Modeling Enterococci in the Tidal Christina River

The tidal Christina River is located at the lower portion of the Christina River Basin, and connects to the Delaware River. The enterococci level is one of the major concerns for watershed management. The tidal Christina River receives enterococci loads from Brandywine Creek, Red Clay Creek, White Clay Creek, non-tidal Christina River, and 38 Combined Sewer Overflows (CSO) from the city of Wilmington, DE. The Environmental Fluid Dynamics Code (EFDC) model was modified and configured to address the fate and transport of enterococci in the tidal Christina River. Enterococci loads from the creeks were provided from four watershed loading models using the Hydrological Simulation Program FORTRAN (HSPF). The loads from CSOs were calculated from the output of XP-SWMM model developed by the city of Wilmington. The hydrodynamics were calibrated with water surface elevations at two USGS tidal stations, temperature and salinity time series at five water-quality monitoring stations. The EFDC code was modified to include impacts of solar radiation, temperature, and salinity on the die-off of enterococci. The modeled enterococci were compared against field data at five monitoring stations. The model results reveal that the tidal Christina River is mainly affected by the enterococci from the upstream creeks due to wash off of manure on agricultural land during storms and animal direct deposit of waste in streams in warm weather under existing conditions. CSOs only affect limited areas around the outfalls since the dilution is strong in the tidal Christina River. However, to achieve the water quality standard for the entire tidal Christina River, significant reduction of CSO loads is still required.