3D characterisation and quantification of an offshore freshened groundwater system in the Canterbury Bight

Aaron Micallef (), Mark Person, Amir Haroon, Bradley A. Weymer, Marion Jegen, Katrin Schwalenberg, Zahra Faghih, Shuangmin Duan, Denis Cohen, Joshu J. Mountjoy, Susanne Woelz, Carl W. Gable, Tanita Averes and Ashwani Kumar Tiwari
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Aaron Micallef: University of Malta
Mark Person: New Mexico Tech
Amir Haroon: GEOMAR
Bradley A. Weymer: GEOMAR
Marion Jegen: GEOMAR
Katrin Schwalenberg: Federal Institute for Geosciences and Natural Resources (BGR)
Zahra Faghih: GEOMAR
Shuangmin Duan: GEOMAR
Denis Cohen: University of Malta
Joshu J. Mountjoy: National Institute for Water and Atmospheric Research (NIWA)
Susanne Woelz: National Institute for Water and Atmospheric Research (NIWA)
Carl W. Gable: Los Alamos National Laboratory
Tanita Averes: Christian-Albrechts-Universität zu Kiel
Ashwani Kumar Tiwari: Politecnico di Torino

Nature Communications, 2020, vol. 11, issue 1, 1-15

Abstract: Abstract Although offshore freshened groundwater (OFG) systems have been documented in numerous continental margins worldwide, their geometry, controls and emplacement dynamics remain poorly constrained. Here we integrate controlled-source electromagnetic, seismic reflection and borehole data with hydrological modelling to quantitatively characterise a previously unknown OFG system near Canterbury, New Zealand. The OFG system consists of one main, and two smaller, low salinity groundwater bodies. The main body extends up to 60 km from the coast and a seawater depth of 110 m. We attribute along-shelf variability in salinity to permeability heterogeneity due to permeable conduits and normal faults, and to recharge from rivers during sea level lowstands. A meteoric origin of the OFG and active groundwater migration from onshore are inferred. However, modelling results suggest that the majority of the OFG was emplaced via topographically-driven flow during sea level lowstands in the last 300 ka. Global volumetric estimates of OFG will be significantly revised if active margins, with steep coastal topographies like the Canterbury margin, are considered.

Date: 2020
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DOI: 10.1038/s41467-020-14770-7

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