Understanding dynamics of groundwater flows in the Mississippi River Delta

•Complex hydrogeological setting in the Mississippi River Delta is revealed.•Groundwater flow rate is 3 to 4 orders smaller than Mississippi River discharge rate.•Groundwater and surface water interacts actively during severe hydrologic events.•Sharp groundwater recharge-to-discharge reversion occurs right after hurricanes pass.•High pore water pressure during and after hurricanes may destabilize surficial sediments. Groundwater is a major component of the water cycle in the river-deltaic environment. However, the dynamics of groundwater flows in the river delta remains an open question owing to the complexities in the hydrologic and geological settings. This study employed a hydrogeological modeling approach to investigate groundwater dynamics in the Mississippi River Delta (MRD). A detailed groundwater model for the top 50 m of the MRD was constructed from the Head of Passes (RK 0) to Jesuits Bend (RK 108) using geotechnical borings and topobathymetric maps. Mississippi River stage and tide stage data were the key hydrologic data, which reflect hydrologic forcing (floods, storms, and hurricanes) to the groundwater system. By investigating the 2012 hydrologic year, the study presents several intriguing findings. Groundwater discharge and recharge rates to the river and surrounding bays were estimated 3 or 4 orders of magnitude smaller than Mississippi River discharge rate to the Gulf of Mexico. Nevertheless, the model showed strong surface-groundwater interactions controlled by local hydraulic gradient at the river and bay interfaces during severe hydrologic events. Specifically, during Hurricane Isaac pore water pressure was estimated 4–6 times higher than the normal condition and peak groundwater recharge was reached due to storm surges. Peak groundwater discharge sharply occurred a few days later after the hurricane passed. The study found such a dramatic change in surface-groundwater interactions was due to quick surface water receding and delayed groundwater response. As a consequence, more areas are likely exposed to harmful high pore water pressure and low factor of safety condition, which would destabilize sediments, enhance erosion, and compromise safety of coastal infrastructures such as the ring levees.