The Delaware Geological Survey (DGS) has published a report on the inundation of tidal wetlands in the Murderkill Estuary in southeastern Kent County, Delaware. The report entitled Characterization of Tidal Wetland Inundation in the Murderkill Estuary, Report of Investigations Number 81, presents the results of research by Thomas E. McKenna in cooperation with the Murderkill River Study Group convened by the Kent County Department of Public Works and the Delaware Department of Natural Resources and Environmental Control (DNREC).

The Murderkill River and Estuary are located in southeastern Kent County, Delaware and discharge to Delaware Bay at the Town of Bowers Beach. The study area is the lower part of the Murderkill Estuary defined by a 12-km river reach, its tributaries, and adjacent tidal wetlands between Route 1 at Frederica and the Town of Bowers Beach. Approximately 90% of the study area is tidal wetlands.

This study is driven by the need to better understand the causes of low dissolved oxygen concentrations in the tidal Murderkill River that do not meet Delaware’s Total Maximum Daily Load (TMDL) Regulations. A TMDL sets a limit on the amount of a pollutant that can be discharged into a waterbody with the waterbody still meeting water-quality standards. Subsequent to this study and collaborative work by the Murderkill River Study Group, DNREC proposed amendments to the TMDL Regulation to encompass site-specific criteria for dissolved oxygen for the tidal Murderkill River.

In this study, a parameterization of inundation for the wetlands in the Murderkill Estuary was developed. The parameterization was subsequently implemented in a numerical water-quality model to evaluate scenarios of nutrient loading to the river and its effects on dissolved oxygen concentrations. Biogeochemical processes associated with high loads of nutrients (nitrogen and phosphorus) and the biochemical oxygen demands in the river are the likely causes of low dissolved oxygen concentrations. The periodic inundation and exposure of tidal wetlands, as characterized in this study, are primary controlling variables in many biogeochemical processes in estuaries with extensive wetlands.

From Bowers Beach (downstream) to Frederica (upstream) there is a decrease in marsh elevation with mean elevation decreasing from 0.86 m to 0.60 m. Upstream marshes are flooded more frequently and for longer duration than downstream marshes and these patterns are also in agreement with site-specific accretion data and other literature where more frequent and longer inundation results in higher accretion rates.

DGS Report of Investigations No. 81 is part of the Delaware Geological Survey’s ongoing mission to understand geologic and hydrologic systems and to advise, inform, and educate Delawareans about the results of such investigations. The report is available in PDF format from the DGS web page at www.dgs.udel.edu/publications.