3D characterisation of an offshore freshened groundwater system in a passive siliciclastic margin: Canterbury Bight, New Zealand

We integrate controlled-source electromagnetic and seismic reflection data with borehole data and hydrological modelling to quantitatively characterise a previously unknown freshened groundwater system offshore of Canterbury, South Island of New Zealand. The offshore freshened groundwater (OFG) system consists of one main, and three smaller, low salinity aquifers. The main aquifer has a wedge-shaped geometry; it extends up to a distance of 60 km from the coast, has a maximum thickness of at least 250 m, and an estimated volume that ranges between 56 km³ and 213 km³. A meteoric origin and present-day terrestrial connection are inferred, although OFG emplacement has mainly taken place during sea level lowstands. The extent and distribution of the OFG across the margin are primarily controlled by the sedimentary framework. OFG occurrence is associated with a permeability contrast between the high permeability coarser-grained sediments in the aquifer, and the low permeability finer-grained sediments in the overlying aquitard. Where such a contrast is absent, fluid exchange between the offshore aquifer and the overlying seawater column has occurred due to haline convection, in the form of free convective density flows and localised diffuse seepage. Along-shelf differences in OGS salinity are attributed to multi-scale heterogeneity in the sedimentary and permeability architecture of the margin, particularly in relation to braided alluvial deposits and infilled buried valleys. Normal faulting constitutes a barrier to the lateral flow of OFG. Our approach can potentially revise global volumetric estimates of OFG and support their sustainable exploitation.