Understanding Peat Soil Deformation and Mechanisms of Peat Collapse Across a Salinity Gradient in the Southwestern Everglades

Saltwater intrusion is a significant threat to the structure and ecology of inland and coastal freshwater wetlands, altering biogeochemical cycling and disrupting important ecosystem services. As sea level rises along the Florida coast, the saltwater interface progressively moves further inland reaching freshwater soils previously unexposed to saltwater. One result of this soil salinization is a phenomenon called peat collapse which is currently observable along the South Florida coastline. While previous studies have proposed conceptual models to explain peat collapse many uncertainties still exist regarding the physical mechanisms that trigger this phenomenon. In this study we use a unique combination of laboratory and field-based measurements using geophysical methods, deformation rods, gas traps, time-lapse photography, and hydraulic conductivity measurements to investigate the effects of salinization on the physical properties of peat soils across a salinity gradient in the southwestern Everglades. Our results show that freshwater peat soils have about three times greater degree of pore dilation when compared to peat soils previously exposed to saltwater conditions. Differences in soil surface deformation were also observed and related to differences in the soil matrix physical integrity across the salinity gradient. This work also uses electrical resistivity imaging surveys to image areas of peat collapse and demonstrates: (a) the role of salinity in the development of collapse features; and (b) the apparent lack of evidence to suggest subsurface lithological controls on the development of collapse features. A simplified conceptual framework for better understanding peat collapse and pock formation is presented.