The hydrogeological drivers of marsh migration

Abstract

The Delmarva Peninsula is a hotspot for sea level rise making it especially vulnerable to coastal ecosystem change. The Peninsula is fringed by salt marshes and experiencing inland movement of marshes (marsh migration) as rising sea levels create wetter and saltier conditions, displacing upland freshwater vegetation. This dissertation investigates how sea-level rise and other hydrogeological mechanisms drive marsh migration by examining how terrestrial and oceanic forces interact to shape salinity dynamics and hydrological conditions at the marsh-upland boundary. Across three forest and three agricultural field study sites, space-for-time transects were instrumented with shallow wells, soil moisture sensors, and redox probes to capture fine-scale spatial and temporal variability in salinization, flushing, and flooding. Three years of high-resolution observational data reveal that terrestrial groundwater decline is a key, and often overlooked, driver of marsh migration. Inland groundwater declines reverse hydraulic gradients, inducing inland movement of saline groundwater and resulting in lateral salinization that exceeds storm salinization. In fact, storms often deliver enough rainfall to rapidly flush and freshen groundwater, challenging the assumption that storm events mainly contribute to salinization by overland saltwater flooding. However, storm surges remain important contributors to vertical salinization in shallow soils, where recovery can take months and groundwater flooding can produce prolonged anoxia. To further untangle the relative roles, timing, and interactions of compound drivers, a numerical model was developed to quantify salinization under both isolated (e.g., storm surge or drought alone) and sequential forcing scenarios (e.g., drought before storm surge or rain before storm surge). Findings show that the order of events significantly effects salinization; for instance, a storm surge following a drought leads to deeper salinization than the storm surge following a rain event. This work advances our understanding of the spatial and temporal complexity of hydrogeological stressors and provides new insight into the mechanisms driving marsh migration under a changing climate.