Geophysical monitoring for quantifying changes in soil saturation and salinity along coastal interfaces during flooding

Hydrological disturbances, including intense precipitation and sea level rise along coastal interfaces, lead to freshwater flooding and saline water overwash that changes the soil saturation and salinity and, in turn, alters subsurface biogeochemical reactions and soil-plant-atmosphere exchanges. The extents of these state changes are not well known, limiting their accurate representation in earth system models. In this study, we combined the spatial-temporal parameter measurements advantage of geophysical imaging with discrete in-situ and laboratory measurements and petrophysical relationships to quantify changes in soil moisture and salinity at an improved spatial scale. 

We simulated a concurrent freshwater and estuarine water flooding at two adjacent 2,000 m² forested plots by inundating them with 265 m³ of freshwater and estuarine water with salinities of 0.06 and 8.1 practical salinity units (PSUs), respectively. The flooding experiment was conducted over a single 10-hour cycle for year 1 and for two and three flooding cycles for years 2 and 3, respectively, with a 14-hour pause between each cycle. During each flooding experiment, repeated electrical resistivity and induced polarization measurements were used to image the water and solute infiltration fronts along two 100 m and 42 m transects while soil moisture, temperature, and electrical conductivity were monitored every 15 minutes with soil sensors installed at 5, 15 and 30 cm depths and co-located with the geophysical transects. Petrophysical models derived from laboratory multi-salinity electrical measurements were used to estimate changes in soil moisture and fluid salinity from field measurements of real and imaginary conductivity during the flooding experiment. 

During flooding, the real electrical conductivity increased by ~100% in the freshwater plot and ~570% in the estuarine water plot. The change in imaginary conductivity in the freshwater plot was < 1 mS/m, whereas that of the estuarine water plot was ~5 mS/m. The real conductivity shows a dependence on soil moisture content with a coefficient of determination (R²) >0.7, while the imaginary conductivity shows a dependence on soil salinity with R² >0.6. Repeated monitoring over 3 years shows >60% change in ambient soil electrical conductivity at the estuarine water plot, indicating an increase in soil salinity over time. 

These results validate the use of electrical resistivity for estimating changes in coastal soils' moisture content in response to flooding. Combining the electrical resistivity with induced polarization measurements provides the possibility to account for changes in pore fluid conductivity. The intermittent geophysical monitoring limits the comparison of geophysical data with in-situ soil parameters measurement to develop a more robust petrophysical model. This study would benefit from the use of continuous automatic electrical resistivity and induced polarization monitoring, which is becoming increasingly popular for ecohydrological studies.