Did Cyclone Gabrielle increase the risk of pathogen contamination for drinking water supply wells in the confined aquifer of the Heretaunga Plains, New Zealand?

During flood events, due to extreme hydraulic loading in recharge areas of aquifers, groundwater flow dynamics can change, causing a risk of pathogens being flushed into aquifers used for drinking water supplies. Extreme flood events, as they are increasingly experienced with climate change, have potential to cause impacts not seen before, and drinking water sources that were free of pathogen contamination in the past may become contaminated in the future.

As an example, in the Heretaunga Plains, Hawkes Bay, contaminated water from heavy rain inundated paddocks entered an unconfined part of the aquifer and drinking water wells in it, resulting in >6260 cases of illness including 42 hospitalizations, and Campylobacter infection contributed to at least four deaths.

Most of the c. 30 drinking water wells in the Heretaunga Plains, including those supplying the cities of Hastings and Napier, are, however, in the confined part of the aquifer and these were not affected by pathogen contamination. But will more extreme flood events, predicted with climate change, eventually also compromise drinking water sources in the confined aquifer which were deemed safe in the past? Wells in the confined aquifer have shown indications of changing groundwater flow dynamics, for example variable water age, and changing hydrochemistry after flood events, which might be associated with younger water, bearing the risk of pathogen intrusion.

On 13 and 14 February, 2023, Cyclone Gabrielle lashed Hawke’s Bay, with record rainfalls causing rivers to burst their banks causing a death toll of 11. To improve understanding of the impact of the extreme hydraulic loading on the aquifers through such events, specifically changes to the water flow dynamics with potential for new, previously unrecognised contaminant pathways and associated risks for drinking-water supply wells, we measured age-tracers in selected wells again, two months after Cyclone Gabrielle. Comparing the results of this survey with age-tracer data from just three months prior to the cyclone provided an opportunity to test how extreme events like
Cyclone Gabrielle change groundwater flow dynamics in confined aquifers.

On 12 and 13 April 2023 we re-sampled for age tracers a selection of drinking-water supply wells in partnership with Hastings District Council and Napier City Council, and of private and monitoring
wells in the central and marginal confined parts of the aquifer system in partnership with
Hawkes Bay Regional Council.

The data indicate that groundwater ages in these wells have not changed significantly because of Cyclone Gabrielle. The wells that showed slight changes in age-tracer concentrations consistently showed older water after Cyclone Gabrielle. Other wells, despite showing no detectable changes in age-tracer concentrations, contained water that was more evolved after the cyclone, indicated by decreased dissolved oxygen and elevated methane, ammonia, and phosphorous concentrations.

These observations all point toward older (probably deeper) groundwater having been
pushed into the active groundwater flow paths by the increased hydraulic loading. With no younger water detected in the investigated wells following Cyclone Gabrielle, there is no indication of increased risk of pathogen contamination in the confined aquifer system following extreme flood events.