TIDAL AND WATER QUALITY VARIABILITY IN AN URBANIZED ESTUARY

Joseph L. DiLorenzo, Ph.D., Senior Scientist, Najarian Associates, 1 Industrial Way West, Eatontown

NJ, 07750, dilorenzo@najarian.com, (732) 389-0220;

 

Ronald J. Filadelfo, Center for Naval Analyses, Alexandria, VA;

 

Vajira Gunawardana, Director of Environmental Engineering and Water Resources, Najarian Associates, Eatontown, NJ;

 

Clem Surak, Senior Scientist, Najarian Associates, Eatontown, NJ;

 

Howard S. Litwack, Senior Analyst, Najarian Associates, Eatontown, NJ; and

 

Tavit O. Najarian, President, Najarian Associates, Eatontown, NJ.

 

 

ABSTRACT: During 1988, tide and water quality data were collected intensively in the Hackensack Estuary -- a tidal tributary of the New York/New Jersey Harbor Estuary. Tidal elevations were monitored continually at four estuarine stations and over a six-month period; current velocities were measured concurrently at one station near the mouth. Discrete water quality samples were collected at six main-stem estuarine stations and at 2- or 3-hour intervals. Harmonic analyses of tidal elevation data indicate that Hackensack Estuary tides are predominantly semi-diurnal, though modulated by diurnal and fortnightly components. The tide propagates over the entire length of the HRE in approximately 1.1 hours, with slight (2-9%) amplitude amplifications in the upper reaches. Also, current fluctuations in the lower estuary lead tidal elevations by nearly one-quarter of a tidal cycle, corresponding to near-standing-wave tidal characteristics. An average tidal excursion of 8 km (at the mouth) is calculated from the current velocity data. This distance is comparable to tidal translations displayed in longitudinal profile plots for several sampled water quality constituents. Both lag correlation analyses and spectral analyses of the water quality time series reveal significant ammonia-nitrogen variability at dominant tidal frequencies. These analyses suggest that tidal variations in Hackensack Estuary ammonia concentrations are dominated by major point-source loadings and by horizontal advection processes. A simplified, one-dimensional model is used to characterize such variations in relation to relevant hydrodynamic/hydrologic controls. Other parameters, such as dissolved oxygen and biochemical oxygen demand, exhibit less variability at semi-diurnal frequencies, suggesting a greater influence of biochemical processes acting over longer time scales. Uncertainties due to the selected water quality sampling frequencies are quantified, and sampling criteria are developed for resolving semi-diurnal tidal variability. Overall, the analyses highlight the importance of tidal variability in estuaries where concentration gradients are large (due to major point source discharges) and tidal excursions are moderate.

 

 

 

 

 

 

 

 

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