Multi-Layered High Permeability Conduits Connecting Onshore and Offshore Coastal Aquifers

Abstract

Groundwater resources in coastal regions are facing enormous pressure caused by population growth and climate change. Few studies have investigated whether offshore freshened groundwater systems are connected with terrestrial aquifers recharged by meteoric water, or paleo-groundwater systems that are no longer associated with terrestrial aquifers. Distinguishing between the two has important implications for potential extraction to alleviate water stress for many coastal communities, yet very little is known about these connections, mainly because it is difficult to acquire continuous subsurface information across the coastal transition zone. This study presents a first attempt to bridge this gap by combining three complementary near-surface electromagnetic methods to image groundwater pathways within braided alluvial gravels along the Canterbury coast, South Island, New Zealand. We show that collocated electromagnetic induction, ground penetrating radar, and transient electromagnetic measurements, which are sensitive to electrical contrasts between fresh (low conductivity) and saline (high conductivity) groundwater, adequately characterize hydrogeologic variations beneath a mixed sand gravel beach in close proximity to the Ashburton River mouth. The combined measurements-providing information at three different depths of investigation and resolution-show several conductive zones that are correlated with spatial variations in subsurface hydrogeology. We interpret the conductive zones as high permeability conduits corresponding to lenses of well-sorted gravels and secondary channel fill deposits within the braided river deposit architecture. The geophysical surveys provide the basis for a discharge model that fits our observations, namely that there is evidence of a multilayered system focusing groundwater flow through stacked high permeability gravel layers analogous to a subterranean river network. Coincident geophysical surveys in a region further offshore indicate the presence of a large, newly discovered freshened groundwater system, suggesting that the offshore system in the Canterbury Bight is connected with the terrestrial aquifer system.

Groundwater erosion of coastal gullies along the Canterbury coast (New Zealand): a rapid and episodic process controlled by rainfall intensity and substrate variability

“…With regards to the G-TEM slingram time gate plots (Fig. 8), we interpret the higheramplitude responses on the time gate 1 plots that are preferentially located upslope of recently active gullies as being zones of relatively high electrical conductivity in the subsurface at depths of ∼ 10 m. These zones are suggestive of buried groundwater conduits made up of gravel and/or sandy units (Weymer et al, 2020) or tunnels formed by subsurface groundwater flow in sand units. Further analysis of the G-TEM data, including 2D modelling and inversion, is required to ascertain the subsurface hydraulic geometry responsible for the along-profile amplitude variations.…”

“…This is a standard 1D TDEM inversion code that has previously been successfully used in coastal hydrogeophysical studies (e.g. Pondthai et al, 2020).…”

“…We also modelled the groundwater flow and pore pressure distribution within the soil using the Poisson equation, which is the generalised form of the Laplace equation (Whitaker, 1986) as follows:…”

“…Figure 8a displays a 1D model (I) that contains a conductive layer of 200 m between a 10 and 20 m depth in a homogeneous 1000 m background. The 1D model (i) is motivated by the inversion results of deep-penetrating 40 × 40 m TX loop TDEM soundings carried out on top of the cliffs several tens of metres inland (Weymer et al, 2020), which revealed such a conductive zone at these depths. Unlike the slingram profiles, the deeper-penetrating, larger-loop sounding curves are readily fitted by a 1D model.…”

“…The spatial analysis of the time gate plots is not a conventional approach in time domain electromagnetics, but it is somewhat analogous to the spatial analysis of the apparent resistivity profiles in frequency domain electromagnetics using terrain conductivity meters (e.g. Weymer et al, 2016). This is based on the idea that the G-TEM response at a fixed time gate carries information similar to that of a terrain conductivity meter response at a fixed frequency.…”