Tidal variability in the water quality of an urbanized estuary

Tide and water quality data were collected concurrently in the Hackensack River estuary (HRE), a tidal tributary of the New York-New Jersey Harbor estuary. Harmonic analyses of tidal elevation data indicate that HRE tides are predominantly semidiurnal, though modulated by diurnal and fortnightly components. Nearly uniform tidal ranges (averaging approximately 1.6 m) were observed at three stations within the HRE. Periodogram estimates reveal significant tidal variability for the water quality parameter NH4-N under dry weather conditions. Lag correlation analyses associate NH4-N concentration variations with water level fluctuations. Longitudinal profile plots for NH4-N reveal a consistent pattern of tidal translations, with peak concentrations oscillating about a major wastewater discharger. These analyses suggest that the distribution of NH4-N concentrations in the HRE is controlled primarily by major point source loadings and horizontal advection. A simplified, one-dimensional model is used to describe this distribution. Effects of tidal variability in masking water quality status and weak trends are also analyzed. These analyses highlight the potential importance of short-term water quality variability in tidal estuaries where concentration gradients are large.