The dynamics of groundwater within beaches has implications for sediment transport in the swash zone and mass transport across the land-ocean boundary. Groundwater forcing by nearshore processes (e.g., wave runup, setup and ocean tides), and regional and local aquifer flows, result in the complex exchange of fresh and saline waters across the beachface boundary.
The new application of a cross-borehole resistivity imaging technique was used to observe the salinity structure along a 20 m shore-normal transect within the upper 3 m of an open-coast beachface. Several intriguing (and unanticipated) observations were made. These include the horizontal layering of saline and fresh pore-waters, the presence of salty water overlying fresh water, and evidence of significant advective/dispersive mixing, within a groundwater environment where the prevailing hydraulic gradients did not suggest this to be the case.
An advantage of the resistivity imaging technique over direct point sampling with spear points to extract pore-water, is that a dense and fully 2-D profile of the subsurface salinity structure can be mapped. The cross-borehole technique is intrusive in the sense that some disturbance is caused during installation, but this occurs only in the immediate vicinity of the vertical electrode strings, and post-processing is effective to remove the influence of these data. A high correlation achieved between measured bulk resistivity and fluid conductivity confirmed that the results of resistivity imaging can be used to calculate the pore-water EC (salinity), and hence the relative contributions from the adjacent freshwater aquifer and ocean sources. Though not attempted at the deployment described here, the burial of all electrodes also makes the technique suitable for examining the variation through time of saline-fresh pore-waters within the beachface.