HR: 1330h
AN: OS32B-0249    [PDF]
TI: Effect of tidal phase on solute flushing from a strait: SF$_{6}$ tracer study in the East River, New York
AU: * Caplow, T
EM: tc144@columbia.edu
AF: Dept. of Earth & Environmental Engineering, Columbia University, 500 West 120 Street, New York, NY 10027 United States
AU: Schlosser, P
EM: peters@ldeo.columbia.edu
AF: Dept. of Earth & Environmental Engineering, Columbia University, 500 West 120 Street, New York, NY 10027 United States
AU: Schlosser, P
EM: peters@ldeo.columbia.edu
AF: Dept. of Earth & Environmental Science, Columbia University, New York, NY 10027 United States
AU: Schlosser, P
EM: peters@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Ho, D T
EM: david@ldeo.columbia.edu
AF: Dept. of Earth & Environmental Science, Columbia University, New York, NY 10027 United States
AU: Ho, D T
EM: david@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AB: Flow in the East River, a 25 km tidal strait connecting Long Island Sound with New York Harbor, is driven by a tidal phase lag between the two ends of the strait. The direction and rate of solutes transported in the strait, including natural materials as well as anthropogenic contaminants, has important implications for the environmental management of Long Island Sound and other fragile local ecosystems. Sulfur hexafluoride (SF$_{6}$) is a successful deliberate tracer for rivers, estuaries, and coastal areas. It is non-reactive, inexpensive, and offers an extremely low detection limit. High-resolution transport studies of complex coastal and estuarine areas up to 100 km$^{2}$, and lasting up to two weeks, have recently been achieved using a boat-mounted SF$_{6}$ measurement system with a sampling interval of 1 min and a detection limit of 1 x 10$^{-14}$ mol L$^{-1}$. In June 2003, two injections of 6.2 mol sulfur hexafluoride (SF$_{6}$) were made 8 days apart in the East River to study residual circulation and rates of solute dissipation at different states of the tide. Both injections were made at the same location, but the first injection occurred at the slack before flood (northward flow), and the second injection occurred at the slack before ebb (southward flow). Tidally synchronized surveys of the SF$_{6}$ tracer patch were made by boat for 7 days following the flood injection and for 5 days following the ebb injection. For the flood and ebb injections, respectively, mean displacement of the center of tracer mass within the East River, a proxy for residual circulation, was northward at 0.31 $\pm$ 0.35 and 1.5 $\pm$ 1.0 km day$^{-1}$, mean fractional tracer loss due to tidal flushing was 0.32 $\pm$ 0.06 day$^{-1}$ and 0.52 $\pm$ 0.10 day$^{-1}$, and mean residence time was 2.6 $\pm$ 0.4 days and 1.3 $\pm$ 0.6 days. These tracer loss rates include a small correction for air-water gas exchange, which was estimated by a combination of previously established relationships between gas transfer velocity and wind speeds, river flow velocities, and rain rates. Residual circulation appeared to have little impact on tracer fate, while the state of the tide at the time of injection had a large impact. Tracer injected on the ebb tide dissipated more rapidly, indicating a large differential between the mixing power at the two ends of the strait. (In the case of the East River, New York Harbor offered more rapid mixing than Long Island Sound). These results suggest that tidal phasing of contaminant discharges in a strait of this kind (where the tidal excursion is comparable to the length of the strait) has the potential to reduce ecological impacts, by increasing flushing rates and directing a greater fraction of solutes away from ecologically sensitive areas.
UR: http://www.columbia.edu/cu/tracer
DE: 4235 Estuarine processes
DE: 4251 Marine pollution
DE: 4546 Nearshore processes
DE: 4558 Sediment transport
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting