RELATIONSHIP BETWEEN GROUNDWATER FLOW, TIDES, AND SALT POND FORMATION AT THE DELAWARE SALT MARSH

Salt marshes are economically important coastal landforms that provide a variety of ecosystem services. However, salt marshes are disappearing around the world at an alarming rate. Estuarine marshes around Delaware Bay are being lost with a rate of ~ 2.9 km² per year (Partnership for the Delaware Estuary, 2017). In the United States >96% of salt marsh loss is attributed to conversion of marsh to open water. Recent studies of 5.5 km² of salt marsh at Slaughter Beach, DE documented 2,762-5,000 salt ponds that changed in size and numbers seasonally and on a decadal scale. While some changes were correlated with major storms, rising sea level, and anthropogenic factors, the role of ground water flow in salt pond development was poorly understood.

Using four monitoring well installed along the transect crossing ponds of different sizes we measured groundwater discharge at 1 m, 3 m, and 6 m depth below marsh surface during two lunar cycles. We collected a high-resolution aerial imagery at peak high and low tides using a drone.

Our results document the presence of 2 aquifers; deep (3m) and shallow (1m). Groundwater flow is strongly fluctuating in response to tides in the deep aquifer and not so much in the shallow. The direction of groundwater flow is downward with little lateral movement. Analysis of drone imagery reveal no changes in the shape or size of the pond during tidal cycle. Groundwater elevation decreases in proportion to distance from tidal channel towards the coastline. We suggest that the deep aquifer is confined and tidal ponds formed above the semi-impermeable unit of tidal mud. Therefore, their changes are related to changes in the volume of tidal flow and hydrologic regime of a shallow tidal drainage network. Rapid development of salt ponds could reduce the marsh resilience to sea level rise.