Geophysical imaging of the salt content at the soda lakes from the National Park Neusiedlersee – Seewinkel

Abstract

The soda lakes of the National Park Neusiedlersee - Seewinkel harbor rare and endangered waterfowl species and plants adapted to these extreme biotopes only found in this area within Austria. The high salt content at the surface is driven by capillary forces that permit the up-flow transport of salt from the subsurface through a shallow soil layer characterized by low hydraulic permeability. Such a shallow soil layer also hinders the percolation of surface water. Nonetheless, salt transport to the surface is only possible if the low permeable layer remains wet during the summer periods when evaporation reduces the volume of the surface water. Hence, soda lakes are sensitive to climate change because long dry periods or changes in the groundwater levels may degrade the soda lakes, endangering the rich biosphere that depends on them. The design of adequate methods to restore and preserve the soda lakes requires a good understanding of the surface-groundwater interactions that control the salt dynamics. In particular, it is required to understand the spatial variations of the water content, geometry, and hydraulic properties of the shallow soil layer, which ultimately control the salt dynamics in the soda lakes. To date, subsurface investigations in the soda lakes are still based on the analysis of soil and water samples, thus, they rely on the interpolation of discrete data and may be biased by the characteristics of the samples. To overcome this limitation, we propose applying electrical and electromagnetic geophysical methods, which permit to map variations in subsurface electrical conductivity based on non-invasive measurements in an imaging framework. The electrical conductivity is mainly controlled by water content, porosity and salinity, three relevant parameters driving salt dynamics. The combination of different geophysical methods enables an improved interpretation of the results and permits the estimation of relevant parameters for managing soda lakes. In particular, we demonstrate the application of frequency-domain electromagnetic methods to map variations in the salt content at the lake-scale, while temporal changes in water content at different depth can be resolved through monitoring measurements with electrical resistivity tomography. We obtained deeper information, as required to understand the geometry of the aquifers, by combining dc-resistivity measurements and transient electromagnetic soundings. We also deploy a joint inversion strategy combining induced polarization and refraction seismic to directly solve for porosity and the hydraulic conductivity in the soil layer and the aquifer in an imaging framework. Interpretation of the geophysical images was evaluated through the analysis of soil sediments in the laboratory.