Impacts of Sea Level Rise and Tidal Restoration on Groundwater Dynamics in the Herring River Watershed, Cape Cod, USA

Sea levels are expected to rise due to global climate change, leading to significant impacts on coastal systems. One of these impacts is the decrease in the depth to groundwater in shallow coastal aquifers. This decrease could result in changes to groundwater discharge, increased groundwater shoaling, and the loss of dry land due to groundwater emergence and inundation.  

In the past, many tidally influenced coastal drainage networks were diked and drained for human use. However, there is now growing interest in restoring these diked systems to their natural tidal conditions. Tidal restoration will change the surface water levels of estuaries, which will affect the underlying groundwater in ways similar to sea level rise but potentially more rapidly. 

The Herring River watershed in Cape Cod, MA, USA is currently undergoing a tidal restoration project. We have chosen the Herring River as a case study to determine how tidal restoration and sea level rise will change conditions in the surrounding shallow aquifer. To predict the changes in groundwater conditions due to tidal restoration and sea level rise, a MODFLOW 6 groundwater model was coupled with a DELFT3D hydrodynamic model. The hydrodynamic model provided surface water boundary conditions for the groundwater model. Sea level is expected to rise by approximately one meter by the year 2100. Therefore, the model was run for several sea level rise scenarios, including the current sea level and projected sea level rise of 0.6 meters and 0.9 meters, for tidally controlled and restored conditions.  

The preliminary results of the model indicate an increase in groundwater shoaling and emergence as sea level rises and restoration occurs. This shallowing of the water table may also lead to more overall groundwater discharge to the drainage network and coastal waters. If tidal control does not allow sufficient drainage, increased groundwater discharge may become impounded behind similar diked systems leading to more overall flooding than if tidal conditions were restored. Showing how groundwater emergence, shoaling, and discharge will change with sea level rise and tidal restoration can help coastal stakeholders decide which management practices are best implemented during restoration efforts.