Long-term monitoring of a high-energy beach using joint salinity-based inversion of different ERT and sampling data

High-energy beaches represent dynamic biochemical reactors at the boundary between land and sea, driven by salinity changes. We study the temporal dynamics of the saltwater-freshwater distribution on the North beach of Spiekeroog, Germany, over a period of 18 months by continuous logging of pore water salinity in multi-level monitoring wells, campaign-based direct-push measurements of porewater salinity, a quasi-continuously measuring electrode chain and repeated ERT measurements both obtaining the bulk electrical conductivity. Instead of handling each data type independently and in particular converting the inversion results of the bulk electrical conductivity of the ERT measurements into the salinity, we analyze all data jointly and directly invert for salinity. As the temperature distribution in the subsurface plays a crucial role to derive salinity from bulk electrical conductivity, it needs to be measured and incorporated in the inversion. This approach proves superior to classic workflows. As a result, we are able to study the dynamics of subterranean estuaries at depth and provide 2D images of the seasonal change and extent of the upper saline plume and observe how remnants of winter storm floods penetrate into the subsurface. These findings are useful for calibrating groundwater models and improving our understanding of the deeper salinity dynamics. • Investigation of High-energy beaches driven by salinity changes. • Logging, campaign-based direct-push, a quasi-continuously measuring electrode chain and repeated ERT. • All data are inverted jointly directly for salinity, which is a novel technique. • The result are 2D images of the seasonal change of the upper saline plume and winter storm floods.