Overview

*Hollywood Tracer Study: Contour plot of SF6 concentrations encountered during the survey on June 7-9, 2004. The sampling locations along the cruise track are indicated by the + symbols.*

The fate of effluent from point sources into the coastal ocean is of Interest because of the recognized potential impact of excess nutrients and other chemical entities on coastal ecosystems. In particular, coral reef tracks along the east coast of Florida are thought to be sensitive to water chemistry perturbations. Moreover, attribution of the far-field sources of low level contaminants is not straightforward as there are several major outfalls along the Florida coast and numerous non-point source inputs. Determining the origin of the material is often controversial because of lack of unequivocal markers of point sources. One method of following the flow from specific point sources is to tag the effluent with a substance that has low background levels in the environment and can be measured at low concentrations. Fluorescent dyes such as rhodamine-WT have been used extensively for this purpose and are approved in EPA protocols to quantify near-field dilutions. Another tracer with a lower limit of detection is sulfur hexafluoride (SF₆). Ease of injection, in situ detection, and reasonable stability in natural water along with low toxicity at dilute concentrations make these good tracers for this purpose.

An outfall tracer study was conducted on the Hollywood treated-wastewater (TWW) outfall in June 2004 by injecting SF₆ gas into the outfall pipe over the course of six days. Under the physical and meteorological conditions encountered during the study, the surface SF₆ concentrations showed that the discharged water flowed northward parallel to the coast with a broadening of the width of the plume to about 3 km at the farthest point sampled, 66 km from the outfall. The discharge was fully mixed throughout the water column within 13 km of the outfall terminus. In the first 20 km from the outfall, SF₆ surface concentrations were highly variable, while beyond this the SF₆ concentrations decreased monotonically going northward. Maximum concentrations decreased by about 200-fold per kilometer from the outfall to the northern end of the study area.

Reference: Wanninkhof R, K Sullivan, W Dammann, J Proni, F Bloetscher, A Soloviev, and T Carsey. Farfield tracing of a point source discharge plume in the coastal ocean using sulfur hexafluoride, Environ. Sci. Technol. 39: 8883-8890. 2005 (PDF).

Additional tracer studies have been performed at the Boynton Inlet and at the South Central Regional Wastewater Treatment and Disposal Plant ocean outfall during 2007. At both sites, two chemical tracers (sulfur hexafluoride and rhodamine-wt were injected into outgoing flow and were then tracked by two research vessels. Two tracking two boats were used, the Coral Reef II and the NOAA research vessel Cable. Samples obtained from both boats were analyzed for a variety of chemicals (in addition to the tracer chemicals) including nitrate, nitrite, ammonium. In addition, acoustic instrumentation was employed on the boats and at various locations in the vicinity to gather information on ocean current velocity and direction. A side-looking acoustic instrument was installed at the Inlet to measure the current flowing in and out during daily tidal cycles. These data and other related data are presented in a NOAA Technical Report listed in the Publications page.

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