Soil salinization due to saltwater intrusion in coastal regions: The role of soil characteristics and heterogeneity
Authors: Sobhi Gollo Vahid , Sahimi Muhammad , González Eva , Hajati Mithra-Christin , Elbracht Jörg , Fröhle Peter , Shokri Nima 
Published: June 2024

Soil plays a vital role in maintaining ecosystem functionality, supporting biodiversity, facilitating successful crop production, and ensuring socio-economic stability.  Soil quality is, however, constantly threatened by various factors, such as adverse climate conditions, hydrogeological processes, and human activities. One particularly significant stressor is soil salinity, which has a detrimental effect on soil quality. This study focuses specifically on understanding how soil properties contribute to the accumulation of surface soil salinity in the presence of shallow saline groundwater. To achieve this objective, advanced groundwater modeling techniques are employed to simulate saltwater intrusion in a riparian area known as Altes Land in northern Germany. A realistic representation of the salinization process is created and evaluated using a comprehensive dataset of hydrogeological information specific to the region. Additionally, the study examines the influence of soil heterogeneity on regional soil salinity by varying soil properties through devising six distinct scenarios for generating the numerical models that represent variations in soil texture and structure. The study reveals that regional soil texture and layering arrangement significantly influence the availability of water and the propagation of saline water in the vadose zone, and are major contributors to surface soil salinity. Subtle alterations and simplifications, often inconspicuous or deemed inconsequential in the context of small-scale experiments, may carry substantial ramifications for the formulation of enhanced management strategies in regions characterized by low elevation and influenced by groundwater salinity. Furthermore, the insights gained from this research provide valuable information for applications in agricultural practices and environmental conservation.
Plain language summary
Saltwater intrusion occurs when seawater enters coastal groundwater. In low-lying coastal regions, saline groundwater can rise close to the soil surface, leading to soil salinization that negatively impacts soil health and plant growth. The extent of soil salinization can be impacted by soil texture and heterogeneity, which is not fully understood at regional scales. In this study, we developed a new decision-support framework capable of describing and predicting salt transport through unsaturated zones lying over groundwater affected by seawater intrusion, and evaluated it against field measurements. This enabled us to investigate soil salinity under a variety of conditions and quantify the effects of important parameters, including soil texture, heterogeneity, and layering arrangement, on salt deposition close to the surface. Our study offers new quantitative insights into and tools for revealing the mechanisms governing the spatial distribution of soil salinity, as well as its health, hence contributing to global efforts for sustainable resource management and United Nations Sustainable Development Goals, particularly UN SDG15.