Investigation of Subsurface Stratigraphy and Groundwater Dynamics in the Mississippi River Delta

Abstract

The Mississippi River Delta (MRD) is socioeconomically important to the state of Louisiana and the United States. Various types of land-water system data have been collected in the MRD. However, very few efforts have been made to utilize these datasets in modeling regional stratigraphy and groundwater dynamics in the MRD, especially for the upper 50 m of the depth. In this interval of depth, the Mississippi River and surrounding interdistributary bays intensively interact with the groundwater system. The lack of knowledge in regional stratigraphy and groundwater dynamics hinder an understanding of how hydrogeological setting affects processes such as surface-groundwater interaction, subsidence, and sediment erosion. In this study, topobathymetric, geological/geotechnical, and hydrological data were used to construct multiple 3-D stratigraphy models and a groundwater flow model in the MRD. Ordinary kriging, compositional kriging, and multiple indicator methods were found to be efficient in regionalizing different types of geological/geotechnical data. The stratigraphy models and groundwater model reveal a complex hydrogeologic setting in the MRD. Mississippi River channel cut through clayey delta plain deposits into buried sands between -10 m and -35 m. Sands deposited at depth and near the surface provide pathways for groundwater to interact with surface waters. Groundwater flow rate is 3-4 orders of magnitude smaller than the river discharge rate. The groundwater system actively interacts with surface water system in the Mississippi River and in the surrounding bays, especially during flood, storm, and hurricane events. Dramatic increase in pore water pressure and sharp groundwater recharge-to-discharge reversion are estimated to occur during hurricane and right after hurricanes respectively. High pore water pressure during and after hurricanes may destabilize sediments and compromise safety of coastal infrastructures such as the ring levees. Groundwater activities may contribute to vertical movement in the delta.