The Wairau River hydrologic system: where are the old-water stores, and do floods push groundwater faster through the coastal aquifer?

Our current knowledge about the water dynamics through groundwater systems is primarily based on observations of celerity - pressure responses at wells and flow increases at springs. However, it is velocity of the water that characterises hydrologic systems, regarding water quantity and transport of water contaminants.

Tritium data from the Wairau River around Blenheim, New Zealand, indicate 50% of baseflow has a mean transit time (MTT) of 8 years (Taylor et al., 1992; Morgenstern et al., 2019). The other 50% is younger water. The groundwater storage to provide such long transit times was attributed to large deposits of scree and alluvium infilling U-shaped glacial valleys in the headwater areas of the Upper Wairau catchment.

However, after collecting baseflow samples from these scree discharges, we did not find dominance of old water with MTT=8 years. The old water storage must therefore be attributed to the deep groundwater flow system in the entire Wairau River catchment.

In the coastal Wairau Fan, where the river loses water into the aquifer, with extremely high hydraulic conductivity and groundwater MTTs of only a few months, we traced a seasonal ¹⁸O spike from the river through the aquifer to the discharge of the aquifer, Spring Creek. This revealed that even after extreme rain events which cause immediately elevated water levels and aquifer discharges, the discharging water remains old. After the extreme Marlborough flood in August 2022, with rivers showing the highest flows on record, the elevated flows at Spring Creek appeared to be nearly unchanged in their natural annual cycle of water transit time. This implies that the elevated flow following the flood and associated with much elevated water levels in the aquifer, was caused by old water flow paths activated due to the increased hydraulic loading and supplementing the normal shallow flow.

It came to a surprise when we found a similar activation of old-water flow paths in drinking water supply wells in the confined aquifers in the Heretaunga Plains, with a change to slightly older water in the wells following the extreme flooding caused by Cyclone Gabrielle. To find such an old-water flow activation as the main cause for the elevated flows also in the unconfined aquifer of the Wairau Fan was another surprise.

References

Taylor CB, Brown LJ, Cunliffe JJ, Davidson PW. 1992. Environmental tritium and ¹⁸O applied in a hydrological study of the Wairau Plain and its contributing mountain catchments, Marlborough, New Zealand. Journal of Hydrology. 138(1):269–319.

Morgenstern U, Davidson P, Townsend DB, White PA, van der Raaij RW, Stewart MK, Moreau M, Daughney C. 2019. From rain through river catchment to aquifer: the flow of water through the Wairau hydrologic system. Lower Hutt (NZ): GNS Science. 83 p. (GNS Science report; 2019/63)