4/13/23 AEG Event: Modeling Coastal Groundwater in New Jersey

DINNER MEETING ANNOUNCEMENT

Modeling Coastal Groundwater to Solve Environmental Problems Associated with Sea Level Rise: Case Studies from Sandy Hook and Forsythe Wildlife Refuge, NJ

Thursday, April 13, 2023

At the Clarion Hotel in Somerset, New Jersey

Alex Fiore, Hydrologist, USGS

  
 Simulated change in depth to groundwater with 0.2-meter sea-level rise above baseline conditions, Sandy Hook, New Jersey

 

ABSTRACT
     The availability of fresh groundwater resources in coastal areas play a crucial role in determining the type of ecosystem regime that can survive at a given location. The inundation and intrusion of saltwater into unconfined aquifers due to sea level rise (SLR) threatens habitats that rely on fresh groundwater for their survival. Results of two studies by the U.S. Geological Survey are presented here that use groundwater flow modeling to assess the effects of SLR on coastal groundwater conditions at locations in the New Jersey Coastal Plain that contain vulnerable habitats: (1) Gateway National Recreation Area, Sandy Hook Unit (hereafter Sandy Hook), a sand spit in Monmouth County managed by the U.S. National Park Service, and (2) Forsythe National Wildlife Refuge (NWR), which occupies more than 48,000 acres in Atlantic and Ocean counties and is managed by the U.S. Fish and Wildlife Service.

Conceptual model and hydrogeologic framework development for Sandy Hook and the Forsythe NWR included collection of borehole geophysical logging, ground-penetrating radar, groundwater levels, and groundwater sampling. These were incorporated into two numerical MODFLOW models (one for each location) utilizing the Saltwater Intrusion Package (SWI2) that simulates multi-density groundwater flow, using the half-seawater surface (17.5 parts per thousand of total dissolved solids) as a sharp interface representing the freshwater/saltwater transition zone. The models included three scenarios, simulating 20 cm, 40 cm, and 60 cm of SLR. Model results from both locations indicate that SLR will lead to corresponding rises of the water table closer to land surface, which will affect ecosystems farther inland than would be expected from inundation from surface flooding from SLR alone.

At Sandy Hook, the groundwater table is projected to rise above land surface across 7.4 percent of Sandy Hook’s land area. Sandy Hook contains no surface streams, with spring-fed ponds and wetlands providing the only fresh surface water, so the depth to shallow fresh groundwater is a major determinant on which ecosystem regimes can be supported. Thus, a rising water table is likely to create emergent wetlands in areas previously occupied by other habitats, including low-lying inland areas away from the shoreline where habitat destruction from a rising water table may precede surface inundation of these areas from SLR itself. Increased water table height also reduces the thickness of the freshwater lens and causes the half-seawater surface to become shallower, which may affect freshwater-dependent ecosystems closer to the shoreline.

More than 82 percent of the Forsythe NWR are wetlands, 78 percent of which is salt marsh. Salt marshes flourish in areas where saltwater inputs from the bay or ocean is balanced by fresh groundwater discharge coming from the mainland. Across the Forsythe model, fresh groundwater discharge to the salt marshes and bay is projected to decrease by about 50% between baseline and the 60 cm scenario, causing water levels and freshwater discharge to increase in upland areas and create emergent wetlands similar to the results from Sandy Hook. The lens of shallow salty groundwater beneath the salt marsh becomes thicker and moves inland as the upward vertical flow of groundwater into the marsh decreases and can no longer keep it perched near the marsh surface, turning freshwater wetland habitats into salt marshes.

BIOGRAPHY
     Alex R. Fiore is a hydrologist with the USGS New Jersey Water Science Center specializing in aquifer characterization, groundwater-surface water interaction, and groundwater modeling. He has contributed groundwater hydrology expertise in a variety of applications, including assessments of contaminant transport in fractured rock and karst aquifers, hydrogeologic controls on landslide initiation, and effects of sea level rise in coastal aquifers. He has a M.S. and B.S. in Geological Sciences from Rutgers University-New Brunswick.