of contaminants. The methodology used an initial rapid, low-cost sampling for screening a large number of wells for indicator parameters of potential contamination. Wells located in areas where anomalous measurements occurred were subsequently sampled for a more comprehensive suite of contaminants.

Between 28 April and 11 May 2023, 105 wells were visited in the screening survey. Field measurements (temperature, conductivity, pH and turbidity) and samples for E. coli testing could only be collected at 62 sites. The remaining wells were deemed unsuitable for a range of reasons (e.g. lack of power or sampling access prior to a storage tank), including two wells buried under sediment in the Esk Valley. The screening survey results showed anomalies in several geographic areas with, e.g. E. coli detection, elevated groundwater temperature, lower conductivities and/or more acid waters compared with wells in other areas. Wells in these areas were selected for a more comprehensive analysis as part of the targeted survey. The more comprehensive suite included: groundwater chemistry comprising 16 major and minor anions and cations; and anthropogenic contaminants comprising 22 acid herbicides, 112 organochlorine and organonitro-phosphorus pesticides, glyphosate and 64 volatile organic compounds. The targeted sampling was undertaken by HBRC between 6 and 12 June and included 12 wells (one in Ōtāne and the remainder in the Heretaunga Plains). There were no occurrences of detection of either pesticide or volatile compounds, and comparison of the major chemistry with existing records (State of the Environment [SoE] data collected within a few days preceding the cyclone) did not indicate significant changes in chemistry at any of the wells. The results showed that groundwater contamination was not widespread and did not warrant further investigation or remediation.

The methodology developed and applied in this project to respond to flood events and identify contamination of groundwater drinking-water supplies provided valuable insight for similar surveys in the future, including on: fieldwork efficiency, sampling site suitability screening, collection of floodwater or baseline data, the suite of analytes to use, inter-organisation involvement and communication, and event-response cost. For instance, in this project, total cost was NZD \$127,000, including \$30,000 for laboratory analysis and field expenses.

Recommendations from this project cover pre-event preparedness, event response design and response activities.