Farfield Tracing of a Point Source Discharge Plume in the Coastal Ocean Using Sulfur Hexafluoride

Abstract

Pathways and dilution of a point source ocean discharge in the farfield (approximately to 10-66 km) were measured using the deliberate tracer sulfur hexafluoride (SF6). The injection of SF6 was performed by bubbling the gas over a period of 6 days into an ocean outfall pipe discharging into the southeast Florida coastal ocean. The surface SF6 concentrations show 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. The currents were measured during the study with a bottom-mounted acoustic Doppler current profiler (ADCP) located 5.5 km from the outfall. Velocities were variable in magnitude and direction but showed a net northward flow during the 6-day study. Maximum concentrations decreased by about 200-fold per kilometer from the outfall to the northern end of the study area. The study shows that SF6 is an effective method to trace point source releases far from their origin.

The Use of SF<sub>6</sub> and GIS to Study Farfield Modeling of Ocean Outfall Plumes in Florida

“…The development of the plume model in this study used data gathered from the experiment conducted by Wanninkh of et al [68] on the Hollywood outfall as well as data other NOAA operations [83] [95].…”

“…SF 6 was used because it is a good tracer to study point source releases as it has low background levels in the environment. It is nontoxic, stable, and measurable at low concentrations [68]. The injection of SF 6 began on June 4, 2004 (10 a.m. EDT) and ended on June 9, 2004 (3 p.m. EDT) as described in Wannikhof et al [68].…”

“…SF 6 was suggested as the tracer due to NOAA's success in using it in the open ocean. Its usefulness in the coastal ocean had not been tested until the Hollywood experiment discussed here in 2004 [68]. The detection limit of SF 6 is less than 1 part per trillion by volume (pptv) using headspace analysis, and two orders of magnitude lower still using trap and purge techniques [69] which has made it possible to perform tracer studies over large space scales and long time scales.…”

“…Wannikhof et al [68] notes this application adds to the utility of the tracer, which had only previously been used as a tracer of gas exchange processes in the ocean [70]- [72], lakes [73] [74], and in rivers [75]; as a surface water dispersion tracer [76], as a tag for surface water in Lagrangian studies ([77]- [79]; and to determine mixing in the deep ocean [80] [81]. Because Wannikhof et al [68] noted success with the Hollywood tracer in June 2004, a second tracer study of the Boynton outfall was conducted in 2007. The intention was to use these data points to develop a model of the farfield plume to evaluate the potential effects of the outfalls on benthic communities.…”

“…The sampling survey of the ocean waters commenced on three days after initial dosing commenced on the Hollywood outfall in a continuous mode using a towed "fish" containing a Seabird Micro Cat C (onductivity)-T (emperature) recorder and submersible water pump and samples as described in Wannikhof et al [68]. The same basic procedures were used in February 2007 for the second SF 6 tracer experiment conducted by NOAA and Florida Atlantic University (FAU).…”