INFLUENCE OF CANALS ON THE EXTENT OF SALTWATER INTRUSION DURING WIND TIDE EVENTS IN A COASTAL WETLAND SYSTEM

The effects of man-made canals on hydrologic systems have been studied for over a decade, particularly along the coasts, where the majority of the U.S. population resides. At the Emily and Richardson Preyer Buckridge Coastal Reserve saltwater intrusion in the freshwater system is facilitated by the presence of man-made canals that provide pathways by which saltwater can enter wetlands in the reserve. Wind tide events that occur in the region typically lead to corresponding spikes in specific conductivities in surface water in the canals.

This research project examines the extent to which brackish water propagates into the interior of the reserve under the wind tide events.

Following previous research activities in the reserve, monitoring wells equipped with water level and specific conductivity loggers were installed in two canals at different locations in the reserve. The first monitoring station was located 1.7 km from the immediate source of the brackish water (the Alligator River), whereas the second station was located 6 km from the source. The loggers were set to sample at 30 minute intervals over a period of 3.5 months. The two monitoring stations were located roughly 1.6 km apart in the northern and central parts of the reserve.

Preliminary results show that brackish water travels from the Alligator River into the interior of reserve, reaching as far as 6 km upstream from where one of the canals empties into the Alligator River. This is far enough into the reserve to reach locations where the canal runs adjacent to agricultural fields. Results indicate that elevated conductivities are as high as 2.5 mS (background ≤ 1.0 mS), at this location and persist in the water column for several weeks (three to four) before returning to background levels. Results from the station located closer to the Alligator River reveal conductivities as high as 3.2 mS that persist between 2 to 9 days.

The results suggest that the canals in the reserve may facilitate movement of brackish water over larger distances than previously thought. The presence of brackish water upstream from the Alligator River may threaten agricultural fields that are adjacent to the reserve. Future studies, will focus on groundwater-surface interactions to assess the extent to which brackish waters may infiltrate into the groundwater system from surface water in the canals.