Transport mechanisms in the Upper Delaware Estuary
Creators: name: Duzinski, Philip J.
Publisher: name: No Publisher Supplied
Year: 2023

description: The City of Philadelphia and nearby communities rely upon the freshwater section of the tidal Delaware River as a source for drinking water for Philadelphia’s 1.6 million residents and millions living outside the city. Philadelphia is also located at the outlet of a more than 10,000 sq. mile watershed and benefits from upstream water management and policy decisions that guarantee adequate river flows to preserve water quantity and quality in the Schuylkill and Delaware Rivers. Complex water quality dynamics in the Delaware Estuary, water supply planning priorities, and the proximity of numerous sources of pollutant discharges near several of these drinking water intakes provides the societal motivation for the study of transport mechanisms in this end of the Delaware Estuary. Pollutant inputs exist in the study area including several wastewater treatment plants, stormwater from point and non-point sources, as well as permitted industrial discharges located along both shorelines of this river. Other potential pollutant sources creating risks to water suppliers include industrial and transportation accidents and spills. During low flow conditions, which may be only 85 m3s-1 at Trenton during typical dry periods and 71 m3s-1 during declared droughts, the oligohaline section of the river can move upstream ~30 km from its mean observed fall location near river km 115. Saltwater intrusion during these flow regimes, can reach the lower boundary of the City of Philadelphia. The transport dynamics in the oligohaline reach of this estuary have not been studied extensively under these conditions yet have great importance to the region’s community source of drinking water. The Delaware River Basin Commission oversees a 4-state agreement with New York City to release water from upstream reservoirs when discharge at Trenton, NJ, falls below 3,000 ft3s-1 (85 m3s-1). Prior to that agreement, the oligohaline section of the estuary moved upstream ~50 km from its mean location during the drought of the 1960’s. This study seeks to characterize and quantify dispersive mechanisms in the tidal fresh and upper oligohaline reaches of the Delaware River using a field dye study and a 3-D numerical model. The results can be used to predict a range of threats to potable water in this region including those from pollutant spills and salt intrusion during extreme droughts, storms and rising sea-level. Moreover, the results are applicable to other systems where salt-water intrusion threatens drinking water supplies. In Chapter 2, results from a numerical and a dye field study were analyzed to quantify and diagnose dispersion in the freshwater portion of the Delaware River to assess the magnitude and dynamics of dispersive processes in the reach of the river near Philadelphia. Results from a Regional Ocean Modeling System (ROMS) numerical model were analyzed to explore potential mechanisms of dispersion including vertical and lateral oscillatory shear, and tidal trapping through analytical models. Results found that oscillatory lateral shear dispersion contributed most strongly to along channel dispersion with a secondary contribution from tidal trapping.In Chapter 3, a numerical model of the upper Delaware Estuary was used to analyze the potential mechanisms of dispersion in the oligohaline section of the Delaware Estuary. While the model domain is largely within the tidal-fresh upper estuary, the domain below Philadelphia becomes oligohaline during low-flow events, and there is evidence that this intrusion is facilitated by a combination of steady vertical shear dispersion and tidally oscillatory salt flux, with the latter becoming increasingly important approaching the upstream intrusion extent. Frontogenesis is also important in the intrusion process in the vicinity of specific bathymetric features. In Chapter 4, a review of the early and recent history of the Philadelphia Water Department (PWD) through the 1972 Clean Water Act is discussed to provide motivation for why this water utility is supporting research on estuarine transport mechanisms. A range of watershed protection programs of PWD are discussed related to pollution control and drinking water protection including from salt intrusion. A review of sea level rise impact studies in the Delaware Estuary is discussed to assess future climate change implications of this research.