EFFECT OF A TIDE-CONTROL STRUCTURE ON GROUNDWATER FLOW AND CONTAMINANT TRANSPORT AT THE GROUNDWATER-SURFACE WATER INTERFACE OF A SMALL WATERSHED DRAINING TO DELAWARE BAY

Red Lion Creek is a small coastal plain tributary with a history of contaminant input from multiple chemical plants and a landfill located within the lower half of its basin. Creek stage is regulated at the confluence with Delaware Bay by a tide-control structure, a levee with five 48-inch culverts having flappers on the outlet end which allow discharge at low tide but prevent entry of saline bay water during high tide. The final one mile reach of the creek is a broad, shallow, mud-bottomed waterway fringed by a Phragmites dominated wetland punctuated with areas of preferential, often contaminated, groundwater discharge. Evidence from well logs and geophysical surveys suggest that the Pleistocence drop in sea level during the last glacial maximum provided a gradient that allowed the paleo-Red Lion Creek (much larger drainage area) to cut through confining clay layers. The creek was subsequently refilled as sea level rose and melting glaciers provided abundant water and sediment. The infilled Quaternary deposits include 0-40 feet of silt, underlain by more than 100 feet of medium grain sand.

Groundwater discharge occurs within the wetland at toe-slope seeps and along the transitional area near the creek. These areas have been identified using hand-held and helicopter-based infrared cameras, and remote-controlled boat-based temperature, pH, and specific conductance probes. Where confining layers are absent, strong vertically downward gradients measured in nearby well pairs may indicate possible recharge of the deeper aquifer system from contaminated surface and shallow groundwater.

Recent damage to the tide-control structure from large coastal storms has led to malfunction, allowing tidal fluctuation and inundation of more wetland area, leading to redistribution of contaminated water and changes in shallow water chemistry. Limiting tidal influence in the creek is necessary to minimize mobilization of PCB contaminated sediments and for assessing current contaminant distribution and baseline geochemical conditions, particularly for an ongoing evaluation of potential bioremediation of wetland areas. A restoration of stability in creek stage also will help improve implementation of remedial actions within the wetland areas, and a lower stage will decrease potential recharge to the semi-confined aquifer.