Investigation of Submarine Groundwater Discharge at Holts Landing State Park, Delaware: Hydrogeologic Framework, Groundwater Level and Salinity Observations

Abstract

Monitoring wells and groundwater sensors were installed and monitored in and around Holts Landing State Park on the Indian River Bay, eastern Sussex County, Delaware, between October 2009 and August 2012. Data from test drilling, geophysical logging, geophysical surveys, and well testing characterized the hydrogeological framework and spatial and temporal patterns of water pressure, temperature, and salinity in the shallow, unconfined Columbia aquifer. The work revealed a plume of freshened groundwater extending more than 650 ft into the bay from the shoreline. Groundwater salinities intermediate between baywater and inland groundwater are present both offshore and on land adjacent to the bay and tidal tributaries.

The fresh groundwater plume, as observed in wells and borehole geophysical logs, decreases in thickness from more than 40 ft nearest the shoreline to less than 20 ft farthest from the shoreline. Saline water is found above and below the plume and the freshwater-saltwater interface is spatially complex. Characterization of the hydrogeologic framework was critical to explaining the distribution of fresh groundwater. Fresh water is trapped near the bay bottom by an overlying confining bed composed of the low permeability sediments of a Holocene paleovalley fill sequence and the Beaverdam Formation. This complex, heterogeneous geological framework also causes multiple stacked interfaces in one location at the study site.

Groundwater levels, temperatures, and specific conductivity respond to climatic, seasonal, and storm-related weather forcing patterns as well as to forces caused by astronomical tides. The relative importance of these forces to groundwater levels, the flux of fresh groundwater, and groundwater salinity varies with location. Ranges in groundwater levels are more than 6 ft at an inland location and are clearly controlled by seasonal recharge patterns. Extreme weather events have a secondary effect on groundwater levels. In comparison, ranges of groundwater levels are much smaller in near shore and offshore wells, and are more closely related to tidal forces. As a result of this difference in ranges of groundwater levels, seasonal variations in water levels at inland locations are the primary variable controlling bayward-directed groundwater gradients, fresh groundwater flux, and groundwater salinity distribution. Shorter duration weather and tidal events have a secondary role. The freshwater-saltwater interface and associated mixing zone moves upward and/or landward during extended periods of low freshwater flux into the bay, and downward and/or bayward during extended periods of higher freshwater flux.