Transport

1
Transport Dispersion in New York Harbor A
High-Resolution SF6 Tracer Study
http//www.columbia.edu/tc144
Ted Caplow1, Peter Schlosser1,2,3, David T.
Ho2,3 corresponding author Mudd 918, 500 W 120
St, NY, NY 10027, tc144_at_columbia.edu 1Dept. of
Earth Environmental Engineering, Columbia
University 2Lamont-Doherty Earth Observatory,
Columbia University 3Dept. of Earth
Environmental Sciences, Columbia University
Nicholas Santella, Columbia University John
Lipscomb, Riverkeeper Megan Garrison, East Side
Middle School Rica Enriquez, Johns Hopkins
Method Sulfur hexafluoride (SF6) has been used
successfully as a deliberate tracer for rivers,
estuaries, and coastal areas, due to its inert
nature, non-toxicity, and extremely low detection
limit. An automated, high-resolution SF6
measurement system mounted on a boat was recently
developed (Ho et al 2002). The system has a
sampling interval of two minutes and a detection
limit of 1 x 10-14 mol L-1. A single injection
of approximately 0.9 mol of SF6 was made in the
northern end of Newark Bay on July 14, 2002 to
investigate the hydrodynamics of the Bay, the
Kills, and the tidal portions of the Passaic and
Hackensack Rivers. The spread of the tracer plume
was tracked for 11 consecutive days following
injection (time-series images below). Most
measurements were taken at a depth of 1 m, with
deeper profiles at certain locations.
Approximately 200 CTD casts were made to examine
the halocline and thermocline at different
locations. The study period coincided with dry
(10th percentile of river flow volume over past
20 years) and relatively calm conditions. Results
Initially, SF6 spread quickly through Newark Bay,
the Kills, and up into the New Jersey rivers,
indicating high-energy mixing driven by strong
tidal currents. Bulk advection of the tracer was
almost negligible, indicating very low river
flows during the study period and suggesting long
residence times (several weeks) for most
dissolved contaminants under these conditions.
Ultimately, tidal mixing and gas loss at the
air/water interface dispersed most of the plume.
The Kill van Kull (see map below) was the primary
dilution pathway. A combined SF6 mass inventory
was achieved for the inner harbor area,
including the Kills, Newark Bay, and the lower
Hackensack and Passaic rivers, using detailed
bathymetry and daily composites of SF6
measurements. Total SF6 inventory within this
area exhibited exponential decay (? 0.3 day-1
see figure at lower right). Daily wind data from
Newark airport (u10, mean 4.5 m s-1) was used
to estimate gas transfer velocity over the study
period based on quadratic relationships developed
by Wanninkhof (1992) and Nightingale et al (2000)
(both formulas yielded k600 6.2 cm s-1,
normalized to a Schmidt number of 600 using ScSF6
819 at Tavg 24.9 C, Salavg 24). However, a
paucity of data in the literature from this low
wind regime, together with results from a recent
SF6 study of the lower Hudson (Ho et al 2002),
suggest that the k600 derived from this estimate
should be taken as a lower bound. References Ho
et al 2002, Environ. Sci. Tech. 36, 3234-3241
Nightingale et al 2000, Glob. Biogeo. Cyc., 14,
1, 373-387 Wanninkhof 1992, J.G.R., 97, C5,
7373-7382.
High-Resolution, Automated, Real-Time Measurement
System uses a continuous pump, a flow-through
membrane contactor, and a gas chromatograph
equipped with an electron capture detector
(GC-ECD). The system is deployed on an 11 m
wooden workboat with an enclosed cabin. A laptop
computer controls all functions, collects data,
and displays results in near-real time, allowing
on-the-fly revisions in sampling strategy.
New York is the third-busiest seaport in the
United States. Most of the shipping facilities
are in Newark Bay and in two connecting channels
Kill van Kull (8 km long) and Arthur Kill (20 km
long). The study area is subject to extensive,
ongoing navigational dredging.
Water is pumped through the membrane contactor,
where gases are stripped via counterflow N2.
The boat (Riverkeeper) in Nyack, NY. The sampling
pump attached to the bow is tilted up for storage.
SF6 injection. Gas is bubbled into the water
column via a perforated hose.
Fate of Tracer and Conclusions

• Tracers in
• the Estuary
• Looking Forward
• Verification tool for numerical flow models
• Develop predictive relationships for dispersion
  and gas exchange
• Tag chronic outfalls (CSO, industry, leachate)
• Disaster simulation

• High-energy tidal mixing, on the same order as
  the Hudson River estuary.
• Very slow residual advection. Non-volatiles can
  reside in the Kills system for weeks, and upriver
  penetration can be strong.
• Gas transfer velocity (k600, cm s-1) 6.2 lt k600
  lt 12.6
• Transport direction and rate is dominated by
  tidal dispersion at low flow periods, and cannot
  be accurately predicted from residual sub-tidal
  circulation.

Vertical profiles for SF6 were taken at selected
times and locations. The section above (4
profiles), from the day following injection,
suggests that the initial SF6 mass was
concentrated in the upper part of the water
column. Subsequent mixing reduced, but did not
eliminate, this bias.
The Arthur Kill displays typical estuarine
structure, with a salt wedge evident at the
southern (left) end of the section. Dense water
near the 23 km point in the above figure arrives
via the Kill van Kull, which is considerably less
stratified (see below).
Typical surface salinity.
SF6 concentrations throughout the experiment
showed a slight inverse correlation with depth.