ESTIMATING RELATIVE FLOWS OVER TIME TO AN URBAN COMBINED SEWER OUTFALL (CSO) FROM CONTAMINANT MIXING

One of the principal reasons why New York City Harbor waters often do not meet U.S. EPA ambient water quality standards for bathing and fishing is the frequency of sewage discharges in wet weather from the combined sewer system. These sewage discharges at combined sewage outfalls (CSOs) have a detrimental impact on urban coastal watersheds, and potential human health consequences as fecal pathogens, nutrients and heavy metals are released to the environment. Analysis of microbiological and heavy metal indicators at one such outfall in a tidal wetland creek draining an urbanized watershed in eastern Queens indicates that sewage is illegally discharged even in dry weather.

At this site, a minor contributor to the overall sewage loading to the coastal waters, different combinations of parameters suggest discharges from different sources: sewage, groundwater, surface-water and stormwater, at different times under different weather conditions. Sewage tracers include Enterococcus, Na and Fe, while stormwater contains Zn, surface water is identified by Ba and groundwater by Cr. Mass and volume-based methods have been incorporated into equations for estimating pollutant loading into urban watersheds. The concentrations of the CSO discharge and the background water quality data provide information on the mixing of source-water chemical compositions. Using basic hydrological data over the short term, assuming constant source-water compositions and limited reactions between constituents, the relative proportions of sewage, groundwater, surface water and stormwater discharged into this urban watershed may be discerned. Estimating these different discharges under different meteorological conditions provides a tool for assessing the impacts of such sewage discharges on similar urban wetland environments in the New York area.